What Is the Advantage of the Wound-Rotor Over the Squirrel-Cage Motor?

The company is the world’s best Wound Rotor Induction Motor Vs Squirrel Cage supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

The wound-rotor motor addresses these issues better than the squirrel-cage motor. This type of motor requires lower inrush current at the start-up phase, and starts up with a higher starting torque than its counterparts. This pullout torque is also enhanced by the wound-rotor design, making it an efficient speed-controlling motor.

Variable-speed drives

When comparing the wound-rotor and squirrel cage motor, the latter offers more energy efficiency. However, the wound-rotor is more expensive to manufacture and has fewer advantages over the former. For this reason, VSDs are not ideal for high-speed applications. These motors are not recommended for hazardous environments. In such areas, special equipment must be used.

The wound-rotor motor has greater starting torque and low starting current, making it better for applications where inrush currents are problematic. Unlike squirrel cage motors, wound-rotor motors' speed and torque characteristics can be altered by varying the external resistance. The squirrel cage motor, on the other hand, has fixed characteristics and is not suitable for variable-speed applications.

A wound-rotor motor is also more efficient for variable-speed applications. It consumes less power, and can start at a lower speed than the squirrel cage motor. It also has a higher capacity for overloading than the squirrel cage motor and has a lower starting current than its counterpart. It is also better for applications with inertial loads, as the wound-rotor motor develops high starting torque at standstill and low inrush current.

Another type of adjustable-speed drive is the slip-controlled adjustable speed drive. This type of drive uses slip rings in the external circuit to transfer torque to the output rotor. However, these slip rings can result in a low starting torque and high inrush current, causing the motor to fail. Furthermore, the slip-ring design can be prone to damage, including from heat, vibration, or centrifugal force.

More expensive

If you're looking for a high-quality AC motor, consider a squirrel cage induction motor. These motors are popular because they're inexpensive, robust, efficient, and reliable. You can also look into slip ring motors, which cost more, but have very few advantages. Slip ring motors are prone to copper losses and require frequent maintenance. This article will discuss the advantages and disadvantages of squirrel cage induction motors and help you choose the best one for your needs.

There are many types of induction and squirrel cage motors. They can be classified by size and torque, and are rated by the International Electrotechnical Commission (IEC). Single-phase motors need an initial 'shove' to start, while three-phase motors have synchronous speeds. Single-phase motors do not have these features. These motors can run on either a single-phase or three-phase power supply.

A squirrel cage motor uses a semi-closed rotor containing a core of copper or aluminium. The rotor has slots that are not parallel, which reduces humming and offers smooth torque curves. The rotor has a permanent slot at one end. Both types of squirrel cage motors have a rotary core with parallel and skewed slots. The squirrel cage motor is the most efficient of the two.

Cheaper

There are many benefits to a wound-rotor motor. It uses less current, is cheaper, and has a higher starting torque. It also has adjustable speed and torque. It is also more versatile and is often used in low-voltage applications. However, if you're considering buying one for your home, you should consider what the differences are. Here are some of the main differences between squirrel cage and wound-rotor motors.

First, a wound-rotor motor is more complex. The secondary circuit introduces more possibilities for errors. In addition, the slip-ring brushes are more prone to sparking, resulting in an increased fire risk. This lowers the overall efficiency of the motor, which is another disadvantage. The wound-rotor motor is also less efficient than the squirrel-cage motor.

While the squirrel-cage motor is less expensive, it's a better choice for some applications. The squirrel-cage motor is ideal for applications that require low torque or high torque. It's easy to install and relatively low cost compared to a wound-rotor motor. Compared to an induction motor, it requires less maintenance and is more affordable. A wound-rotor motor has more internal resistance, which can affect its efficiency.

Jessica Watson is a PHD holder from the University of Washington. She studied behavior and interaction between squirrels and has presented her research in several wildlife conferences including TWS Annual Conference in Winnipeg.

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Wound Rotor vs Squirrel Cage

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Wound Rotor vs Squirrel Cage

Wound Rotor vs Squirrel Cage

OhioAviator

(Electrical)

(OP)

14 Dec 03 18:48

Hi All,

I have a basic question regarding wound rotor motors vs squirrel cage motors in high horsepower (4,000 HP+)  automobile shredder applications. Automobile shredders, like any large rock crusher, experience very high shock loading. Which type of motor is better suited for this application, and why?

Thanks!

RE: Wound Rotor vs Squirrel Cage

Marke

(Electrical)

15 Dec 03 05:06

A wound rotor motor, with an appropriate secondary reistance starter is able to produce a high starting torque from zero speed through to full speed. This will result in a higher acceleration rate than you will achieve with a squirrel cage motor. The starting current will be lower and the motor will be able to start in loaded situations where a standard cage motor will not.
The negatives, are that both the motor and the starter will require a lot more maintenance than a standard cage motor, and the purchased price is higher.
Best regards,

Hello OhioAviatorA wound rotor motor, with an appropriate secondary reistance starter is able to produce a high starting torque from zero speed through to full speed. This will result in a higher acceleration rate than you will achieve with a squirrel cage motor. The starting current will be lower and the motor will be able to start in loaded situations where a standard cage motor will not.The negatives, are that both the motor and the starter will require a lot more maintenance than a standard cage motor, and the purchased price is higher.Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Wound Rotor vs Squirrel Cage

OhioAviator

(Electrical)

(OP)

15 Dec 03 07:51

Hi Mark,

Thank you for your reply to my question, I certainly appreciate it.

That leads me to my second question...
Can I achieve the benefits of a wound rotor motor (high starting torque w/ lower starting current) along with the added benefits of reduced maintenance by using a squirrel cage motor and an electronic soft starter?

Again, thanks for your help.

--John Ruble

RE: Wound Rotor vs Squirrel Cage

jbartos

(Electrical)

15 Dec 03 08:29

Suggestion to the previous posting: The soft starter and induction motor will approximately accomplish the similar output to the wound rotor motor. However, the soft starter may be more expensive and more demanding on the service. Also, MTBF may be lower for the soft starter.

RE: Wound Rotor vs Squirrel Cage

OhioAviator

(Electrical)

(OP)

15 Dec 03 08:37

Thank you jbartos,

I'm not sure about the MTBF being lower for a soft starter; they seem to be getting more and more reliable these days. But I do not have experience with medium voltage starters in the HP+ range, either.

Back to wound rotor motors...
Are liquid rheostat starters still the current, most reliable technology? If not, are there other types of wound rotor starters out there that are more reliable and less maintenance? If so, what manufacturers?

