Why don't electric motors use exciter coils like generators do?

  • #1
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I remember (long ago, in college physics) learning about electric motors and generators, specifically about the "exciter" coils in generators which did a better job than permanent magnets, because of a sort of "turbo" or force-multiplier effect.

I notice, however, that while basically ALL generators use two sets of coils, NO electric motors do that. I would GUESS that it would be a simple way to make a stronger motor: Put your power into both sides of the attraction/repulsion rather than leaving one to a permanent magnet.

But no one does it that way.

What am I missing?

-Jeff
 

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  • #2
sophiecentaur
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Summary: Why don't electric motors use exciter coils like generators do?

I notice, however, that while basically ALL generators use two sets of coils, NO electric motors do that.
It is not as simple as that and it has changed over history. In terms of numbers, there are many more permanent magnet motors than motors with field windings these days. But the statistics are slanted for another reason. The biggest motors are mostly Induction Motors and most of the biggest of those are Three Phase motors. For induction motors there is no 'field' winding and the rotor is 'passive' and not fed, relying on the current 'Induced' in it to produce its magnetic poles which follow the rotating field of the stator winding.
Before very strong magnets were available, motors used to have Field Windings in addition to the Armature Windings. (Look up AC/DC motors) You will still find such a setup in fractional horsepower motors but more and more brushless motors are found in all sorts of equipment. The Stator in a brushless motor is driven to produce a rotating field that's electronically controlled to 'pull' a strong permanent magnet around at the required speed. Motor Car motors are brushless and deliver kWs of Power.

Now look at generators. The big generators are bigger than any available motors (someone may point out an exception to that statement) because they tend to be needed to supply several big motors amongst all the other loads. Those Alternators use a (wound) rotor for the field and the power it taken off the stators. This allows the output power to be varied by regulating the Field Current.
Most small Alternators do not have field coils but use a strong multipole magnet. I'm talking about small Wind Generators, for instance.

So, basically there ain't no rule these days!!

PS We all (in the old days) were taught about conventional motors with brushes. Series and Shunt Wound motors always appeared in Exams. They're almost as old fashioned as steam engines these days.
 
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  • #3
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Thank you, thank you for a thorough, clear, and succinct reply.

I take my hat off to you, sir.

-Jeff
 
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  • #4
anorlunda
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Some of the largest an most powerful motors (i.e. 10-20 MW in size) are AC synchronous motors. Their construction is nearly identical to synchronous generators, and they do use exciter windings. They use a second motor for startup to get them to synchronous speed before connection to the grid, and before application of mechanical load.
 
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  • #5
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As @sophiecentaur pointed out, AC/DC motors do have windings in both rotor and stator. But I disagree about their prevalence, - such motors today are very wide spread in fact. Look at any (cheap) power tools, vacuum cleaners, miscellaneous kitchen devices, etc. Most of such things uses so-called universal motors (one more article)

I should mention one of the most important reason, why such motors rarely used for high power and high reliability applications. The reason is the commutator block: commutator itself and brushes. Both is not very reliable thing. Furthermore, the commutator creates a lot of electrical "noise" (electromagnetic interference).
 
  • #6
sophiecentaur
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As @sophiecentaur pointed out, AC/DC motors do have windings in both rotor and stator. But I disagree about their prevalence, - such motors today are very wide spread in fact. Look at any (cheap) power tools, vacuum cleaners, miscellaneous kitchen devices, etc. Most of such things uses so-called universal motors (one more article)

But, I should mention one of the most important reason, why such motors rarely used for high power and high reliability applications. The reason is the commutator block: commutator itself and brushes. Both is not very reliable thing. Furthermore, the commutator creates a lot of electrical "noise" (electromagnetic interference).
Those AC/DC motors are on the way out, I am sure. Old technology can always be found in cheaper goods and the AC/DC motors will be around for a while yet. It’s only a matter of time, imo.
 
  • #7
Hi everyone,

Jeff, perhaps you are seeing squirrel cage rotors, which look like they aren't really coils, but in fact are. Both single and three phase induction motors can use these kinds of rotors. I'm not aware of any externally excited AC motor other than the synchronous motor or synchronous condenser. DC motors are excited for speed/torque control, and it's these motors which are being reengineered with permanent magnets. Many AC motors use brushgear as well in order to allow speeds above the line frequency, but they aren't usually externally excited i don't belive, only the rotor windings are shorted through the brushes. But as already mentioned, whatever configuration of windings you can think of, some bloke's built it and there's probably a good application for it due to the resultant speed/torque relationship.
 
  • #8
sophiecentaur
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“whatever configuration of windings you can think of, some bloke's built it and there's probably a good application for it due to the resultant speed/torque relationship.”
That’s true. Or at least it was true before the advent of beefy solid state control, I don’t see why any chosen torque etc. characteristic couldn’t be chosen for a permanent magnet motor - and at a good price, eventually.
 
  • #9
anorlunda
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Have a look at this post. It was inspired by this thread.
https://www.physicsforums.com/threads/remarkable-new-marine-propulsion-systems.972646/#post-6186452

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