Wound Rotor Induction Motor 2

[tmcgett]

When looking in the motor terminal box ( WRIM ) is it correct to assume that the group of wires with the ground is the stator. And can the rotor be wired wye or delta.

THANKS

 

[jbartos]

Suggestion: Yes, the group of wires connected to the ground may be the stator, if it is wired wye. However, verify this with manufacturers since there may be a group or wires connected to ground, not necessarily the stator wires.
Rotor is normally wired wye since there are three slip rings only. Delta wired rotor would require six slip rings. This is not normally done.

 

[edison123]

jb,

Can you tell me why would a delta connected rotor require six sliprings? Any rotor can be designed for star or delta connection with only three leads brought out.

 

[aolalde]

I have been working for 30 years with electric motors, manufacturing and repair business. Wound rotors can be either wye or star connected, regularly with 3 slip rings for external rotor connection to resistive banks. The manufacturer designs the internal connection that matches the operating voltage, turns, groups, poles, etc. The external resistive bank could be “V” or open delta connected.

The stator winding is regularly connected to a three lines power supply, so regularly there is no neutral terminal involved with the stator. The ground should be connected to the frame structure for protection and fault detection purposes.

You should do simple continuity test from the rotor slip rings to find if your leads are from the rotor or stator. See Name Plate data and look for current figures and voltage. the motor leads should match those two parameters. Good Luck.

 

[aolalde]

Sorry, I meant rotors are wye or delta connected with only three rings.

 

[tmcgett]

The motor in question is a Patay hoist motor,105 hp with eddy-current brake(1st 2 steps)and large external resistance bank.
The problem was hoist motor running very slow along with deep growling noise only in 1st and 2nd step ( 5 steps hoist ). Runs good in 3rd thru 5th

I'm a technician in training so I brought in a professional today and here's what he found.
The rotor leads coming from the resistor bank were connected to the stator terminals at the motor ( wye )
The stator leads ( 3 wires + ground ) coming from the control panel were connected to the rotor terminals at the motor. Ohm out the stator and found an open leg !
( there also was wrong wiring in the resistor bank – 1st step wired to 2nd run of resistors )

This is a hoist motor on a tower crane I've been operating for six months this way!!!! ( it came to the job wired like this) It has always run to fast on the first 2 speeds up, going down has been OK. In the last couple of months I have replaced or repaired 8 resistors
Every one I’ve talked to finds it hard to believe that the crane was able to run this way for 6 months. Has anyone else ever heard / seen anything like this.

 

[DougMSOE]

Obviously, WRIM stators may be connected wye or delta.
Further, the rotors may be connected wye or delta as well.
The only thing that you have to be careful of and this goes for the SCIM, WRIM and the starting winding (rotor bars and shorting rings) of the sync motor is that electrically non-symetric rotor windings MAY lead to torsional vibration problems.

 

[aolalde]

WRIM are like a transformer. You can connect the line either to the stator or the rotor windings as far as you put the design required voltage and according to this, the resistors could go to the each other circuit, the resistance values must match the circuit requirements too.

For this case it seems that the stator and rotor voltages are in a close ratio to 1 ( similar voltages) but the best option is that matching the nominal or nameplate voltages.

When the applied voltages are mixed and far apart, the results are magnetic saturation if the voltage is excessive or too week motor if the voltage is lower. Humming could result from unbalanced phases due to uneven resistance per phase.

 

[jbartos]

Suggestion to edison123 (Electrical) Jan 13, 2004 marked ///\\jb,
Can you tell me why would a delta connected rotor require six sliprings? Any rotor can be designed for star or delta connection with only three leads brought out.
///Yes that is correct. However, any motor is not exactly wound rotor motor for variable speed control. How would one change the current in the delta winding? Perhaps, there might be some sophisticated way the would allow to imbed resistor in each rotor delta winding leg to have a smooth speed control from the zero speed to the rated speed.\\\

 

[edison123]

jb,

Any WRIM rotor can be designed and wound for star/delta with only three sliprings, including variable speed application. I repeat there is no necessity nor have I seen rotors with 6 sliprings for delta connection.

 

[atvargas]

tmcgett: Yes I repair two overhead cranes.
These new cranes arrive to plant with a very high noise in motor hoists, in first and second step (five step each) but in 3 4 y 5th step operate normal. Silence.
Motor hoists worked like a two phase condition and high amps in wound rotor.
Then I review conections and i found an opened section of resistors bank.
When resistors bank were wired, maybe technicians forgot to put a wire to complete series resistor.
After connect this section the noise was off.

 

[jbartos]

Suggestion to edison123 (Electrical) Jan 18, 2004 marked ///\\jb,
Any WRIM rotor can be designed and wound for star/delta with only three slip rings, including variable speed application.
///Yes, I essentially agree. The only weak point will be the initial speed since the delta will have some finite impedance in each leg and the external controlling resistor may be open (infinite resistance) or closed with a very high resistance at the motor start. The wound rotor motor with y-rotor-winding does not have this drawback since the variable resistors controlling the motor speed are in series with the rotor coils.\\ I repeat there is no necessity nor have I seen rotors with 6 slip rings for delta connection.
///Yes, but it depends.\\

 

[aolalde]

For WRIM the same problem will arise if the resistance bank is open or with very high resistance. It does not matter if the connection of such a rotor is delta or wye, the rotor circuit is open and you will get zero torque. I disagree with jbartos statement.

 

[tmcgett]

ATVARGAS

I haven't been able to find who tinkerd with this crane last. Only one row of (3)resistors ( out of 12 rows )was skipped. I'm not sure if this was done intentionally.The motor was for sure wired backwards ( stator to rotor / rotor to stator ) and to boot it worked this way for almost 6 months !! GO FIGURE
We replaced the motor yesterday ( wired it correctly ) and she's running sweeeeeeeeeet !!!!!!!!!!