Induction vs. wound rotor motors

[Jim Harmon]

Hello,

What's the easiest way to discern whether a motor is a single phase induction motor or a wound rotor motor? Is there an easily identifiable external feature that wound identify what I am looking at?

Thanks
Jim

[Mike Henderson]

A wound rotor motor will have a commutator or slip rings. But it's almost impossible to find a wound rotor motor for AC (except for "Universal Motors", such as in power hand tools). They're very common for DC motors but not for AC - a DC motor requires a commutator and wound rotor. They're only used in special applications for AC and nowdays you're more likely to see a "special purpose" VFD used with an inverter duty induction motor than a wound rotor motor.

At least, that's the best of my knowledge. Maybe someone else will have different experience and comments.

Mike

[Oops, I forgot about synchronous AC motors. You don't see them a whole lot, but they do exist and they have a wound rotor.]

[Tom Veatch]

There are a zillion different designs of electric motors and I'm not going to try to pass myself off as an expert on any of them. But, the induction motors that I've known were indeed induction motors fell into one of two categories, Capacitor Start-Induction Run, and Capacitor Start - Capacitor Run.

In both those cases, the motor had at least one large "bump" on the outside of the casing that contained at least one capacitor. A capacitor is used to induce a phase shift in the motor's start coils to create a starting torque when the motor is at rest.

Based on that, I'd say that any motor with one of those capacitor housings on the case is an induction motor of one type or another. If there is no capacitor housing, I can't say for a fact that it is not an induction type.

As far as the motor being a "wound rotor" type, look for brushes and a commutator. If they are present, it will have rotor windings. The brushes and commutator are needed to feed current to those windings.

No capacitor and no brushes? One of the induction types would be my guess.

Is there some reason other than simple curiosity for asking the question?

[Rod Sheridan]

Many single phase motors do not have a starting or run capacitor, so they won't have a case on the motor to hold the capacitor.

A wound rotor motor will always have brushes and a commutator, so that's the best method of determining whether it's a wound rotor motor.

Wound rotor motors often have external caps to allow replacement of the brushes. These are 180 degrees apart.

Wound rotor motors have higher frictional losses due to the brushes, so that's another way to check, spin the motor with your finger. If it coasts, it's an induction motor, if it doesn't, it's either a wound rotor motor, or needs new bearings.

Regards, Rod

[Jim Harmon]

Thanks for the info.

Not just curiosity, although I would like to understand more about electrical motors. My questions mostly stem from a desire upon occasion to slow the RPMs down on AC electric motors.

This came up most recently on a stock feeder and I was warned not to throw a variac on it and slow it down because it was an induction motor.

I have been known to slow down my huge Dayton circulation fan with my 10A variac before because that thing will part your hair backwards! Now I wonder what kind of motor it is - and how I can tell by looking at it - and whether that move was ill-advised. I see no brushes nor caps or bumps where brushes would go. There is a conical shaped cover over the back end of it - maybe the brush access is in there....

So, what will happen if I do put a variac on an induction motor? My fan passed the smoke test and it certainly does slow the RPMs down.

Thanks for entertaining all these screwy questions. I have learned a lot just with a few postings so thanks a lot for that!

Cheers
Jim

[Mike Henderson]

Thanks for the info.

Not just curiosity, although I would like to understand more about electrical motors. My questions mostly stem from a desire upon occasion to slow the RPMs down on AC electric motors.

This came up most recently on a stock feeder and I was warned not to throw a variac on it and slow it down because it was an induction motor.

I have been known to slow down my huge Dayton circulation fan with my 10A variac before because that thing will part your hair backwards! Now I wonder what kind of motor it is - and how I can tell by looking at it - and whether that move was ill-advised. I see no brushes nor caps or bumps where brushes would go. There is a conical shaped cover over the back end of it - maybe the brush access is in there....

So, what will happen if I do put a variac on an induction motor? My fan passed the smoke test and it certainly does slow the RPMs down.

Thanks for entertaining all these screwy questions. I have learned a lot just with a few postings so thanks a lot for that!

Cheers
Jim

I would definitely recommend you not put a voltage control device on an induction motor. If you run an induction motor at less than the rated voltage, you will see an increase in current to levels above rated current (assuming you have a load on the motor). This will cause increased heating which will trip the thermal breaker, or if you don't have a thermal breaker, will burn the motor out.

The only safe way to speed control an induction motor is with a VFD.

Mike

[That conical shaped cover is probably covering a fan used to cool the motor.]