Wound Rotor Motor (Rotor Voltage) 1

[TommyOO7]

Hi all!

I have a question about open rotor voltage for 2.3kV Wound Rotor Induction Motor (WRIM).
What are the standard (or common) open rotor voltages for 2.3 kV WRIM? The motor is standard NEMA motor, 3000HP, 1500rpm.

Thanks for help!

Tommy

 

[aolalde]

Hi TommyOO7

As far as I know NEMA Standards do not define the rotor voltage for induction machines. Normally the voltage is selected into a compromise of current amount and insulation for the voltage. Your motor voltage should be between 900 to 1500 volts.
A simple test will give you the secondary voltage of your motor. The speed of 1500 rpm shows that you are referring to a 50Hz motor.
With the rotor stand still and open leads (after insulation tests are satisfactory) energize the STATOR LEADS WITH 460 Volts 50 HZ or 60 Hz (for this test frequency does not matter).
Measure the real applied voltage phase to phase and the induced voltage ring to ring. WARNING. THESE VOLTAGES CAN BE LETAL FOR UNQUALIFIED PERSONS.
Your rotor voltage will be very close to;

V2= Vtest rotor/Vtest stator*2300

Note that for operation intended at 60 Hz you should analyze that the motor and load performances are still satisfactory.

 

[TommyOO7]

Thanks aolalde,

You are rights, NEMA doesn't specify rotor voltage for MV motors, MG-1 specify only rotor voltages for LV motors (up to 460V).

But I would like to know what are the common Turns ratios (or Vs/Vr) for the MV WRIM? If the motor voltage is 2300V, what are common rotor voltages, any value from 900 and 1500V, as you said, or certain (common) values?

I am working on the spec for the motor, so I can't measure it or read it fromm nameplate! :-(

Tommy.

 

[aolalde]

Secondary voltage of WRIM is defined by the ratio rotor to stator of slots, turns per coil, winding factor, number of circuits and connection. If you have to specify one, for 3000 HP; I suggest 1500V +/- 20% for around 910 Amps at full load. This will allow optimum matching for the stator and rotor parameters. Limiting this value to a tight figure could lead to a marginal motor design. Having a guide line, the motor manufacturer will suggest the optimum rotor voltage for a specific motor design.

 

[TommyOO7]

Thanks aolalde !

 

[electricpete]

Won't the voltage measured during open circuit test depend upon position of the rotor? (I have seen wound rotors used as variable voltage sources using the principle that voltage ratio depends on rotor position.)

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[TommyOO7]

electricpete you are right.

Actually, when I was a student we used WRIM as a transformer with variable (secondary) voltage in Electric Drives Laboratory. The voltage change is continouos (not in steps).
But the question is how to measure the open circuit rotor voltage. The Rotor shall be in a position which gives the max. rotor voltage (which is when the stator coils and rotor coils are aligned).

Tommy.

 

[aolalde]

Electricpete.

The magnetic field produced by a 3-phase stator winding is rotating at synchronous speed.

To induce voltage in a stand still rotor, the stator and rotor windings DO NOT NEED TO BE ALIGNED as Tommy007 stated since the field rotation cuts the rotor winding conductors; -e = N*d(phi)/dt

Expected variations on induced voltage for different rotor positions are due to Reluctance changes for different stator rotor teeth alignments. When the rotor or stator are skewed this variations are minimum.

 

[edison123]

I agree with aolalde that the rotor induced voltage does not change much with the rotor position.

What Tommy says is an induction regulator, which though resembles a WRIM is different in the following ways

1. It usually does not have slip-rings since the rotor moves about only a pole pitch. It uses a flexible connectors.

2. They are mostly vertical machines.

3. The rotor is turned (within a pole pitch) by some gearing.

4. Variation in output voltage is got by phase shift not by a change in secondary voltage, which as I said remains nearly constant.

* If you have never succeeded in anything in first attempt, skydiving isn't for you. *

 

[TommyOO7]

You are right guys. It was a long time ago when I was a student and I didn't remember all details. But I checked my old books.

Let me first correct what I said in my previous message. When stator is connected to AC voltage and rotor circuit is open, rotor voltage does not depend on rotor angle at all (let's neglect stator rotor teeth alignment efffect).

But WRIM can be used as phase shift transformer (or induction regulator). Stator windings are open, one side connected to nominal 3ph voltage. Rotor is Y connection and through sliprings and brushes connected to the same terminals as one end of the stator windings. Now we measure the voltage on the other stator windings side (terminals). Measured voltage is a vector sum of the induced stator and rotor voltages. Angle between vectors is regulated by turning the rotor. So, the total voltage range is from 0V to 2 x connected voltage.

This is what I tried to explain, but as I said didn't remember all details. Sorry for that.

TommyOO7

 

[fsmyth]

Resurrecting an old topic: how about a rotor with only
two slip rings? Still have not found an answer.
I cannot find the thread with the details; maybe someone
a bit more versed on searching this forum could post a
pointer.
<als>

 

[motorspert]

fsmyth
Two sliprings is a synchronous motor? Or have you come across an induction motor with two rings?

 

[petronila]

fsmith,

The Baldor´s 20 HP motor thread is 237-130091. Sept 30/05.

Regards

Petronila

 

[waross]

motorspert.
My first thought was an old two bearing generator seperated from its prime mover, but your example is probably more common.

 

[Fintan]

Here is a simple equation which works for most slipring motors:

RV=(HP*470)/RA

Regards
Fintan