Motor Torque

Hi All,

Just looking for advice on a problem we are having. We have a crane hoist that had a Yaskawa G5 15Kw drive with a 15kw slipring motor(slip rings shorted). From what I understand is that a while back the G5 drive went faulty and was replaced, then a while later the motor was replaced with a 15kw Squirrel cage induction motor. A while after that I was called as the when lifting a load that was already suspended the motor would lose torque and run downwards. When looking at the drive I noticed it was an E7 drive which is meant for HVAC. I then stated that the drive was not the correct drive. we approached the company that supplied the drive and they say that the drive is an F7 with an E7 Cover(Apparently the agent did not have a complete assembled drive) and they say the problem is with the motor being changed to a low efficiency(89%) squirrel cage induction 15kw motor. According to the company that replaced the drive they say it was working fine before the motor was chnaged. talking to the guys for our company they do not know when the problem started as they say both were changed a long time ago.

My question is this with a the above situation can a slipring motor generate more torque than a squirrel cage induction motor?

Yes, the motor matters.

The drive would need to have flux vector capabilities (HVAC drive probably would not have this).

The drive would need to be setup for flux vector operation AND BE TUNED TO THE MOTOR THAT IT IS RUNNING.

Your problem could be any of the above. For me, changing the drive with a mislabeled drive and changing the motor with a different type of motor would all be unacceptable. I hope no one is at risk of being hurt.

Motor torques vary with the type of motor. At full speed and hp the torque is always the same. However, pull up torque and breakdown torque vary with the motor design designation. There are squirrel cage motors (Design D is one I think) with better stall torque characteristics than a standard Design B motor. Also, an HVAC drive is rated for variable torque loads like a fan, and may not have enough current capacity to maintain high torque at reduced output speed without overheating and tripping out.

You need to verify that the new motor torque characteristics and the new drive current capability match the old unit's.

If the drive really is an F7 (questionable) and is capable of sensorless vector control, then when changing the motor, you must also re-run the motor ID so the drive can build a motor model that matches the actual motor. Trying to run a sensorless vector drive without a matching motor model can result in extremely poor performance.

As to the motors, a wound rotor motor (3 slip rings) with the rings shorted compared to a standard NEMA Design B motor will be a close match from a peak torque standpoint. But, without careful drive programming specific to the motor being used, the capabilities will be far less than peak.

Bottom line: Don't blame the motor

The torque developed by asynchronous induction motors varies with the speed of the motor when its accelerate from full stop or zero speed, to maximum operating speed.A high Starting Torque is more important for application or machines hard to start - as positive displacement pumps.

I think I would drop back a few steps and check my facts - such as
1. Does the motor now installed have the proper torque rating for crane service. Check with factory and get factory rating sheets
2. Is the VFD the correct one for this application. Check with factory. Your local vendors seem to be doing nothing for you except confusion and "it is not my fault" Get rid of em.
3. I would then check to ensure all parameters are correctly set in VFD.
4. Of interest to me and should be of concern to you - why when the VFD "loses control of motor" and lower the load uncontrolled - is not the mechanical brake set - assuming there is one of course.

Dan Bentler