Wound Induction motor Rotor Voltage

See these

http://en.wikipedia.org/wiki/Electric_motor

http://www.ee.lamar.edu/gleb/power/Labs/Lab%2013%20-%20Wound%20rotor%20induction%20motor.pdf

If there are brushes, then it is not a true induction motor. In an induction motor the rotor's magnetic field is induced by the stator magnetic field. The current in the rotor cage is a transformer like current that has a frequency related to the slip of the rotor.

So I suspect that you actually have a synchronous motor. The DC current applied to the rotor windings are to create the rotor magnetic field.

Brushes in a induction motor rotor are used for connecting starting resistor.

This is the old fashion to start an induction motor.

you can also connect any device to control the rotor current and therefor motor torque and speed. also this is the old fashioned speed controller if rotor is open, motor act as a transformer,

There are two kinds of AC motors that have brushes; Wound Rotor Induction Motors (WRIM) and Synchronous Machines (motors or alternators, or both).

In a WRIM, you do not APPLY a voltage to the rotor windings, the voltage is induced onto them by the stator, the rotor brushes are used to connect an external resistance in order to change the torque / current profile.

In a Synchronous Machine, a DC voltage is supplied to the rotor in order to establish and control the magnetic field in it, which can be used to either create a voltage in the stator windings from a prime mover applying force to the rotor (alternator mode) or to alter the Power Factor when used as a motor, allowing the Synchronous Machine to act as a Condenser to add VARs to a system with a lot of other induction machines.

Friends,

JRaef has given a good explanation. For the two types of machines there is a significant difference: WRIM will have three slip rings and three sets of brushes, while Synchronous motors have two slip rings and two sets of brushes. For either one, there are often more than one brush on a ring, because of the currents being handled. Also, on synchronous motors, the slip rings are preferably made of iron or steel while on WRIM's they are brass. Wound rotor starters were made with a drum switch that would have multiple positions to change the amount of resistance connected to the rotor---higher resistance was a slower speed and zero resistance was the maximum speed. If you open-circuited (infinite resistance) the rotor, the motor would not be able to turn. Under these conditions, there would be no back emf on the stator windings, so you would see "locked rotor" current and trip out the thermal overload on the starter within a few seconds.

There are other classes of motors with brushes--DC motors, pole-changing motors, and universal motors. Universal motors are usually seen in small tools and similar equipment; DC motors can occur in a wide range of sizes (some are permanent magnet types with no brushes). Pole-changing motors are really a breed of their own. The slip ring assembly on one end is mounted with a gear-motor to rotate it over about a 90° range, to change the speed of the motor; there is a set of slip rings on the other end with their own brushes also.

--John M.

D'oh! Forgot about Universal motors. Thanks for catching that...

"If you open-circuited (infinite resistance) the rotor, the motor would not be able to turn. Under these conditions, there would be no back emf on the stator windings, so you would see "locked rotor" current and trip out the thermal overload on the starter within a few seconds"

You've never tried this then? The motor acts as a transformer, with no mechanical load on the motor you could turn the shaft with your hands.

I've only done a transformer test once in 35 years of electrical engineering, we suspected the rotor windings of a 2000HP motor were faulty so we removed the brush gear and measured the rotor votage on the slip rings all showed a balanced voltage.

Guest,

You are correct, that I have never done it. Probably I should. Am I incorrect about locked rotor current?

--John M.

Sorry, but yes you are incorrect.

The one danger of a transformer test is you have the full OC voltage on the slip rings. This could cause flash over if the slip rings aren't scrupulously clean. For a normal start up there is always some resistance connected to the brush gear, which lowers the voltage to about 80% of OC.

PS, I wasn't a member when I posted as a "Guest"

Refer any text book.It will show the equivalent circuit of I.M.

"Under these conditions, there would be no back emf on the stator windings, so you would see "locked rotor" current and trip out the thermal overload on the starter within a few seconds."

Huh ? When the rotor is open circuited, the motor acts like a transformer on open circuit. The stator current will then be just the magnetizing current (very close to the normal no-load running current). No way thermal overload will act unless there is a faulty stator/rotor winding.

Friends,

I believe I have been well-corrected on that one.--JMM