In one of our crane application where wound rotor motor has been used. Motor has rotor conected in delta and external resistance is connected in star to the rotor.
Rotor resistance is cut through contactors. but in a particular case single phasing is done in rotor ckt (one phase open). It is definately to reduce the torque.
But my question is when rotor is single phased the torque produced is reduced by what percentage?

 

Pkumar,

Your explanation needs to be further detailed. The delta-connected rotor winding remains connected in delta.

1) By rotor single-phase, are you referring to one element of one-phase of the resistance bank? Is it open-circuited for both directions?

2) How is the 'star' point of the resistor bank sequenced? That is, how is the final step of short circuiting accomplished? Between just two of the slip rings? Or, all three?

3) The delta winding is mathematically converted into a wye. Its equivalent phase-to-star midpoint resistance is determined. Finally, it is added to the external resistance for each of the acceleration or deceleration steps.

4) There is no "rule-of-thumb" calculation that will yield a "percentage" torque reduction value. Instead, he following can be used. The resistance obtained from 3) above is used to calculate the per-phase resistance-to-slip ratio. This ratio is usually kept constant for each step during the entire acceleration or deceleration period. Then the total per phase torque, can be determined using the sqrt rule.

5) Last, but not least... are any of the resistors permanently connected to the rotor? That is, do they continually dissipate I^R loss?

Regards, Phil Corso, PE Boca Raton, FL [[email protected]] ([email protected]) {[email protected]}

 

hi phil corso
thanks for the reply
as i told the rotor is delta conntected so even if the external resistance one phase is open there will not be single phasing.infact there will be unbalance rotor resistance.
2. the external resistance is generally made star but in my case only two phase are connected and one phase is kept open at end of external resistance.this is only for one step.when the rotor contactors are energinsed then the external resistance is made star.
3.some part of the rotor resistance is connected permanently with rotor.
Pkumar

 

Kumar,

The connection you detail is (typically?) used to reduce the duty on the mechanical brake during lowering of the load. Apparently, someone felt that the mechanical brake wear was unusual.

Have you posted the original query because you feel that motor performance, or life expectancy been negatively affected?

Do you still want to determine the effect on net torque? It will require knowing motor and resistor parameters.

Regards, Phil Corso, PE Boca Raton, FL [[email protected]] ([email protected]) {[email protected]}