Electronics > Beginners

Will induction motors get replaced by synchronous motors?

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bonzer:
I don't know exactly why they don't care about them, maybe because there's not enought time. But the professor himself said that industry now is full of asynchronous motors so if you want we can have a lecture about them but we don't include it in the program because it's a matter of time when they are gonna be replaced with what we study.

But from the driving point of view like the power electronics - is there a big difference from working with an asynchronous motor? Is it more difficult to control them? If I tried to understand them by my own? (After all synchronous motor knowledge)

hermitengineer:
Did they already drop classical computing too, because quantum computing is the wave of the future?

Unless they see strong indicators that these reluctance motors are poised to drive induction motors into extinction before you graduate, that's a very impractical, elitist attitude they're taking.

atmfjstc:

--- Quote from: bonzer on November 19, 2019, 08:57:23 pm ---But the professor himself said that industry now is full of asynchronous motors so if you want we can have a lecture about them but we don't include it in the program because it's a matter of time when they are gonna be replaced with what we study.

--- End quote ---

University professors have a habit of being excessively liberal and "progressive", but this just takes the cake. So instead of preparing you for reality, the professor is preparing you for what he has unilaterally decided is the future, against all evidence and common sense....

T3sl4co1l:
An induction motor, you drive flux into the stator, which induces a current in the rotor.  The induced current in turn reflects a magnetic field, which serves as the permanent magnet in a PMAC machine.

The key observations are:
1. The reflected field is proportional to applied field.

The most direct consequence of this is, because flux is limited by stator cross section and flux density, the induced current is small at low frequencies.  In other words, a VFD must reduce voltage in proportion to frequency, and the available torque (before stall) is proportional as well.  So power goes as freq^2, and induction motors are not very useful at low RPM.

This does mean they are very useful above rated speed, given that core losses and ultimate RPM limits are respected, of course!

A 1kW size motor isn't going to last very long driven at say 400Hz+... but will be an impressive show when it grenades. ;)  More modest frequencies (say 25-100Hz) are the useful variable range.


2. The field is not permanent.  It is trapped in a conductor, but a nonideal one, so it slips.

After all, if you had a superconducting rotor, it would be a synchronous machine (although a slightly odd one where the reflected field is proportional to applied field).  Or possibly it wouldn't work because the induced current needs to lag behind the applied field, but I don't think so?  (I'd have to refresh my knowledge of both motors and superconductors to verify that.)

Of course, if we charge the superconductor with a current (and thus field), it will be permanent, and will act exactly like a PM rotor.  (Superconductors exclude external currents, so you can't quite do this in a single step, from the stator field.  It would be an ordinary electromagnet rotor, synchronous machine, that happens to use a superconductor.)

Anyway, the rotor is synchronous with the stator's magnetic field as usual, but because the rotor's own field is not trapped -- it rotates within the rotor itself -- the total RPM is a bit less than the applied mains.  This is why induction motors are labeled 1425 RPM or thereabouts (1750 RPM in US).  The difference is called slip, and is the rate the field rotates in the rotor.  Yep, the eddy currents flowing in the rotor, rotate quite slowly indeed -- this corresponds to an L/R time constant (of the rotor) of a few Hz, not bad, eh?

You can also demonstrate this at ~0 RPM, by biasing the stator with some DC current, and turning the rotor.  This is a highly effective magnetic brake, and this method of operation is sometimes used to stop machinery.

Indeed, the rate of slip is proportional to applied torque, so this could also be used to some extent to gauge load factor; it also means that an induction machine can be used perfectly symmetrically as a generator, you just need to push it faster than the synchronous speed.

You can use an induction machine as a generator (alternator), after supplying some seed current.  Usually there's enough residual magnetism in the rotor steel to get things going, and a capacitor is used (I think) to maintain current between cycles.  Starting a motor from no current, the generated voltage increases exponentially until limited by saturation (which will be close to nominal output voltage, because the stator is designed to run just shy of saturation), and then it's about as good as a synchronous machine; give or take the waveform, which may be ratty due to the saturation, and of course having poor frequency stability because of slip.

Tim

bonzer:
Tim thanks a lot for your explanation.

Yes, I do agree with you talking about university system and these feelings have always followed me during the years. Sometimes the academic world feels a bit parallel to the reality and not part of it. And I'm sure it's not only here in my university. You can find some papers from different parts of the world writing on google something like "reluctance synchronous motor vs induction motor" . Being cheaper than permanent magnet version, they consider them to be able to replace asynchronous motors in future but as I said the real world is different and complex, we can't be certain about what's gonna happen, I think only if there's a huge cost difference it usually matters.

Whatever, I will try to understand them when I'll have some free time from the internet and by studying the details of what Tim said.

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