Wound Rotor Motor Upgrade

leskranes; ihave done the same thing on a 144" roller & works fine. would making less bend on the first sweep,reverse direction add more bend? perry

WHAT?

The torque-curve characteristics of a wound rotor motor is very poor. In other words, when you get close to maximum RPM, your torque begins to drop off rapidly. Shorting the rings to use a VFD doesn't change those characteristics. With that in mind, I always pick the drive that matches the current characteristics (not HP characteristics) of the motor and then purchase the next size up from the one that matches. Since you've already installed the drive, it sounds like you may need to reduce the top-end RPM by reducing your maximum frequency (start with 55 HZ and use a trial & error method). That should solve the problem, but if not, you may need to upsize your VFD & DBR.

Hi Leskranes,

I've read the replies and agree with them (with the exception of "WHAT" :-)) in principle. However, you mention specifically that the motor "stalls". I infer this as meaning the motor itself stalls with the VFD still "supplying" power. In other words, it is not a drive trip or fault. If I am correct then this means that the VFD is not producing the right voltage level (or purity of waveform) to allow the motor to produce the full output torque. If the motor was capable of doing this before fitting the VFD (and if the motor itself is physically stalling), then it can (in my opinion) only be the equivalent (rms) voltage of the VFD not being high enough for the motor to produce this torque.

Different Wound Rotor induction motors have different torque versus speed characteristics - it is not strictly correct to just say that WRIM's have torque-speed characteristics that are unsuitable for the application.

Check the equivalent (rms) output voltage of the drive, transformer ratios if introduced, output voltage purity (ask supplier). To check just in case, establish whether the VFD is "current limiting" and what you are referring to as a stall is not a physical motor stall.

I hope this helps.

Rob Melaia (melaia@mweb.co.za)

Thanks for everyone's thoughts. Since original post, I have reconnected the motor "T" leads directly to line (480) via reversing contactors and the secondaries "M" leads through the original resistors with 2 shorting contactors at mid and zero resistance points. This did not increase my torque output but did allow me to read an imbalance on the motor "M" lead currents. Removed the motor and sent to rebuild shop. No obvious winding problems were observed other than a low (.5 meg) resistance to ground. They cleaned, baked & re-enameled everything and reassembled with new bearings. Re-installed and wired 10-18-08. Amperage readings on the "M" leads are nominal at 320 peak to 270. Will be testing 10-20-08 under loaded conditions. Will report findings as time permits.

Update.....10-29-08 ....

Motor was sent to motor shop, baked, balanced, bearings, etc.... no problems observed.

Installed & reconnected using original fwd/rev "T" contactor & OL's. "M" leads through resistor with shunting contactors at midpoint and end of bank. Motor runs but still "bogging" at approx 60-70% of full design load. !@#%#$#^$%^%$^%!

Power went out here Saturday. They got it back on after 6 to 8 hours and now the motor appears to be running at 100% !@#$@%$%^$@$!!~!

Has anyone had similar experience? Is there a cheap way of checking the "quality" of the three phase power from the mains?

Given time after catching up to productions requirements, I will be attempting to drive the motor with the 250 HP VFD and see if the torque improves with better power.

thanx