Operating Wound Rotor Motor As A Synchronous Motor 2

[OhioAviator]

Hello all...
Here's a question for the group, and an opportunity for someone to tell me I'm crazy. Here's the situation: We are operating a 4,000 HP wound rotor motor that's connected to a dynamic load with quite significant load torque variations. The rotor at present is connected to a liquid rheostat that is used for 'soft' starting and for limiting stator current during heavy overload. Here's the question: Can we remove the liquid rheostat and in its place install a PWM drive to inject current into the wound rotor, thereby causing the wound rotor motor to operate as a synchronous motor? Can I use the PWM drive to control the wound rotor current and cause the wound rotor motor's stator to produce leading VAR's, just like a regular synchronous motor?

If not, why not? Thanks in advance for your thoughts!

 

[electricpete]

Sorry, a clarification in bold:

and that the rotor spins mechanically at exactly that frequency, adjusted for poles, regardless of load.

should have been:

and that the rotor spins mechanically at exactly that frequency(applied stator frequency +/- applied rotor frequency), adjusted for poles, regardless of load.


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(2B)+(2B)' ?

 

[electricpete]

Figure 2.32 of above link shows this. Interesting to note that when machine speed is controlled above stator field speed, the rotor power adds to the stator power. When machine speed is controlled below stator speed, rotor power subtracts from stator power. Seems quite inefficient when reducing speed below stator speed.

Inefficient probably wasn't a good way to say it. At low speed, the power from rotor side in that figure may be returned to the power system.

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(2B)+(2B)' ?

 

[OhioAviator]

Thanks to you all. I'm working out on the road and only now got to crack open my laptop to check these responses. Since my original post I have since discovered that TMEIC sells a "Slip Power Recovery" system that does much of what I'm trying to accomplish. I agree that p.f. caps should be a cheaper and maybe more reliable way to correct power factor. However, by my way of thinking, I think the firmware of TMEIC's SPR drive could be modified to drive the WRIM super-synchronous and also control rotor voltage in order to export VARs. If this project goes forward I will post results here on this forum.

 

[LionelHutz]

Well, my thought was that you woul basically have to source the VARs to the rotor in order to export the VARs from the stator. So, the VARs would basically circulate at the motor and be useless for what you wanted. After all, the rotor circuit would eventually be connected to the same power source as the stator.

Now today, I'm remembering that the VAR's will flow between the motor and the DC capacitor bank inside a standard VFD. In other words, a standard VFD will source the magnetizing current of a motor without drawing any current from the line source. This means it might actually be possible to do what you want without the exported VARs simply going back to the rotor circuit. But, the converter you choose will have to be capable of sourcing the VARs internally or else it won't work.

 

[purplepete]

We are installing Siemens slip-recovery drives on three 2.7MW 593 RPM motors driving pumps, presently started by Liquid Starters. Main issue has been the slip rings, not rated for continuous operation (the motors have shorting rings.)

 

[LionelHutz]

That seems odd. The motor rotor should run at it's rated current when the motor is at full load, even if the rotor is shorted. Does the slip enery drives increase the rotor current above the rated current?