Synchronous vs Induction Motors: A Comparison Guide

To compare synchronous and induction motors, one must understand some key characteristics that describe their performance and behavior. These include the rotational speed of the motor, measured in revolutions per minute (RPM). Synchronous motors have a fixed speed that depends on the supply frequency and the number of poles, while induction motors have a variable speed that depends on the slip. The rotational force of the motor is measured in newton-meters (Nm) or pound-feet (lb-ft), with synchronous motors having a constant torque that is independent of the load, and induction motors having a variable torque that depends on the load and the slip. The mechanical output of the motor is measured in watts (W) or horsepower (HP), with synchronous motors having a constant power that is proportional to the torque and the speed, and induction motors having a variable power that is proportional to the torque and the square of the speed. Lastly, power factor is the ratio of real power to apparent power, which indicates how efficiently the motor uses electrical energy. Synchronous motors have a high power factor that can be adjusted by changing excitation current, while induction motors have a low power factor that decreases with load and slip.

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Synchronous motor question

Synchronous motor question

rmw

(Mechanical)

(OP)

3 Apr 04 23:01

Fellow members, I am a Mechanical engineer, who has to deal with motors attached to mechanical devices, and who knows just enough to be dangerous about motors.  I have even posted to threads in this forum.

However, one thing I cannot remember from my old college days is the difference between induction motors, (which I have a lot of familiarity with) and synchronous motors, which I encounter from time to time in small fractional HP as well as 1-10 HP ranges.

I have looked in all the text books and reference books I have in my possession, as well as googling the words.  Still I don't know what I would like to know about synchronous motors.  Differences, advantages, disadvantages, etc.

I'll bet you electrical types can straighten me out.

Replies continue below

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RE: Synchronous motor question

jbartos

(Electrical)

3 Apr 04 23:25

Comment: The differences, advantages, disadvantages, etc. should be available in textbooks, e.g.
1. M.G. Say "Alternating Current Machines"
2. S. R. Slemon "Magnetoelectric Devices, Transducers, Transformers, and Machines"
3. A. E. Fitzgerald, Charles Kingsley, Jr., Stephen D. Umans "Electric Machinery"
4. Etc.
Differences: Asynchronous motor runs with slip, does not have field winding supplied by a power source called "Exciter", has a different speed - torque characteristics, it is less expensive, etc.
Advantages: Asynchronous motor is simpler and rugged, less expensive, cannot drop out of synchronism, is often simply started with Direct On Line (DOL) start, etc.
Disadvantages: Asynchronous motors usually do not power very high HP loads, experience slip, tend to burn rather than drop out of synchronism and stop, etc.

Also, visit technical library and read various Encyclopedia, Technical Dictionary, and popular magazines/journals similar to popular mechanics.

RE: Synchronous motor question

electricpete

(Electrical)

4 Apr 04 10:45
Sync motor operates at syncronous speed.  A loaded induction motor operates slightly below sync speed.

Sync motor is a little more complex.  There is more to maintain and more to fail.

Sync motor power factor can be adjusted. This is an advantage particularly for low-speed motors where induction motor tends to have very bad power factor.

Therefore sync motors are sometimes selected for low-speed high horsepower applications.

Lots more info at
http://www.geindustrial.com/products/reference/GEZ-.pdf

The basic difference:Sync motor operates at syncronous speed. A loaded induction motor operates slightly below sync speed.Sync motor is a little more complex. There is more to maintain and more to fail.Sync motor power factor can be adjusted. This is an advantage particularly for low-speed motors where induction motor tends to have very bad power factor.Therefore sync motors are sometimes selected for low-speed high horsepower applications.Lots more info at

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RE: Synchronous motor question

aolalde

(Electrical)

4 Apr 04 22:21

Besides the speed difference the synchronous motor needs field excitation with DC. Then you have a three phase supply for the armature and a Dc supply for the field.

The synchronous motor has constant speed from no load to full load or until it is pulled out of synchronism, that speed is exactly 120*Line Frequency/Poles.

Synchronous motors can be applied to very large loads of constant type achieving high efficiency and adjustable power factor. One disadvantage of synchronous motors is the lack of accelerating
Torque, then auxiliary windings or pony motors are required to start.

RE: Synchronous motor question

jbartos

(Electrical)

5 Apr 04 08:57

Comment: Sometimes, the low HP rated synchronous motors are required by the process not permitting small any speed slip. Typical very small HP motor is the clock synchronous motor. Please, notice that there are permanent magnet synchronous motors, where the mentioned complexity of synchronous motors in the above postings does not apply.