Again, thanks all for your help!

--John Ruble

RE: Wound Rotor vs Squirrel Cage

jbartos

(Electrical)

15 Dec 03 08:39

Suggestion to the original posting marked ///\\\
I have a basic question regarding wound rotor motors vs squirrel cage motors in high horsepower (4,000 HP+)  automobile shredder applications. Automobile shredders, like any large rock crusher, experience very high shock loading. Which type of motor is better suited for this application, and why?
///The mechanical load profile torque-speed needs to be known to be able to match the motor torque-speed characteristics. Assuming that high starting torque is required, then the squirrel-cage induction motor Nema Design D may be required. This could be the better solution than the wound-rotor induction motor since the motor may be DOL started, if the power distribution allows it, and it will be simpler to maintain than the wound-rotor induction motor. Normally, starting and operating conditions of this size of motors are simulated by using software, e.g. EMTP.\\\
 

RE: Wound Rotor vs Squirrel Cage

2

electricpete

(Electrical)

15 Dec 03 10:23

I don't have knowledge for comprehensive evaluation of your options compared to your application.

Considering only the motors themselves (not the other parts of the starting/control): I agree with jbartos the wound rotor motors require more maintenance. Also, in my experience we have much higher failure rate on wound rotor motors than on our squirrel cage motors (although the applications are not comparable' wound rotor motors are probably used in the more demanding applications).

RE: Wound Rotor vs Squirrel Cage

Marke

(Electrical)

15 Dec 03 13:13

Although you can certainly reduce the starting current by using a soft starter, you will not get the same starting effectiveness (Start torque/start current) using this method.
If you begin with a standard cage motor, particularly at this size, your starting torque will be low compared to that achievable with a wound rotor motor. The initial start torque may be in the region of 100 - 120% at 600 - 800% start current as opposed to to say 250% current for 200% torque with the would rotor.
When we apply a soft starter to a standard cage motor, we reduce the start current and also reduce the start torque by the current reduction squared.
For equal start current, the wound rotor motor and secondary resistance starter will produce many times the start torque of the soft starter and cage motor.

If you do not require a high start torque, then the soft starter and cage motor are definitely a very viable option. The reliability of correctly engineered soft start applications is very high. Some installations that I have been involved in are still operating correctly and without problems after twenty years.

I suspect, that for this application, you will require a high start torque, hence the suggestion of the wound rotor machine. This is based on my experience, but would depend on your actual requirements and parameters.

Best regards,

Hello OhioAviatorAlthough you can certainly reduce the starting current by using a soft starter, you will not get the same starting effectiveness (Start torque/start current) using this method.If you begin with a standard cage motor, particularly at this size, your starting torque will be low compared to that achievable with a wound rotor motor. The initial start torque may be in the region of 100 - 120% at 600 - 800% start current as opposed to to say 250% current for 200% torque with the would rotor.When we apply a soft starter to a standard cage motor, we reduce the start current and also reduce the start torque by the current reduction squared.For equal start current, the wound rotor motor and secondary resistance starter will produce many times the start torque of the soft starter and cage motor.If you do not require a high start torque, then the soft starter and cage motor are definitely a very viable option. The reliability of correctly engineered soft start applications is very high. Some installations that I have been involved in are still operating correctly and without problems after twenty years.I suspect, that for this application, you will require a high start torque, hence the suggestion of the wound rotor machine. This is based on my experience, but would depend on your actual requirements and parameters.Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Wound Rotor vs Squirrel Cage

OhioAviator

(Electrical)

(OP)

15 Dec 03 15:12

Hello Marke,

Thank you very much for your explanation of starting squirrel-cage motors vs wound-rotor motors. What you say makes perfect engineering sense, substantiated by my own personal observations in the field.

What is your opinion of solid-state wound-rotor motor starters? Are they more reliable than liquid rheostat starters? What about maintenance requirements? I've been looking at a couple of solid state wound-rotor motor starters from Benshaw, Inc. Any experience with these types of starters?

Again, many thanks!

--John Ruble

RE: Wound Rotor vs Squirrel Cage

2

DougMSOE

(Electrical)

15 Dec 03 15:20

I have personally been to installations that used soft starters, and they were very sorry that they had them.
Like the previous posts, a wound rotor induction motor (WRIM) will have faster acceleration and an adjustable speed torque curve, which a squirrel cage motor (SCIM) does not have.
One important point that many SCIM manufactures do not understand is the KWH demand billing. What this means is that as the motor is crushing cars, there is a demand limit that you do not want to go over otherwise you will be paying substantially higher energy bills to compensate for that (for example) 5 second period that you went over that value.
Yes, you will get full load torque at standstill with about 70% of your full load (rated) starting current, provided that the liquid rheostat is operating correctly AND the brine solution is correct.

PS The best way to determine if the brine solution is of the correct concentration is to use a hydrometer (density test of the solution). Please take note that the concentration needs to be temperature compensated.

RE: Wound Rotor vs Squirrel Cage

Mendit

(Mechanical)

15 Dec 03 19:07

Auto shredder motors see far more than shock loading, the main area which I did not see covered is the extended period of time where the load on the motor reduces the speed of the driven equipment. This is where the SCIM fail, they do not work well when punished at less than full speed.
I have seen many shredder motors and they are far beyond the capabilities of any other motor. The service factor is typically 2. Schorch make one of the best motors I have seen for this application. They are of course the wound rotor type.  P&H may have a shredder motor also.
On SCIM motors the rotor will usally break up as a result of the rotor running slower than design.

RE: Wound Rotor vs Squirrel Cage

OhioAviator

(Electrical)

(OP)

16 Dec 03 08:14

Thank you, all, and especially to DougMSOE and to Mendit. Both of you obviously have specific knowledge of the scrap metal processing industry (far more than I have, that's for sure!) and car shredders in particular. I originally come from the mining industry and while that industry has large crushers, I've never seen mining crushers (and motors) that take the punishment that car shredders do.

I think I'm convinced now that a WRIM really is the better choice for our applications in this industry.

One thing that still puzzles me though, is the fact that we still seem to purchase and use liquid rheostats instead of electronic starters. Is this because you can continuously vary the resistance of a liquid rheostat while in operation? And if so, what benefits are to be gained from running a WRIM with resistance in the rotor circuit? Does rotor resistance help limit KW Demand?

Again, thank you everyone!

--John R.

