Efficiency: Syncronous Motors vs. Induction Motors

Synchronous motors are used as a means of power factor improvement. If you change to induction motors you will need capacitors to correct the new power factor.

In addition to what wareagle said, synchronous motors are typically used when much lower speeds are needed. When you increase the number of poles on an induction motor in order to get the base speed lower, that is where the comparative efficiency begins to drop. You end up with more i^2R losses in the multi-pole induction motor than in a similar speed synchronous motor. But if you wanted a high speed sunchronous motor, the situation would be reversed.

Additionally what you believe is correct.. Synchronous motors are inherently much more efficient than IMs. It is the losses that is introduced by the slip that are quite a bit responsible for this (rotating magnetic field and rotating rotor are not at same speed, so there is bound to be eddy losses).

The only problem with the SM is the speed and stability in case of overload (instead of warning, if the Synch speed is disturbed, motor simply stalls).

Some links are useful, but does not give the values due to variations in designs, speeds etc. However the difference in efficiency of the two types are quite significant. typically the 3ΦSMs have efficiency > 90% (in fact the efficiency are equal to alternators) and are sometimes further inproved by the spacial motors eg Perm magnet etc.

http://www.allaboutcircuits.com/vol_2/chpt_13/2.html

http://synchronousmotor.specaproduct.com/efficiency_synchronous_motors.html

The general rule is (and you can find it too_ the Synch Motor efficiency = alternator efficiency - and you can find it by search.. IM efficiencies are available everywhere including name plates, standards etc.

And the effect is carried forward in Variable frequencies too.

G.A. to you, sb! Concise, accurate answer to the original question, providing additional references and info in context.

Normally at such ratings with speeds below 750Syn RPM pf of induction motor drops and capacitors are needed.Howver maintenace of cage induction motor i simpler.Efficiency wise at low speeds syn machines are more efficient due to better PF that can be achieved by controlling the excitation.Additional excitor M/c static or rotating and its maintenace need to be considered.you have to compare the combined eff of syn m/c with IM with capcitor banks and its losses and arrive at what is best.

hello

you need not to worry about efficiency of Induction motor , you could achieve more efficiency in combination with MV Drives .

Possible try to replace existing motor by Induction motor with Variable frequecy drive ( VSI )

At least a few of you are going to think I'm picking at minutiae, and the rest may very well conclude that I'm a total dumbo. BUT, it is my understanding that all non commutated motors are refered to as induction motors, and all induction motors, running no load, are operating at their synchronous speed. Shaded pole motors are synchronous motors, capacitor start, and capacitor run motors are synchronous motors. What induction motor is not a synchronous motor? What is it that determines the no load running speed of the "non synchronous" induction motor?

Bob

Guest,

The synchronous speed is only one, resulting from the frequency and the number of poles of the machine (this is the speed of the rotating magnetic field). No induction motor can work at the system's synchronous speed, since at this speed there is no torque developed.

The operation principle of this type of motors (also called asynchronous motors) relies of torque developed by the interaction of induced currents in a winding (usually rotor cage) as a result of cutting magnetic field lines(produced by the stator winding). If rotor and field are rotating with the same speed, there is no line cutting and thus no torque. The higher the speed difference (slip) between synchronous speed (field) and actual rotor speed, the higher the torque (up to a limit).

For instance, the synchronous speed for a 60 Hz, 2 pole motor is 3600 rpm. The no load speed of an induction motor (60 Hz, 2 poles) may be 3540 rpm, the full-load speed 3200 rpm, but never 3600 rpm, while the speed of a synchronous motor (also 60 Hz, 2 poles)(no load to full load) can be only 3600 rpm (synchronous speed).

Shaded-pole, capacitor (start/run) are induction motors (single phase in this case).

No induction motor is a synchronous motor as mentioned above.

tomad,

If shaded pole motors are not synchronous motors, why have they been used as ac clock motors? You're right, the shaded pole and cap start/run motors have their synchronous speed "set" by the number of poles and the ac frequency. I agree that these motors have "zero" torque at their mathematic "synchronous" speed, but if you look at their speed/torque curves, you will see that the shaded pole motor has a very flat curve until it breaks over and stops from overload. Thus, I think it may be refered to as a synchronous motor.

Bob

Bob,

The shaded pole motors, as well as the capacitor motors are induction motors. Please see also the respective definition/descriptions on Wikipedia.

Also, accurate clock motors are all synchronous motors. Normally, the speed at which a motor rotates depends on the torque applied to its shaft. If that torque is constant, the speed will also be constant.

This is under normal circumstances, when mechanical losses due to friction, etc remain constant. If any of the bearings gets damaged the load torque applied to the shaft increases and the speed decreases subsequently.

For small motors this is hardly noticeable, but in time the error adds up.

The problem with the synchronous motors is that they cannot start as synchronous motors. In order to provide a starting possibility, the synchronous motor has to as an induction motor via provided additional rotor bars. In case of single phase motors, they must have either an auxilliary start winding & a capacitor, or a shaded pole (which is basically a low number of turns wound around a part of a pole).

But in all cases, when they reach a speed close to the synchronous speed, their turning torque operates as a synchronous machine torque (i.e. field speed = rotor speed).

A type of induction machine that can operate as a synchronous machine is the so-called "self-starting synchronous reluctance motor".

It seems that I cannot attach the link, but a book where you find additional info is"Electric Machinery" by Eugene Fitzgerald, Charles Kingsley (page 460). It appears under Google Book search. Try to click on the red x.

You mentioned some speed/torque curves, if you have them please attach them in your response.

You have hit the bull's eye100% correct.And as to ref. as many times mentioned it is the best book on electric machinery which I have been reading from my college days till now.