RE: Synchronous motor question

jacekd

(Industrial)

5 Apr 04 09:18
-price (magnets are pretty expensive)
-overheating them can destroy magnets
-if you are not experinenced with handling of them you can usually dissasemble them only twice - first and last at once

But other factors applay to permanent magnet SMs.-price (magnets are pretty expensive)-overheating them can destroy magnets-if you are not experinenced with handling of them you can usually dissasemble them only twice - first and last at once

Regards,

Jacek

Do it right or don't do it at all.

RE: Synchronous motor question

jbartos

(Electrical)

8 Apr 04 21:41
http://labs.ee.psu.edu/labs/powerlab/papers/energy_.pdf
http://www.polito.it/syncrodrive/aquila/Report_Aquila.pdf
etc. for more info on pmsm and their attractive characteristics, e.g. high power factor, absense of field losses.

Suggestion: Visitetc. for more info on pmsm and their attractive characteristics, e.g. high power factor, absense of field losses.

RE: Synchronous motor question

Rodmcm

(Electrical)

26 May 04 04:15

A simple way to describe it is that an induction motor requires both active and reactive power from the supply, a synchronous motor absorbs power but due to external equipment excitation can supply reactive power. They are more expensive, have lower starting currents, and are used when there is an additional advantage (usually monetary)to produce reactive power by this means instead of installing power factor capacitors for instance

RE: Synchronous motor question

aolalde

(Electrical)

26 May 04 09:10

Rodmcm (Electrical); Synchronous motors DO NOT " have lower starting currents"

RE: Synchronous motor question

thinker

(Electrical)

26 May 04 14:21

There is one more advantage of synchronous motor: higher overload capability.

RE: Synchronous motor question

jbartos

(Electrical)

26 May 04 22:06

Suggestion: The synchronous motor speed drive, LCI (Load Commutated Inverter), can start the synchronous motor with low starting current.
The synchronous motors are usually convenient to apply to very large loads, e.g. over HP.
The synchronous motor is more expensive than the induction motor; however, it runs at higher efficiency than the induction motor.

RE: Synchronous motor question

Rodmcm

(Electrical)

26 May 04 22:07

aolalde
Don't they? I've worked with 2MW synch and 2MW induction motors and the synch motors had less starting current in the same application. Is this an aberation?

RE: Synchronous motor question

thinker

(Electrical)

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27 May 04 10:08

Rodmcm,
You might be right. The starting current for sync motors is inversely proportional to p.u. value of its sub-transient reactance Xd", so if this value in your case was 0.2 the starting current could be 5 times of rated.

RE: Synchronous motor question

rmw

(Mechanical)

(OP)

27 May 04 10:59

For you that are contributing to this thread, how does this discussion pertain to the 1-10 HP synchronous motors that I mentioned in the thread starter.  Remember that it is a ME asking the question, so you have to try to couch your answers in such a way that it is understandable to someone outside your disipline.

Do concepts like the reactive power issue, which I actually understand a little, have any real affect in that small HP range.  What I know about the motors that I have in mind in asking the question above, is that they are slow speed, and draw very little current, and can drive into a stall and not burn up very rapidly, (if at all-and that is in question in my application.).

I have already noted above that the reference information readily available to me has failed me on the topic, and the nearest technical library at an engineering university, or such, is some distance away, so I was hoping to get answers and input from peers, rather than having to make the trip.

rmw

RE: Synchronous motor question

jraef

(Electrical)

27 May 04 13:13

The downside is usually cost (significantly higher) and off-the-shelf availability (significantly lower) in one-off applications compared to the ubiquitous induction motor. You will however find synchronous motors more often in OEM applications because the cost and availability issues then become negotiating points.

If you are going to need adjustable speed, the latest generation of vector control drives for induction motors makes synch motors less attractive unless again, slow motor speeds without forced cooling are necessary. They are also less valuable in high speed applications and/or those where output shaft speed is lowered through a gearbox in order to multiply torque, because in most cases you may as well use an induction motor anyway.

Once you get into high HP and power factor issues, they become attractive again, but for different reasons.

Was this what you were looking for?