RE: Wound Rotor vs Squirrel Cage

jbartos

(Electrical)

16 Dec 03 09:04

Suggestion to the previous posting marked ///\\\
One thing that still puzzles me though, is the fact that we still seem to purchase and use liquid rheostats instead of electronic starters.
///The cost and reasonable reliability including simplicity may be the main reasons.\\\
 Is this because you can continuously vary the resistance of a liquid rheostat while in operation?
///Yes, in comparison with wound resistors; no in comparison with soft starters.\\\
 And if so, what benefits are to be gained from running a WRIM with resistance in the rotor circuit?
///The resistors require less maintenance, can have better location, can be remotely controlled, etc.\\\
 Does rotor resistance help limit KW Demand?
///In which context or comparison?\\\
///Please, notice that the above postings did not differentiate among squirrel-cage induction motor Nema Design letters. Nema design letter D has noticeably different characteristics from other Nema design letters. The Nema design letter D motor has substantially higher starting torque than other ones.
Another aspect that has not been addressed yet would be adding a flywheel. However, this would impact the operation of the crusher since it would have to start empty to minimize the starting torque unless the flywheel is disengageable. \\\

RE: Wound Rotor vs Squirrel Cage

TheDOG

(Electrical)

4 Jan 04 19:41

A Variable Speed Drive (VSD) will provide the most amount of torque per amp of any motor starting method available.  If you choose a reputable supplier (of both motor and VSD), reliability will be much better than the WRM option.
An added bonus is that you will get full speed control.

RE: Wound Rotor vs Squirrel Cage

GGOSS

(Electrical)

4 Jan 04 23:30

Hello OhioAviator,

Although I fully agree with and support much of what has been said here, I believe you need to consider each application on a case-by-case basis, particularly if you are aiming achieve best performance with the lowest possible capital outlay.

Wound Rotor Motors are capable of producing high torque when controlled via a 'properly engineered' liquid resistance starter. Their torque/speed characteristics are also well suited to applications that present transient over-load conditions and this can be further enhanced in shredding applications through the addition of a flywheel to the mechanical system. On the down side they do require more maintenance than a Squirrel Cage Motors and this needs to be factored into the decision making process.

Medium (and High) Voltage Squirrel Cage Motors are notorious for their poor start performance, often exhibiting levels of Locked Rotor Torque less than rated or Full Load Torque. As has been advised by Marke the addition of a soft starter will act to reduce starting torque further and therefore it is likely problems will be experienced here. This might also explain comments by DougMSOE! On a more positive note some motor manufacturers will design and manufacture Medium Voltage Cage Motors to your specifications, thus allowing you (possibly) to utilize Full Voltage Starting techniques.

As suggested by TheDOG, a Cage Motor controlled via a Variable Speed Drive will provide the best overall performance......dollars permitting of-course!

If your clients are anything like mine (unlimited dollars are not available to them), an analysis of motor and load curves should assist to determine best starting method for the intended application.

Regards,
GGOSS

RE: Wound Rotor vs Squirrel Cage

2

gsimson

(Electrical)

5 Jan 04 02:45

I will just add only one point.
With VFD with SQ. Cage motor performing satisfactorily there is a possibility of DC motors and Slip ring Motors becoming "Show case items" and limited to academics especially in large industries operating in dusty and humid atmosphere.
As on today there is no application which can not be satisfactorily performed by SC motor with VFD. Considering the maintenance, this option will workout cheaper in the long run

RE: Wound Rotor vs Squirrel Cage

OhioAviator

(Electrical)

(OP)

5 Jan 04 08:26

"Thank You" to TheDog, GGoss, and GSimon for your replies to this thread. While I agree that a SCIM/VFD combination might produce starting torques comparable to a WRIM, the up-front capital costs tend to be prohibitively higher. (Remember, our world is dominated by bean counters who, by and large, look only to the end of the current fiscal quarter.)

What I don't have a good handle on, though, is how it is possible to continuously manage (control) KW Demand with a SCIM/VFD combination like I can a WRIM/Liquid Rheostat. Is it possible?

Thanks,
J Ruble

RE: Wound Rotor vs Squirrel Cage

electricpete

(Electrical)

If you want to learn more, please visit our website Wound Rotor Induction Motors.

5 Jan 04 09:32

Total input power  = motor output mechanical power plus motor/drive system losses.

Output mechanical power can be roughly controlled by adjusting speed for both the wound rotor and SCIM+VFD.

Losses would have to be considered for each system.  I think the wound rotor will have increasing external (rheostat) losses as you attempt to adjust speed lower.

RE: Wound Rotor vs Squirrel Cage

aolalde

(Electrical)

5 Jan 04 12:08

Ohio Aviator.

You should consider both, starting and full speed operation of the motor.

My opinion is that WRIM with a proper matched resistance will provide the higher starting torque with the lower KVA inrush.

At full speed operation, your load application could develop sudden high peak Torque overloads. A flywheel could provide the extra required torque combined with some resistance in the rotor circuit to allow the rotor to slip under a shock load and then gradually accelerate at operating speed again.

The kilowatt demand is due to load plus losses and you have to provide it since it is pure energy ( the flywheel, if you have one, will provide momentary kinetic energy but then the motor will return that energy while accelerating back to full speed ) A variable frequency driver will develop a Volt/Hertz electric input to the motor but the voltage drops following the frequency to avoid magnetic saturation, then your torque could be constant but not larger than under 60 Hz operation.

A Olalde.

RE: Wound Rotor vs Squirrel Cage

rlpuck

(Electrical)

5 Jan 04 13:04

The only advantage the wound rotor motor offers is during start up of high torque applications.  External resisters are used in the rotor circuit to limit starting amps in the rotor.  Once the wound rotor motor comes up to speed  all external resistance is shorted out and the motor performs like a squirrel cage motor.  Your application may not require high starting torque as I would assume the shredder is started unloaded, i.e. without a car in the shredder.  Therefore the wound rotor motor may not be benificial.  The Design class D squirrel motor are specifically design for surge amplications.  

RE: Wound Rotor vs Squirrel Cage

DougMSOE

(Electrical)

5 Jan 04 14:11

Having installed the 3 different types of motors in scrap yards WRIM,SCIM and Synch (and a few DC) the shock loading can take a motor from operation RPM to 1/4 of the at RPM in less than 2 seconds! Be advised, what is constant with scrap-- NOTHING! While in the theory world 'white goods' are only 'white goods', you may also have the "engine block" mixed in with them. By far a WRIM is totally the only motor that can handle 4* rated current for 30m seconds while the solid state world is in smoke. Been there and saw that. Reliabiliy is best when you can fix it youeself.

