ECN Forum Forums Technical Discussion Electrical Theory and Applications Electric Motors

Scott, I wish to ask a HUGE question here and I am willing to help with your reply.
But here is the question.
What is the basic theory behind the workings of the following motors?:

• Induction Motor (Cage motor).
• Synchro Motor.
• Slip-Ring Motor.

I hope that you can explain this, but there were a few in the Chat room that were found wanting, w/ respect to Motor Basics.

Hi again Mike!;

I'll be glad to give some basic answers to this Q.

First off, these types of Motors have one thing in common - they are all AC Induction Motors.
They function similar to the operation of a Transformer - as each has a Primary and Secondary winding (or windings).
For the Induction Motors, the Primary winding(s) are referred to as the Stator, and the Secondary winding(s) as the Rotor.

The two windings are physically separtated by an air gap.

Input KVA to the Stator is induced into the Rotor, which rotates and transduces Torque + Horsepower as required.
The "Windings" on the Rotor (Secondary) are embedded as bars (or other types of configurations) - and the entire assemblidge resembles a "Squirrel's Cage" (like an excersize treadmill).

As to the types, here's a basic description of each:

* Induction Motor (Cage motor):
The basic and common type used. Referred to as "Squirrel Cage Induction Motors" - they are the single or two-speed steady RPM type Motors (normal case) - with the "dual voltage options".
Squirrel Cage Motors range from the 1Ø Split Phase types (Resistance start, Capacitor start, PSC and Shaded Pole) through the Delta and Wye connected 3Ø types. These Motors slip behind the syncronous frequency when developing torque.

* Synchro Motor:
These are Induction Motors which run at the syncrronous speed of the AC frequency.
For a 2 Pole Motor at 60 Hz, this speed is 3600 RPMs.
These Motors are part Squirrel Cage and part DC windings. The DC windings are wound to the Rotor.

* Slip-Ring Motor:
These are Wound Rotor type Induction Motors.
Instead of having the Secondaries short circuited, they are connected together via external Resistances to achieve speed control.
They also slip behind syncronous frequency when developing torque.

Hope this is helpful!

If further information is needed, let me know!

Scott35

Trumpy, Scott35 did a good job, but if you need more on AC motors, see http://www.lmphotonics.com/m_control.htm and http://www.lmphotonics.com/slipring.htm
Mark Empson, the website owner in Christchurch, NZ, is well respected for internet contributions on electrical subjects. He is also very knowledgeable about starting methods and often-questionable energy-saving motor controllers.

Hey Scott's!
Thanks for all your help and info.
While Motors have never really been my strong suit, I wish to learn all that I can about this subject and I am getting better at working with them.
My Trade Cert exam was a nightmare, as most of the questions pertained to motors or transformers or power factor correction, all the Inductive things.
Experience in the field though has taught me to work calc's pertaining to this sort of thing out, either on paper or in my head.
I was taught at school before the advent of the pocket calculator, I think that was a life-saver to be honest!.