Small synch motors such as you describe are typically used when the necessity of maintaining an accurate fixed speed is important to the application without the need for complex speed control electronics, i.e. clock and timer motors, projector drives, turntables, indexing conveyors etc. They are also slightly more efficient and smaller than their induction counterparts, and can turn at lower speeds, again without additional electronics. By simply controlling the field strength, either with permanent magnets or slip rings on the rotor, a specific speed can be accurately maintained with relatively high torque for the physical size. They are sometimes used in servo applications, especially when slow motor speeds are necessary (but of course then you add back in the complex electronics).The downside is usually cost (significantly higher) and off-the-shelf availability (significantly lower) in one-off applications compared to the ubiquitous induction motor. You will however find synchronous motors more often in OEM applications because the cost and availability issues then become negotiating points.If you are going to need adjustable speed, the latest generation of vector control drives for induction motors makes synch motors less attractive unless again, slow motor speeds without forced cooling are necessary. They are also less valuable in high speed applications and/or those where output shaft speed is lowered through a gearbox in order to multiply torque, because in most cases you may as well use an induction motor anyway.Once you get into high HP and power factor issues, they become attractive again, but for different reasons.Was this what you were looking for?

"Venditori de oleum-vipera non vigere excordis populi"

RE: Synchronous motor question

rmw

(Mechanical)

(OP)

27 May 04 21:13

Jraef,

That is a very comprehensive review.  Thank you.  Other comments in the thread helped too.  I did remember some basic facts regarding synchronous motors from an electrical machinery course I had to take in college.  But, what I remembered was pretty sparse.  This helps.  Thanks.

rmw

RE: Synchronous motor question

ScottyUK

(Electrical)

28 May 04 02:40

Not sure if I'm just mis-reading your post here -

By simply controlling the field strength, either with permanent magnets or slip rings on the rotor, a specific speed can be accurately maintained with relatively high torque for the physical size."

How does control of the field strength determine motor speed? Stator frequency governs the speed of a synchronous machine; field strength can be used to vary the speed of a DC machine.

For mains-supplied application, synchronous machines are fixed-speed drives. They normally rotate slightly faster than the equivalent induction machine because the synchronous machine rotates at a speed determined by the supply frequency, while the induction machine rotates at a slightly lower speed - it 'slips' relative to the supply. If an induction motor was going faster than the equivalent synchronous motor, it would in fact be  a generator!

Hi jraef,Not sure if I'm just mis-reading your post here -How does control of the field strength determine motor speed? Stator frequency governs the speed of a synchronous machine; field strength can be used to vary the speed of a DC machine.For mains-supplied application, synchronous machines are fixed-speed drives. They normally rotate slightly faster than the equivalent induction machine because the synchronous machine rotates at a speed determined by the supply frequency, while the induction machine rotates at a slightly lower speed - it 'slips' relative to the supply. If an induction motor was going faster than the equivalent synchronous motor, it would in fact be a generator!

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I'm getting a great education!

RE: Synchronous motor question

jraef

(Electrical)

28 May 04 14:34
Hmmm, I can't believe I wrote that...
This is one of those times when you learn how someone else read your words and you see the completely different and unintended viewpoint they can get from it. My bad.

The intent behind that statement was not to imply that changing field strength changes speed (although I DID apparently say that). It was meant more as a design issue in either selecting the PM or the field coil design. Sorry to be misleading, thanks for pointing it out.

ScottyUK,Hmmm, I can't believe I wrote that...This is one of those times when you learn how someone else read your words and you see the completely different and unintended viewpoint they can get from it. My bad.The intent behind that statement was not to imply that changing field strength changes speed (although I DID apparently say that). It was meant more as a design issue in either selecting the PM or the field coil design. Sorry to be misleading, thanks for pointing it out.

"Venditori de oleum-vipera non vigere excordis populi"

RE: Synchronous motor question

jbartos

(Electrical)

28 May 04 16:20
For you that are contributing to this thread, how does this discussion pertain to the 1-10 HP synchronous motors that I mentioned in the thread starter.  Remember that it is a ME asking the question, so you have to try to couch your answers in such a way that it is understandable to someone outside your disipline.
///Visit for synchronous machines problems and solutions:
http://edt.uow.edu.au/indulkar/chap02.pdf
for a little brain gymnastics around those solved examples.\\\

Suggestion to rmw (Mechanical) May 27, marked ///\\\For you that are contributing to this thread, how does this discussion pertain to the 1-10 HP synchronous motors that I mentioned in the thread starter. Remember that it is a ME asking the question, so you have to try to couch your answers in such a way that it is understandable to someone outside your disipline.///Visit for synchronous machines problems and solutions:for a little brain gymnastics around those solved examples.\\\

RE: Synchronous motor question

rmw

(Mechanical)

(OP)

28 May 04 22:41

Jbartos,

Thanks for the link, but that is

way

beyond what this Mechanical Engineering brain can stand to be teased.

Thanks anyway, though.

rmw

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