RE: Wound Rotor vs Squirrel Cage

OhioAviator

(Electrical)

(OP)

5 Jan 04 14:38

Thank you, DougMSOE.
I'm finding myself pretty much in complete agreement with you, especially as time goes on and I gain more and more operating experience with these shredders. You are so right that 'white goods' ain't always 'white goods'. And I doubly agree with your statement that "Reliability is best when you can fix it yourself"! Been there, done that... too many times. I've spent way too many long nights and weekends learning how to fix the supposedly superior 'latest and greatest' electronic gadget, wishing I had a simple molded case circuit breaker and an across-the-line starter, instead.

Gentlemen (and ladies, too) thanks for your input on this thread. I think I have enough info for now. If a new posting comes up I'll check it out but I can't promise I'll respond to further postings.

Again, many thanks!

John R.

RE: Wound Rotor vs Squirrel Cage

Aquarius

(Electrical)

8 Jan 04 03:16
I was on holidays and missed the whole discussion
Apologies to jbartos, electricpete,dougmsoe, gsimson if I repeate what you have already said; you guys provided v.good replays.

OhioAviator- you did not state at the beginning what is the application you are looking for, speed control or soft start ?
Anyway, here is my contribution:

Solid State Soft Start Systems reduce the available full voltage start torque when applied to SQ or WR induction motors. Hence applications requiring a maximum starting torque cannot  be used with a soft starter. However, where the WR motor has been used to reduce and control the starting torque application of soft start control can be readily adopted.

The characteristic of increasing torque with decreasing current frequently determines the selection of the WR motor when starting current limitations are severe. I.e if a power company regulations limits LRC to 1.5 FLC , WR motor would still produce a LRT of 1.5 FLT.

Each addition of R in the rotor circuit reduces motor speed . Speed reduction is practical only to 50 % of synchronous speed. Beyond that speed becomes unstable because high slip characteristics are produced in the a rotor that operates with a high resistance in its circuit. For this reason application where speed is to be reduced <50% are used only where a constant loads are involved( bridge, trolley, crane)
Normally, a WR motor is designed to operate with small slip and high efficiency at full load. So, when speed is reduced , slip increases and efficiency decreases.

When operating at anything other then max speed the resistance control of WR motor consumes power that would otherwise be used to move the load. This inherent inefficiency of WR design has lead to its obsolescence in variable speed application.
Consequently, applications requiring variable speed 3 ph motors now use a variable frequency drive (VFD) to adjust the speed of a standard SQ motor. The VFD electronically changes the frequency of the AC current supply , thus changing the  synchronous speed of the motor. This has a huge benefits in power consumption over the resistance control.

The other problem with application of WR motor in variable speed application is that if the secondary resistance value of each phase become unbalance, the vibratory torque is generated.

WR motor can be applied on either constant speed or adjustable-speed drives. They are particularly suitable for smooth acceleration of loads in application which require high starting torque with low starting current or impact load (elevators, ventilating fans, printing press, pumps, compressors, conveyors, pulverizers, stokers, positive pressure blowers, crushers, shredders .
In summary major reasons for selecting WR motors are:

1.    The load can be started at the max torque
2.    Large starting torque can be obtained in comparison with low starting current
3.    In case when starting frequency is too high for  the thermal resistivity of the an ordinary SC motor
4.    In case GD2  of the load is too large for the thermal resistivity of an ordinary SC motor
5.    In case load requires cushion starting

Today, owning to their complicated construction and hence high maintenance and cost, WR motors are used mainly in applications when relatively high starting torque is required, but in which the starting current may not exceed the FLC much-application 2 above.
 
Note ' in very large applications + kW a solid state slip energy recovery (SER) system in conjunction with the conventional slipring resistors is used to perform variable speed control.

I know you've said you do not want any more emails, so please do not feel under obligation to replay.I was on holidays and missed the whole discussionApologies to jbartos, electricpete,dougmsoe, gsimson if I repeate what you have already said; you guys provided v.good replays.OhioAviator- you did not state at the beginning what is the application you are looking for, speed control or soft start ?Anyway, here is my contribution:Solid State Soft Start Systems reduce the available full voltage start torque when applied to SQ or WR induction motors. Hence applications requiring a maximum starting torque cannot be used with a soft starter. However, where the WR motor has been used to reduce and control the starting torque application of soft start control can be readily adopted.The characteristic of increasing torque with decreasing current frequently determines the selection of the WR motor when starting current limitations are severe. I.e if a power company regulations limits LRC to 1.5 FLC , WR motor would still produce a LRT of 1.5 FLT.Each addition of R in the rotor circuit reduces motor speed . Speed reduction is practical only to 50 % of synchronous speed. Beyond that speed becomes unstable because high slip characteristics are produced in the a rotor that operates with a high resistance in its circuit. For this reason application where speed is to be reduced <50% are used only where a constant loads are involved( bridge, trolley, crane)Normally, a WR motor is designed to operate with small slip and high efficiency at full load. So, when speed is reduced , slip increases and efficiency decreases.When operating at anything other then max speed the resistance control of WR motor consumes power that would otherwise be used to move the load. This inherent inefficiency of WR design has lead to its obsolescence in variable speed application.Consequently, applications requiring variable speed 3 ph motors now use a variable frequency drive (VFD) to adjust the speed of a standard SQ motor. The VFD electronically changes the frequency of the AC current supply , thus changing the synchronous speed of the motor. This has a huge benefits in power consumption over the resistance control.The other problem with application of WR motor in variable speed application is that if the secondary resistance value of each phase become unbalance, the vibratory torque is generated.WR motor can be applied on either constant speed or adjustable-speed drives. They are particularly suitable for smooth acceleration of loads in application which require high starting torque with low starting current or impact load (elevators, ventilating fans, printing press, pumps, compressors, conveyors, pulverizers, stokers, positive pressure blowers, crushers, shredders .In summary major reasons for selecting WR motors are:1. The load can be started at the max torque2. Large starting torque can be obtained in comparison with low starting current3. In case when starting frequency is too high for the thermal resistivity of the an ordinary SC motor4. In case GD2 of the load is too large for the thermal resistivity of an ordinary SC motor5. In case load requires cushion startingToday, owning to their complicated construction and hence high maintenance and cost, WR motors are used mainly in applications when relatively high starting torque is required, but in which the starting current may not exceed the FLC much-application 2 above.Note ' in very large applications + kW a solid state slip energy recovery (SER) system in conjunction with the conventional slipring resistors is used to perform variable speed control.

