Transiet Torque - Chiller Power Fault

[ndwade]

Hello all,

I'm working at a chiller plant and a few of our chillers are shutting down due to a "Power Fault" everyone once in a while. This error message continuously comes on the chiller control panel and according to the manual, it is a protective control circuit measure for the compressor and driveline to protect against transient torque damage. We have York centrifigul chillers approx 2000tons capacity, 4160V, 3600RPM, operate @ 197A.

The manual says that the circuit anticipates the transient torque condition by detecting a momentary interruption in the motor current and denergizes the starter. If the motor runs for > 75secs and the motor current decreases to < or equal to 10% FLA, a Power Fault shutdown is initiated.

Can anyone explain the concept of Transiet Torque as my understanding is that it has to do with the magnetic flux and current of the stator and rotor in this situation and how the current is rapidly reduced from high to low which causes problems.

Also, has anyone else encountered this problem and is it a function of incorrect operation in reducing the load too quickly, a case where some electrical settings may need to be adjusted or a case where something is faulty with the circuitry.

Any help or experience on this matter will be greatly appreciated.
Thanks

P.S See the attached link for the exact wording for the "Power Fault" condition.

 

[dpc]

I think the concern is a very brief power interruption - a few cycles. When power is lost, the motor acts as an induction generator but immediately starts slowing down. If the power is restored before the motor's generated voltage decays away, it is similar to closing in a generator out of synchronism and can cause very high shaft torques due to the out-of-phase condition.



David Castor

http://www.cvoes.com

 

[waross]

I agree with David. The problem is when the voltage is re-applied. The loss of voltage initiates a trip so that you won't have the voltage being re-applied. It is the reapplication of voltage to a spinning motor that may cause the damaging transient torques.

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

 

[electricpete]

The manual says that the circuit anticipates the transient torque condition by detecting a momentary interruption in the motor current and denergizes the starter.

I agree with the others on meaning of "transient torque" and logical action upon detection of loss of voltage is to open the starter to prevent damage which could otherwise occur if power comes right back.

....If the motor runs for > 75secs and the motor current decreases to < or equal to 10% FLA, a Power Fault shutdown is initiated.

I can't make sense of this. Starter should be open at which point sensed current is zero. What is this 75 seconds and 10% FLA... something completely unrelated?

=====================================
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[ndwade]

Thanks all for your answers to date.

Electricpete,

The 75sec is basically the run time setpoint in the control sequence that initiates the "Power Fault" condition. The actual "Power Fault" shutdown can only occur when the current drops below 10% FLA on any of the phases. So basically, both conditions have to be satisfied; the chillers running for a minimum period (75sec)and if imbalance is sensed where one phase significantly drops (below 10% FLA). Sorry if I caused any confusion.

All,
Based on the general consensus, it is clear that there is some loss of voltage and imbalance for a short time.

Does anyone have any methods for solving this problem? Should the PQ be looked at or should the time delay possibly be increased?

My boss is hinting at replacing the control module as he thinks it might be reading something that isn't there.

What type of issues should we be concerned about when trying to find a solution to investigating this imbalance as it seems to be an intermittent problem? Any input as to the best course of action in this scenario is welcomed.

Thanks for the feedback

 

[rbulsara]

You can check with an independent PQ monitoring and see if it finds something that coincides with the events. My hunch is that your boss might be right.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

If only one phase is going down your issue is an unbalance condition.
Transient torque occurs if three phases go down.
If you lose one phase, the healthy phase will usually keep the motor in sync and avoid transient torques.
You may consider a time delay of some seconds and then an automatic reset and restart.

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

 

[LionelHutz]

FYI, a lot of chillers can easily damaged by even smaller torque transients that wouldn't affect other types of motor and loads. They are basically building these things without any extra torque handling capacity built-in. I believe they now require a line disconnect that will occur within something like 5 or 8 line cycles. It's a real trick to accurately detect this and it can also require things like fast opening contactors to meet the timing they want.

You should double check that the unit is properly set-up. Obviously, if it is looking for <10% of motor FLA current then you first need to make sure it's measuring the current correctly (CT ratio setting?) and that it is set to the FLA of the motor (programmed value?).

 

[burnt2x]

If the you have tried recording the line currents, you could possibly catch that 10% decrease in the load! Assuming that your recordings are all within the sensing limits of your preset values, something could be wrong with the rectifier circuit or the pre-conditioning circuitry that feed signals to your microboard; a broken pot, loose connections, etc.. Also check CT's as mentioned by Mr. Hutz above. Or have a replacement board as your boss said.

 

[ndwade]

All,

Thanks for your helpful comments regarding this issue. From your comments and questions, I have learned a good bit.

We did a little more probing and finally got the manufacturer to admit that the problem lies in their control board. Supposedly the power fault problem is one that many of the York chillers have been experiencing.

Thanks for your time and help