Problem on Cage Motor with Vector Drive 3

[sunrays]

I recently changed a 55kW 750RPM FLC: 101 Amps, Slip Ring Motor with Rotor Resistance Controls with a 55kW 750 RPM, 101 Amps Squirrel Cage Induction Motor and a Closedloop Vector Control Drive. At full speed and maximum torque requirement the current drawn by the motor was to be 200 Amps i.e. 200% of its FLC. The drive is also of 55kW with a continuous current of 125 Amps & 150% for 60 Sec. What is the reason for the abnormal increase in motor current as compared to the Slip-Ring motor. Could some one help on this?

 

[Marke]

Hello sunrays

Just looking through this thread, one question comes to mind. "Why was the slip ring motor used in the first place"

Are the rotor resistors used in circuit when the system is running, or only to get things running?

The addition of the rotor resistors into the circuit shift the slip where the maximum torque occurs from almost zero to greater than 100% depending on the value of the rotor resistors used. If you use the rotor resistors during part of the operation, you are effectively changing the mode of operation from a "speed source" to a "torque source" where the torque alters with slip to the maximum torque point.
This can mean that under a severe torque transient, the machine can slow down with some of the torque provided by the inertia of the machine and the rest by the motor.
The maximum torque that you will typically get from a SC motor is in the order of 200 - 250% whereas the WR motor can deliver in the order of 300 - 350%. If you add in the energy that is effectively stored in the momentum of the machine, you have a very high transient capability albeit with sagging speeds (assuming the rotor resistors are in circuit).

If you operate this machine with the rotor resistors shorted except during the initial start, you could need to increase the size of the SC motor and drive by 50% in order to provide the same transient torques. If you operate the machine with rotor resistors in circuit during transient operation, you may need to oversize by a greater amount.

I always investigate the hows and whys of the original use of the WR motor as often, someone did their homework and either required torque at a lower speed, or a high transient overload torque, or a moderately high "speed impedance", or a torque source.

It is common to oversize the SC motor when replacing a WR motor.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[TommyOO7]

This is what I would do:

1. Check the motor direction of rotation, ie. motors are helping each other not fighting each other. I guess you did this and is out of question.

2. When you select SCIM, you should carefully match the torque curves of a new motor and the old (WRIM) one. Breakdown torque of SCIM must be equal or larger then the old WRIM. Check the motor current at the breakdown torque and size the VSD accordingly.

Maybe your VSD's are not large enough?

Tommy

 

[DickDV]

Two things, sunrays. They may be minor mis-communications or major problems. Stick with me on this.

In your last post, you say that the follower drive is in torque regulator mode and the control signal is motor current from the lead drive. If this is so, the control scheme is not at all optimal. With the follower drive in torque mode, the lead drive must provide a torque output signal to operate the follower. A current output is not going to do the job.

I had asked for the drive output voltage, amps, and frequency for both drives when STALLED. You seem to have given me those values at full speed. Please provide output voltage, current, and frequency for both drives at the point the motor reaches stall or zero speed.

Just for curiosity, what kind of tach or encoder are you using for speed feedback?