RE: Wound Rotor vs Squirrel Cage

jbartos

(Electrical)

8 Jan 04 08:50

Suggestion/question: It is not clear from the original posting whether or not the cost is a factor. Please, would you address that point? Any better solution will probably be much costlier; especially, considering modern power electronics options aligned with the squirrel-cage induction motors and wound rotor induction motor.

RE: Wound Rotor vs Squirrel Cage

OhioAviator

(Electrical)

(OP)

8 Jan 04 09:38

Hello Aquarius and Mr. Bartos...

Thanks for your postings to this thread.

Aquarius - The primary parameters I'm looking for are current limited starting, kW Demand control, high slip capability to accommodate very high short term shock loads, and reliability with simplicity. Variable speed control really isn't a goal, just high slip capability.

JBartos - Cost isn't the primary factor, but it probably ranks second. Reliability in this particular application (automobile shredders) is paramount. Simplicity is also important, as we typically don't have highly skilled electrical technicians on the payroll.

As I eluded to previously, I think I'm pretty well convinced that the WRIM is the best option, primarily due to the severity of the duty required for our application. Automobile shredders are "crushers" taken to the extreme. As DougMSOE stated above, a typical automobile shredder is shredding light metal (car bodies) together with heavy chunks of metal (engine blocks). The impact shock loads are tremendous.

Again, thanks!

RE: Wound Rotor vs Squirrel Cage

2

jraef

(Electrical)

8 Jan 04 14:26
I am coming into this late due to the holidays as well, but I would like to add my opinion. I am a big proponent of MV solid state controls, but I, like GGOSS, consider each application on its own merits.

I have applied SS starters to SCIM motors in many shredder applications successfully and they work just fine. However they were waste shredders not AUTO shredders. IMHO, in your application the WR motor is probably better for the shock load capability as mentioned by others. Many of the above posts refer to starting torque issues but may not realize that a shredder NEVER starts with the load already in it (the exception being hydraulic powered shredders, but thats a different discussion). You are looking for a reduction in starting torque since the shredder is always unloaded at start. The slip recovery capability of the WR motor can be "adjusted" by altering the resistance on the secondary, a very useful feature for your application. Look at it this way, a WR motor has all of the capabilities of Design A, B, C and D SCIM motors by simply changing the rotor resistance. With a SS starter on a SCIM, once it is at full output the starter has NO control of torque capability. Your motor is on its own with just the torque and slip recovery capabilities inherent to its design.

One other possibility is what was mentioned by rlpuck on 1/5/04, the use of a Design D squirrel cage motor. It has the slip recovery capability equal to the maximum available in the WR. If you go that route, the RVSS starter would be useful to keep the starting torque to a minimum. If you don't already have a motor you may want to consider this. If you already have a WR motor it would not be worth swapping it out.

By the way, don't be fooled by the Benshaw marketing of their "solid state wound rotor" control. It is just a SS starter for the stator, combined with a fixed resistor in the rotor, which is shorted out at full speed. If you are going to go with WR, have a competent control manufacturer build a starter and resistor control package that matches your needs. SS is kind of a waste on WR motors. Also IMHO liquid rheostat WR controllers are good for things like flow control etc., but the maintenance costs and care required to keep them running would make them poor candidates for your application. Search this forum for the term "Liquid Rheostat" and you will see several discussions pertaining to LR maintenance issues. Shredder operators are not known for being mindful of routine maintenance!

OhioAviator,I am coming into this late due to the holidays as well, but I would like to add my opinion. I am a big proponent of MV solid state controls, but I, like GGOSS, consider each application on its own merits.I have applied SS starters to SCIM motors in many shredder applications successfully and they work just fine. However they were waste shredders not AUTO shredders. IMHO, in your application the WR motor is probably better for the shock load capability as mentioned by others. Many of the above posts refer to starting torque issues but may not realize that a shredder NEVER starts with the load already in it (the exception being hydraulic powered shredders, but thats a different discussion). You are looking for a reduction in starting torque since the shredder is always unloaded at start. The slip recovery capability of the WR motor can be "adjusted" by altering the resistance on the secondary, a very useful feature for your application. Look at it this way, a WR motor has all of the capabilities of Design A, B, C and D SCIM motors by simply changing the rotor resistance. With a SS starter on a SCIM, once it is at full output the starter has NO control of torque capability. Your motor is on its own with just the torque and slip recovery capabilities inherent to its design.One other possibility is what was mentioned by rlpuck on 1/5/04, the use of a Design D squirrel cage motor. It has the slip recovery capability equal to the maximum available in the WR. If you go that route, the RVSS starter would be useful to keep the starting torque to a minimum. If you don't already have a motor you may want to consider this. If you already have a WR motor it would not be worth swapping it out.By the way, don't be fooled by the Benshaw marketing of their "solid state wound rotor" control. It is just a SS starter for the stator, combined with a fixed resistor in the rotor, which is shorted out at full speed. If you are going to go with WR, have a competent control manufacturer build a starter and resistor control package that matches your needs. SS is kind of a waste on WR motors. Also IMHO liquid rheostat WR controllers are good for things like flow control etc., but the maintenance costs and care required to keep them running would make them poor candidates for your application. Search this forum for the term "Liquid Rheostat" and you will see several discussions pertaining to LR maintenance issues. Shredder operators are not known for being mindful of routine maintenance!

Quando Omni Flunkus Moritati

RE: Wound Rotor vs Squirrel Cage

OhioAviator

(Electrical)

(OP)

8 Jan 04 14:41

Hello jraef,

Thanks for your post. And thanks for confirming my suspicions that at WRIM is probably the best choice for our application. And you're right... shredder operators are DEFINITELY not known for routine maintenance.

BTW... thanks for making me look on the internet for the translation to your latin phrase (curiosity, you know)!

Cheers

RE: Wound Rotor vs Squirrel Cage

jbartos

(Electrical)

11 Jan 04 01:55
http://www.elec-toolbox.com/motorchar.htm
for:
Squirrel-Cage Induction Motors - The most simple and reliable of all electric motors. Essentially a constant speed machine, which is adaptable for users under all but the most severe starting conditions. Requires little attention as there are no commutator or slip rings, yet operates with good efficiency.

Wound-Rotor (Slip Ring) Induction motor - Used for constant speed-service requiring a heavier starting torque than is obtainable with squirrel cage type. Because of its lower starting current, this type is frequently used instead of the squirrel-cage type in larger sizes. These motors are also used for varying-speed-service. Speed varies with this load, so that they should not be used where constant speed at each adjustment is required, as for machine tools.
Comment marked ///\\\
Reviewing the above postings and agreeing with the above link, the squirrel-cage induction motor will be the best solution. The flywheel for smoother ride-throughs should not be ruled out.

