WRIM brake application 1

[jraef]

For those familiar with Wound Rotor Induction Motors, I have an odd problem. The end user my DC injection brake has "rewound" his WRIM to be a squirrel cage induction motor. At this point I do not know what he means by "rewound", and I suspect that all he has had done is internally shorted the rotor windings. Typically this results in extremely high starting currents, but in this case he has a solid state soft starter, so that problem is masked. The new problem is that he has now installed a current-controlled DC injection brake on the stator and it continually blows fuses as soon as it is energized. My theory is that by shorting the WRIM rotor, he has created a SCIM with little rotor resistance (thus the high starting currents) which works in reverse as far as the DC injection brake is concerned because the rotor cannot create a strong counter rotating field to apply impedance to the stator windings. The brake current thus rises too fast for the SCRs to control it, and the fuses blow. Any comments?

"Venditori de oleum-vipera non vigere excordis populi"

 

[aolalde]

jraef:

A DC injection brake with partial “single-phase” DC excitation to the WRIM stator windings, creates a “fix amount of DC Flux poles”. The stator winding is now very similar to the shunt field of a traditional DC machine. If the transient condition is not considered the field current is affected only for the DC applied voltage and the winding resistance. (No counter induced voltage as in the rotor armature of DC machines).
The energy induced into the rotor windings will be transformed to heat. Your customer made two changes; being the last one addition of the DC injection brake. It appears to me that he did not matched the current requirements of the brake function with that of the load running condition.

 

[aolalde]

By the way, the rotor winding heat will be proportional to the motor-load rotating kinetic energy.

 

[DickDV]

Most DC Injection Brakes are current controlled devices as you state. It seems to me that the max current setting needs to be lowered so that the fuses do not blow.

If, at that lower current, the braking intensity is not enough to stop the load as desired, and the DC braking current isn't more than 150% of the motor nameplate FLA, the brake module is too small and a larger unit is needed.

Do not be tempted to exceed the 150% of FLA limit or motor damage could easily result. If you are at 150% and the load still isn't stopping fast enough, the motor is too small and, most likely, you will need to either choose a larger motor (expensive!) or add mechanical braking somewhere in the system to help out on stopping (high maintenance item!). Unfortunately, that's the dilemma of dealing with an inadequately sized original design.

 

[jraef]

Thanks DickDV,
The problem here however is that the Current Limit function of the brake takes about 250ms to gain control and limit current. The fact that the fuses blew even though the brake is working fine indicated the possibility that the current is spiking immediately, possibly too fast for the CL feature to catch it. Since originally posting this I have read extensively on what happens in WRIMs with rotor windings shorted. Starting current profiles can apparently range from 1000 - 1400% FLA (with very low torque generated as well). This indicates to me that the rotor, when shorted, applies very little impedance on the stator. A Braking torque/speed curve is close to being a mirror image of an Across-the-Line starting curve. This leads me to the conclusion that with the rotor windings shorted without resistance, extreme currents may flow in the stator before the Brake can affect current limiting.

Now a new problem has cropped up: the brake SCRs have shorted. This has lead to another theory from one of my engineers. Assume the brake could catch and hold the Current Limit. If the combined stator/rotor impedance were so low that starting current would have surged to 1400%, the conduction angle to maintain current Limit would become so short that we run into dV/dt problems with the SCRs. We are cutting open the SCRs to try and determine if they failed on current or voltage.

"Venditori de oleum-vipera non vigere excordis populi"

 

[DickDV]

Yes, jraef, it does sound like you've got some special things going on with that wound rotor motor.

I'm guessing now because I've not run into exactly the same situation but, maybe, a good sized choke in the DC line between the brake module and the motor leads would give enough current rise time for the current limit to work properly.

Otherwise, I'm out of tricks!

One last comment---anyone thinking about shorting the sliprings on a wound rotor motor, please leave a little resistance in the circuit. A hard short is'nt the answer.

 

[jraef]

DickDV,
The choke is not a bad idea! Hadn't occurred to me. Thanks for that suggestion.

"Venditori de oleum-vipera non vigere excordis populi"

 

[Marke]

Hello Jraef

If, as I suspect, you are right and the user has shorted out the rotor, then the current wil be higher because the impedance under high slip conditions will be considerably lower. Typically, the impedance of a WRIM with the rotor shorted is less than 50% of that of a standard cage motor.

If the DC break is a phase controlled SCR based system, then at the same phase angle, you will have twice the current. If you use a feedback system to control the current, you could end up reducing the phase angle, and therebyu reducing the conduction angle, but you will end up with a much higher current pulse for a shorter time to give the same average current. i.e. twice the current for half the time gives the same average, but twice the I2t which will lead to greater SCR stress and also cause fuse failure.

There is always the potential, that the design prevents the SCR from phasing back enough to cope with the lower impedance.

In this application, if you restrict the current to a lower average current, the shorted rotor will result in a vry low braking torque and so the advantage will be severely restricted.
Additionally, I would expect the start torque with a soft starter to be somewhat challenged. All in all, shorting the rotor is not a good option!!

Best regards,

Mark Empson

http://www.lmphotonics.com