Locked rotor test for a 1000 HP DC motor

[veejay729]

The customer who is visiting my facility in a couple of days wants to test his new drive and he s bringing in his 4160 volt drive. We have a GDY-37 generator set coupled to a 1000 HP motor which is capable of running in 4160 volts with an FLA of 124 amps. Now the problem is that, the customer wants to see how a locked rotor condition would affect his drive. We haven't done locked rotor tests in our workshop and since am a new hire, i dont know how to manufacture a brake assembly for such a large motor. Can some1 please guide me on how to lock the rotor (possibly for a time of 15-25 seconds) for a 1000 hp machine?

 

[electricpete]

I’m just thinking out loud here… what if you disconnect the field for the test? That would give equivalent armature current to locked rotor (applied voltage / armature resistance). Or use a resistive load bank?

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[veejay729]

Pete...i was thinking on the same lines...Probably weaken the field sufficiently with a DC drive!! ?? But i dont know if it ll work ..Coz i have'nt tried before..the locked rotor tests that i have performed were on small 20 Hps...N what i did in the lab was use a belt and pulley. n connect it to a wheel which is attached to the shaft... But the problem now is, i dont think i can lock such a huge rotor with a belt and pulley locking method. As mike points out, the vice grips or weight arrangements to restrain the motor have to ginormous. I really dont want to try that method.dunno how to proceed on this one. N thats y i posted here..Coz i know some of you fine folks would have done a locked rotor before.

 

[electricpete]

I'd say whatever you do, think it over twice before you do it. There are a lot of hazards to think about.

I think for large motors, the locked rotor testing is typically done at reduced voltage and results are scaled up.

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[electricpete]

... my last sentence was referring to large AC induction motors.

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[waross]

We commissioned a set of 12 1300 HP motors and 10 1050 HP motors.
To make it more interesting, the motors had to be tested in groups. The largest group was 8 x 1300 HP motors on a common drive that had to be tested simultaneously. We didn't have a large enough set of vice grips although I have seen the shafts of large DC motors clamped for locked rotor tests.
We used jumpers across the brush holders to simulate locked rotor conditions. The drive really can't tell the difference between a jumper and a locked rotor. Yes there may be a slight error but it is conservative. The error may be reduced by selecting jumpers of a smaller gauge the the feed conductors. I would use jumpers sized at about 2/3 of the ampacity of the feeder conductors. This is the equivalent of sizing the feeders for conductors in a conduit, and sizing the jumpers for conductors in free air. The jumpers will be in free air so this will be safe and will reduce the already slight error.
PS, Lift the brushes!

Bill
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"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

It depends a lot what type your drive is.

Thyristor drives need the inductivity in the armature winding to perform properly. If you short-circuit the armature (jumpers), the current controller will not work as intended and your test will not say much about what really happens when rotor is locked.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Thanks for the heads up, Gunnar.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rasevskii]

for veejay729:

You mention a 1000HP DC motor and then a 4160V drive...Sorry, there is no such thing as a 4160V DC motor, you must mean something else.
Why not send a sketch or dwg of the setup.

In any case as suggested by others, jumpering out the armature (rotor) and lifting the brushes of this (unfortunate) DC machine will result in zero torque, as there is then no current at all through the armature (rotor)...no need to lock the rotor, then.

It all sounds a bit flaky, why not explain more to us..before you melt down anything or hurt someone...

rasevskii

 

[waross]

Disconnecting the field may result in the interpole windings developing enough torque to turn the motor. If that happens it may run away and explode from centrifugal forces. You still need to mechanically lock the rotor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottI2R]

Waross, that was always my belief with loss of field. Arent most drives designed to detect loss of field? All the ones I have worked with were.

Scott

I really am a good egg, I'm just a little scrambled!

 

[waross]

I hark back to the motor generator days.
But the suggestion was made to disconnect the shunt field to do locked rotor testing. Even if you disconnect the shunt and series field on a shunt or compound motor there are still the small commutating poles in the armature circuit. I wouldn't want to take a chance without the shaft being mechanically locked.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

I agree 100%. We are not positive how the machine will act in this case, but we should err on the side of caution. A machine flying apart could be a very bad day for someone standing nearby. That is one of the things I had in mind when I followed up my suggestion with: "whatever you do, think it over twice before you do it. There are a lot of hazards to think about."

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[electricpete]

when I followed up my suggestion...

Correction, it was not a suggestion, it was a question: I'm just thinking out loud here... what if you disconnect the field for the test?

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[krsh]

electricpete needs a star for his posts

 

[electricpete]

krsh,

We all take safety seriously (although not every post needs to dwell on it).
I would be mad at myself if I had suggested something blatantly unsafe, and I think we all would rightly judge anyone who did so.
When Bill’s recap indicated (with no bad intentions) that someone else (turns out to be me) had suggested to run the motor without field, I wanted to clarify the original context, which included plenty of cautions on my part.

It is time to move on to more productive things for both of us, I hope.

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[krsh]

It was just a joke on the fact that you hurry to press submit before you collect all your ideas.
Sorry if i offended.

 

[electricpete]

No problem.

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[NIKT1]

Veejay729:
Before testing, you need to answer a few questions:
What is being tested? Is it the control's short-term capability to deliver locked rotor current? Does the control have a bypass contactor for 100% on-time, or is current electronically limited by current sensing and reducing on-time? If the control switches from 0% to 100%, does it need motor and distributed circuit inductance to achieve its switching ability?

Where I'm going with all of this is that if you're testing the control's current capability, and inductance isn't required, why don't you measure the cold DC resistance of the motor, make up a dummy resistive load and test the control without a motor, etc.?

NIKT1

PS: If you must test with a motor at full voltage, you will need a beefy means to lock the rotor. You will need some idea of the anticipated torque and design a means to lock the rotor more securely than a V-belt and pulley system. I have performed many DC motor tests(albeit at <=120 Volts) and we usually test at reduced voltage and with a substantial locking bar.