Suggestion: Visitfor:Squirrel-Cage Induction Motors - The most simple and reliable of all electric motors. Essentially a constant speed machine, which is adaptable for users under all but the most severe starting conditions. Requires little attention as there are no commutator or slip rings, yet operates with good efficiency.Wound-Rotor (Slip Ring) Induction motor - Used for constant speed-service requiring a heavier starting torque than is obtainable with squirrel cage type. Because of its lower starting current, this type is frequently used instead of the squirrel-cage type in larger sizes. These motors are also used for varying-speed-service. Speed varies with this load, so that they should not be used where constant speed at each adjustment is required, as for machine tools.Comment marked ///\\\Reviewing the above postings and agreeing with the above link, the squirrel-cage induction motor will be the best solution. The flywheel for smoother ride-throughs should not be ruled out.

RE: Wound Rotor vs Squirrel Cage

DougMSOE

(Electrical)

11 Jan 04 16:48

With all due respect jbartos.
Squirrel cage motors have been used in the auto shredder applications, BUT the tons/KWH is less for a SCIM than for a WRIM, all things the same.
Unless you have actually been to and worked in an auto/industrial scrap yard where the cars/busses, etc are sherdded from their present form to 'half-dollar size' in less than 10 seconds the true appreciation of the WRIM will never be appreciated. I have worked in the heavy power industry for over 30+ years and watching a shredder do its job is amazing to say the least. A 20,000 HP motor driving a BFP, ID or FD fan is no comparison to what a 7,000HP motor goes through in 10 min of its job in a scrap yard.
Yes, a SCIM is good but will NEVER come close to taking the shock loadings that a WRIM does. Further, the WRIM can be made to operate very closly to the SCIM in this application.
PS I have installed about 10 shredder motors and have worked on over 50 of then.
DougMSOE

RE: Wound Rotor vs Squirrel Cage

SRC99

(Electrical)

5 Feb 04 19:59

The metal type resistive controller for WRIM is more reliable than liquid rheostat starters. The controller consists of only three durable parts: stainless steel resistors, vacuum contactors and PLC.  It's reliable and maintenance free. PLC controls contactor close/open to switch resistor in/out to control the torque and speed. The controller allows the motor to deliver up to its breakdown torque to the driven load during starting or running.The controller is pre-programmed to meet your specific load characteristic

RE: Wound Rotor vs Squirrel Cage

Marke

(Electrical)

5 Feb 04 20:19
A wound rotor starter requires reducing resistance as the motor accelerates so there is an advantage in using the liquid type resistor.
Best regards,

Metal type resistors tend to have a positive temperature constant causing the value of the resistors to increase with heat. Liquid type resisters have a negative temperature coefficient resulting in reducing resistance with heat.A wound rotor starter requires reducing resistance as the motor accelerates so there is an advantage in using the liquid type resistor.Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Wound Rotor vs Squirrel Cage

jbartos

(Electrical)

5 Feb 04 21:06
Reference:
Donald D. Fink, H. Wayne Beaty "Standard Handbook for Electrical Engineers," 13th Edition, McGraw-Hill, Inc., ,
Section: Characteristics of Polyphase Induction Motors on page 20-33 objectively compares various motors characteristics.
Visit
http://www.ecmweb.com/ar/electric_keep_speed_motor/
http://turbolab.tamu.edu/pubs/Pump20/p20pg095.PDF
http://www.dac-3d.com/Knucklebuster/PDFFiles/E410-S02.p...
etc. for more info

Suggestion: The Squirrel-Cage Induction Motor (SCIM) has some room for customization based on its intended application. SCIMs with NEMA Design Letter D can handle very demanding applications, e.g. openings of the rusty valve, punch presses, cranes, hoists, press brakes, shears, oil-well pumps, centrifugals, etc.Reference:Donald D. Fink, H. Wayne Beaty "Standard Handbook for Electrical Engineers," 13th Edition, McGraw-Hill, Inc., ,Section: Characteristics of Polyphase Induction Motors on page 20-33 objectively compares various motors characteristics.Visitetc. for more info

RE: Wound Rotor vs Squirrel Cage

DougMSOE

(Electrical)

6 Feb 04 10:08

The resistor grid system has been used in several WRIM scrap yards and they do work reasonably well. The problem is that they become very hot and with all of the dust and fluff from the car seats you will have a fire. Been there and have installed the liquid rheostats that do not have the problem.
Further, the square footage req'd for the liquid Vs the grid system is much less about 60%. And with the grid system you need large contactors to control the speed torque curve such that the max torque can be placed at the resultant RPM from the load on the shredder.
Yes the SCIM does have some room for customization but does not have the flexibility that the WRIM needs to have to do its job.
I do not care for any book that you might be able to cite that a SCIM is better than a WRIM in this application. There is NO COMPARISON in this application. A NEMA D does not even get close to what a WRIM can do in this application.

RE: Wound Rotor vs Squirrel Cage

jbartos

(Electrical)

8 Feb 04 06:01

Question to the previous posting: Please, would you be more specific in engineering and design terms in your statement:
""Yes the SCIM does have some room for customization but does not have the flexibility that the WRIM needs to have to do its job."" specifically, if the WRIM flexibility is clarified?

RE: Wound Rotor vs Squirrel Cage

DougMSOE

(Electrical)

9 Feb 04 10:24

jbartos, To your question, I hope that this is what you are asking.
"Please, would you be more specific in engineering and design terms in your statement:

I would ask that you might take a look as to what the difference is between a NEMA design A,B,C,D,and F of a 3ph SCIM. What actually is the difference?
Given all other parameters the same the one major difference is the metallurgy of the rotor bars, and they are not Cu, but a Cu alloy!!! The resistivity of the bars AND the shorting rings are what make the difference.
Now thak a look at the equivalent circuit for an induction motor, NOT the equivalent circuit cited to simplify the circuit such that the well known circle diagram is made. You will notice that the maximun power transfer theorm applies, not just by load but also by RPM!
This is why the WRIM does this job better than any other motor or for that matter a diesel engine, which has also been used.
I the WRIM the secondary resistance may be changed at will to a range of resistances 20 times that of a SCIM even mor if required.

RE: Wound Rotor vs Squirrel Cage

aolalde

(Electrical)

9 Feb 04 15:27

The WRIM could work with variable external resistance values connected to the rotor circuit.

That feature makes it feasible to be adjusted to almost any desired Torque-Line current- slip characteristic.

