Slip Ring Motor Current When Used With VFD.

Where are you measuring the current? before or after the starter.

Operationally, you would get much better speed control. The slip ring motor (actually a wound rotor motor) is not really a speed regulating device but rather a torque regulator. No doubt, you have found that, even with lots of resistance in the rotor circuit, at light loads or no loads, the mill will run up to full speed. This will not happen when running on a VFD. I don't see any reason why a properly selected VFD wouldn't work on this application. You would have to size it by peak overload amps, not hp or continuous running amps.

Regards,
Xing

In case of E.O.T Cranes we used fit slip ring motors for various drives. After arrival of VFD we were using Squirel Cage motors with VFD. This has resulted in cost saving as Sq.Cage motors are cheaper than slip ring motors. May i know why you went for VFD for slip ring motor as could have controlled the speeds with this motor?.

As Tony said where are you measuring the current and are you using a clippon ammeter to measure it?

If you are using a clippon ammeter an measuring the output of the drive that is normal, try using the display on the vsd to display the current, you should get a better result.

Or put your clippon on the line side of the VSD and measure from there.

Cheers

Joe

If the applied voltage and frequency and the load is the same as before then the motor current should be quite similar to before. What is the supply and how does this compare to the motor rated voltage (assuming frequency is matched, of course. If the motor current is higher (see other threads for verification and confirm) then there can only be a couple of reasons

1. The applied voltage is higher

2. The load is greater (or work done is higher)

Are you operating at the same frequency / speed? If not, a lower speed may mean your stored energy in the flywheel is less and thus the motor slows more now when under load demanding extra current in this part of the cycle. If it's a higher speed the power requirement goes up and the voltage may not be able to so current increases.

Another possibility is if the VFD does not have it's 'slip compensation' set to zero and is 'fighting' the speed loss when pressing causing extra current again.

If you can supply better details it will be easier to suggest what's happening.

Output current of the drive shown in drive parameter and the current measured with the meter at drive O/p terminal is matching.

This press application is running on 50 Hz all the time.

At the time of actual punching current goes to 72 Amps, otherwise it is 67 Amps.

Customer is not ready to change the existing slip ring motor hence not going for squirrel cage motor.

A potential pitfall is where you short the rotor windings. If you short them on the stationary side where it is easier, the rotor power still must go through the rings and brushes, which means any arcing or resistance in the connections from wear and tear becomes a serious problem with regards to the interaction with the VFD on the stator. So it is highly recommended that the rotor windings be shorted ON THE ROTOR. But that sometimes means having to re-balance the rotor and most people don't want to go through the expense and down time of removing the motor and sending it in to a motor shop for that. However, that could be what you are seeing here if you did not.