Put a high external resistance and the inrush current will drop close or even under full load current and at the same time the resultant Torque becomes more current effective.

Adjusting the external resistance properly,you will get any desired 'Design Performance' from a NEMA design A to a D and much more.

Adjust the external rotor resistance when the motor is running with load and the speed will be reduced with an increase in the rotor slip.

An ISCM has a fix rotor construction and after it is constructed it will have a fixed rotor resistance and performance.

The heat generated in the rotor winding is dissipated into the rotor of an ISCM. For a WRIM most of the rotor heat is dissipated in the external resistors. A NEMA design A (ISCM) has high efficiency at full load and low slip but very high inrush current and only 150% LRT. A NEMA design D (ISCM) has 275% LRT and 400% inrush but generates too much heat and low efficiency under load.

That makes a (WRIM) Wound Rotor Induction Motor performance very superior if compared to an (ISCM) Induction Short Circuited-rotor Motor (squirrel cage).

Costs of Maintenance and Initial investment are certainly much lower for an ISCM, that is why it is the first selection as far as it could be tailored properly to the load.

RE: Wound Rotor vs Squirrel Cage

DougMSOE

(Electrical)

9 Feb 04 18:55

You've got it aolalde!!!
The only addition to your post is that the starting current of the WRIM with a liquid rheostat is usually set at 70 to 75 % of the full load running current (this is where the maximum torque is usually developed). Typically the liquid rheostat uses a solution of soda ash and temperature (more on the soda ash) to adjuse the Istart via the secondary resistance.
Further, the WRIN is made top operate with the resistance in the rotor circuit.
Have a Great Day!!!
Doug

RE: Wound Rotor vs Squirrel Cage

gsimson

(Electrical)

9 Feb 04 23:47

WRIM with resistor grid / Liquid resistor is a "Load dependent" drive. The torque developed at various speeds is dependendent on load hooked. SCIM with VFD is load independent and developes PULL OUT TORQUE at ALL speeds.

The current requirement is based on the load and the power "wasted" in the resistor. With SCIM /VFD there is no wastage of power . The motor draws power only to the extent required. "Solid state going into smoke" is only hypothetical and certainly not required for real applications.

RE: Wound Rotor vs Squirrel Cage

DougMSOE

(Electrical)

10 Feb 04 15:23

Since the secondary resistance can be increased or decreased as required to match, as you put it, the PULL OUT TORQUE, the load WRIM is load dependent. Certainly, as you describe the SCIM with a VFD is, within the limits of the machine, is load independent. And yoe the heat developed by the resistor bank is some what wasted unless like a few scrap yards have done is to provide heating and cooling as required. Un fortunately I HAVE SEEN FIRST HAND secondary energy pump back systems going into smoke as well as the front end drives. What is required to understand in this application of WRIM in scrap yards is,
1.)easy to fix with only simple parts and simple equipment.
2.)TOUGH BEYOND BELIEF i.e. shock, vibration are standard. Again PLCs have had boards shaken out of their cages.
3.)First cost a big factor
4.)Easy to understand

Ultimate reliability is achieved when you can fix what is broken with a Simpson 260, a screw driver and an adjustable wrench.

RE: Wound Rotor vs Squirrel Cage

jraef

(Electrical)

10 Feb 04 16:34

Don't leave out the duct tape!

"Venditori de oleum-vipera non vigere excordis populi"

RE: Wound Rotor vs Squirrel Cage

OhioAviator

(Electrical)

(OP)

11 Feb 04 08:54

The screwdriver (if you don't mind the broken handle or broken shaft), and the adjustable wrench (if you don't mind the finishing nail sticking out of where the knurled shift adjuster lockscrew is supposed to be), and the duct tape (after you warm it up and peel it off the old furnaces and ductwork) are all three easily obtained from the shredder feed pile. The  Simpson 260 will probably need to be purchased new (I probably wouldn't trust a meter that I dug out of the shredder pile; at least not more than once!). As DougMSOE says, we are VERY cost conscious!

In all serious folks, I had no idea that this thread would be so popular. Thank you for all the great and valuable input. I learned a great deal from this thread.

Best Regards,
John Ruble

BTW... I'm thoroughly convinced that a WRIM is all-round the best way to go for a scrap yard shredder application.

RE: Wound Rotor vs Squirrel Cage

jraef

(Electrical)

11 Feb 04 22:04

Spoken like a man forced to improvise on the job trying to get something operating before he is allowed to go home...

Been there, done that, got the T-shirt and the hat.

RE: Wound Rotor vs Squirrel Cage

jbartos

(Electrical)

12 Feb 04 21:05

Questions: How much time/downtime does it take to replace worn out slip rings of WRIM, how soon do they get worn out, and how much does it cost?

RE: Wound Rotor vs Squirrel Cage

DougMSOE

(Electrical)

13 Feb 04 09:45

jbartos,
That depends on;
The design of the slip ring assembly.
I have seen these motors in this duty last 20+ years with only cleaning and a air dry insulating varnish and no ring change outs.
Other machines with a poorly designed slip ring assembly and poor maintenance 5 years.
Some designs can be changed out inplace in about 4 hours, others inplace, 2 to 4 days.

RE: Wound Rotor vs Squirrel Cage

FACB25

(Electrical)

13 Feb 04 22:54

One of the more informative and entertaining threads I've seen.  Thanks to all.
Smaller scale similar problem is the chipper in a sawmill, high shock loads when a chunk of oak or hard maple goes through.  We used a slightly oversize SCIM with a Benshaw SS.  The chipper itself includes a large inertia mass which helps.  I guess my rambling point is that while a WRIM is probably best, look to increase inertia if possible.

RE: Wound Rotor vs Squirrel Cage

aolalde

(Electrical)

13 Feb 04 23:17

FACB25:

I am glad that you mentioned INERTIA, which is a resource that has been forgotten lately. It is interesting the way  it works on heavy peak load applications.

RE: Wound Rotor vs Squirrel Cage

jbartos

(Electrical)

15 Feb 04 02:49

Comment on DougMSOE (Electrical) Feb 13, marked ///\\\
jbartos,
That depends on;
The design of the slip ring assembly.
I have seen these motors in this duty last 20+ years with only cleaning and a air dry insulating varnish and no ring change outs.
///This 20+ years life-expectancy is probably what the automobile shredders WRIM need.\\\
Other machines with a poorly designed slip ring assembly and poor maintenance 5 years.
///The frequent current transients due to variable and impulsing automobile shredder loads reduces the slip ring life expectancy.\\\
Some designs can be changed out inplace in about 4 hours,
///4 hours for the slip ring replacement seems to be optimistically short time; especially, if the rotor balancing is included.\\\
 others inplace, 2 to 4 days.
///This is more like it, if the good unionized workmanship is in place.\\\

RE: Wound Rotor vs Squirrel Cage

gsimson

(Electrical)

16 Feb 04 04:36

Some of the requirements which are required to be met as indicated are better met by SqIM along with VFD.
Easy to fix- No rotor resistance, Rotor cables , Rotor contactors . Only a Sq. cage motor with 3 leads connected to VFD.
Simple parts & simple equipment - What is nmore simpler than SQIM?
Tough beyond belief ( Shock , Vibration) - Sliprings & rotor resistances are certainly not tough.
Cost - A slipring motor with rotor resistance / rotor panel is certainly costlier than a Sq. Cage motor & VFD.
Easy to understand - The requirement is only for maintenance. When SqIM & VFD do not need any maintenance why every one need to understanmd ?
ALL repeat ALL the applications can be met by SQIM & VFD at a cheaper cost both capital & running, with min. maintenace and much better speed /Torque control. Both Dc motor and WRIM will become museum pieces & for academic interests as stated earlier.
Only the cost of VFD is abnormally high for MV applications. Over a period of time this also will come down.

RE: Wound Rotor vs Squirrel Cage

electricpete

(Electrical)

16 Feb 04 12:54
I have to take exception with one thought process:

"Easy to understand - The requirement is only for maintenance. When SqIM & VFD do not need any maintenance why every one need to understanmd ?"

I wouldn't buy from anyone that told me I don't need to understand it because it's never gonna break

gsimson - I appreciate all your expert advice and opinions.I have to take exception with one thought process:"Easy to understand - The requirement is only for maintenance. When SqIM & VFD do not need any maintenance why every one need to understanmd ?"I wouldn't buy from anyone that told me I don't need to understand it because it's never gonna break

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Wound Rotor vs Squirrel Cage

jbartos

(Electrical)

17 Feb 04 22:02

Suggestion to gsimson (Electrical) Feb 16, marked ///\\\
Only the cost of VFD is abnormally high for MV applications. Over a period of time this also will come down.
///ABB rep mentioned sometimes ago that the cost MV VFD for large HP application is coming down so that the LV VFD is not that much better for such applications.\\\

RE: Wound Rotor vs Squirrel Cage

jraef

(Electrical)

18 Feb 04 16:14
You made some good points. I am a big promoter of VFDs, but I beg to differ with you here. As mentioned earlier in this thread, the APPLICATION is not appropriate for a MV VFD. I have been on startups and retrofit projects for plenty of MV VFDs; AB, ABB, Robicon and Ross Hill (now owned by Robicon), and I can report that my experiences lead me to believe that conditions need to be near prefect for them to operate reliably. By that I mean clean, air conditioned spaces, clean, reliable power, clean, knowledgeable operators and clean. If any of those conditions are less than perfect, the equipment tends to cause moe headaches that it is worth. Did I mention it needs to be clean?

An Auto Shredder operation meets NONE of these requirements, and in fact fails them in with extreme prejudice. I have put low voltage VFDs in rock crusher applications for yerars, and only after 10+ years of exposure to them are they finally gaining acceptance by users. That said, I just had an "electrician" at a quarry connect 120VAC to the 4-20ma input on one of mine yesterday, blew up a 250HP VFD. He didn't know exactly what "speed reference signal" meant, he thought it was the Start button! Auto shredder operations are the same if not worse.

gsimon,You made some good points. I am a big promoter of VFDs, but I beg to differ with you here. As mentioned earlier in this thread, the APPLICATION is not appropriate for a MV VFD. I have been on startups and retrofit projects for plenty of MV VFDs; AB, ABB, Robicon and Ross Hill (now owned by Robicon), and I can report that my experiences lead me to believe that conditions need to be near prefect for them to operate reliably. By that I mean clean, air conditioned spaces, clean, reliable power, clean, knowledgeable operators and clean. If any of those conditions are less than perfect, the equipment tends to cause moe headaches that it is worth. Did I mention it needs to be clean?An Auto Shredder operation meets NONE of these requirements, and in fact fails them in with extreme prejudice. I have put low voltage VFDs in rock crusher applications for yerars, and only after 10+ years of exposure to them are they finally gaining acceptance by users. That said, I just had an "electrician" at a quarry connect 120VAC to the 4-20ma input on one of mine yesterday, blew up a 250HP VFD. He didn't know exactly what "speed reference signal" meant, he thought it was the Start button! Auto shredder operations are the same if not worse.

"Venditori de oleum-vipera non vigere excordis populi"

RE: Wound Rotor vs Squirrel Cage

jraef

(Electrical)

18 Feb 04 16:27
Your point about inertia is correct, except that most auto shredders were not designed around a flywheel effect as were chippers. You can't add significant inertia just by increasing the motor size, you need to add mass to the machinery. A wood chipper is inherently a high mass machine. You may have needed to increase HP a bit because in the old days, chippers handled occasional loads of scrap material. When a load came into them, inertia allowed them to chew it up, and the motor simply re-accelerated it after it was gone. Now chippers are generating revenue and are fed a continuous stream of material. The extra HP is sometimes needed on older machines to maintain speed of the mass (intertia) under that more constant loading.

FACB25Your point about inertia is correct, except that most auto shredders were not designed around a flywheel effect as were chippers. You can't add significant inertia just by increasing the motor size, you need to add mass to the machinery. A wood chipper is inherently a high mass machine. You may have needed to increase HP a bit because in the old days, chippers handled occasional loads of scrap material. When a load came into them, inertia allowed them to chew it up, and the motor simply re-accelerated it after it was gone. Now chippers are generating revenue and are fed a continuous stream of material. The extra HP is sometimes needed on older machines to maintain speed of the mass (intertia) under that more constant loading.

"Venditori de oleum-vipera non vigere excordis populi"

RE: Wound Rotor vs Squirrel Cage

gsimson

(Electrical)

19 Feb 04 04:48

Thank you every one for your valuable comments.
Certainly VFDs are understood by engineers. When we are in the age of "Prevent Maintenance" VFDs are preferred as their reliability is very high with minimum or rather no maintenance. As on date the electricians may not understand and considering the reliabilty their understanding is not a must.
MV VFD is a general comment and not relevent to this thread.
As I mentioned 'all' applications, the cost factor was posted for information.
For isolated drives located far away with 1 electrician I agree that the SRIM with resistor is better option.

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