160 KW Motors Tripping

GA.

And that is why, in well-co-ordinated installations, the protection is a motor overload device downstream of motor-rated fuses, the overload being there to protect the motor and the fuses being there there to protect the wiring.

PS: the co-ordination protocol is in British Standard 7671.

Dear sir,The motor is tripping while in running condition.Daily ,in 24 hours,once or twice ,it is tripping.

There's no need to repeat the original problem. It is perfectly obvious why trips happen.

The reason it is tripping is:

• partly that the starting current exceeds the trip setting by a factor of 5 or so, and
• partly that the operating point is so close to the trip setting.

It's time to look at the co-ordination, as given in BS7671, between overload and circuit protective devices.

If in doubt, consult a qualified local Electrical Engineer.

In the subject Schneider breaker, is there any way of knowing which element is operating - Thermal or Short time??

In any case, the Thermal OL relay settings as well as the Schneider breaker protection unit settings need to be adjusted to coordinate with the motor starting current as well as the motor expected maximum load current, it seems.

Dear sir,Since the motor is tripping while in running condition,we are thinking it is long time thermal element.

Good thinking.

• _{Dear sir,The motor is tripping while in running condition.Daily ,in 24 hours,once or twice ,it is tripping.}

It would have been nice to know that up front...

So are your compressors running continuously non-stop, and an unloaded valve is cycling on and off with demand? If so, every time that unloaded closes to build up pressure, it increases the load on the motor from a more-or-less "idling" state and although the motor is running continuously so there is no "starting" current, there is still a rapid step change in loading. That could be pushing your breaker thermal trips into the tripping area. the fact that your currents are unbalanced means that the phase current differential mechanism in your breaker may be stressed already, making it trip sooner than it otherwise would. You need to take steps to try to balance your currents. I would suggest "rolling" the motor leads, meaning take R and connect to Y, Y connected to B, B connected to R. The phase rotation remains the same, but you are changing any internal resistances in the motor to help even out the current balance. You can do that twice (the third time of course puts you back where you started).

But still, take PWSlack's advice and coordinate your protective devices properly.

The Schneider breaker thermal element is set at 320A, whereas the motor load current is reaching even to 356.5A.

Also, the thermal OL is set at 300A.

While we are not aware of the background to these settings, it is obvious you have a problem of protection settings.

Basic thing in overload protection setting is, whether in the breaker or motor protection relay, that the load current shall not exceed the OL setting in the relay/release.

Hope you appreciate that!

The current drawn by the motor varies, mainly due to applied voltage .I could see applied voltage varies. Find out reason for voltage variation, if you could fix the voltage problem, the tripping could be set right.

Name plate current is 289A.

OLR set is 300A

It is low set it for 110 %

i e 318 A

Check cable , Check termination for voltage drop.

As I think about why the breaker would trip instead of the Overload Relay, even though the Overload Relay is set lower, I have a possibility.

Circuit breakers are designed to protect CONDUCTORS. Overload relays are designed to protect MOTOR WINDINGS. The trip curves they function with are not the same. In fact an Overload Relay will have a "pick-up point" built into its design that is commensurate with the typical thermal damage curve of a Motor and for an IEC OL relay is likely anywhere from 110 to 118% of the motor FLA as set on the dial. That is going to be evident when you READ THE INSTRUCTIONS on setting the OL relay dial; it tells you to SET IT FOR THE NAMEPLATE FLA.

Often times people read somewhere that it is ALLOWABLE to have a motor protected at a value higher than the FLA and THAT is what the 110-118% pick-up point is addressing. If YOU then factor in a higher setting already (300A vs 289A), then the OL relay ALREADY has a 112% pick-up point, as the Schneider OL relay does (assuming that's what you have, you didn't say), that means the OL relay will not even BEGIN to start it's trip curve until the current gets to 336A, whereas the Circuit Breaker likely began its tripping curve at 320A.

The circuit breaker wins...

We have the same problem here in the US, because there is a statement in the National Electric Code telling people that a motor can be protected at 125% of the nameplate FLA if it has a 1.15 service factor. So lots and lots of electricians read that and automatically ADD 25% to the motor nameplate FLA value. But again, the OL Relay manufacturers have ALREADY factored that into their design. So when the electrician adds 25%, the OL relay doesn't even BEGIN to protect the motor until the current gets to 156% (1.25 x 1.25) of the nameplate FLA! This issue keeps a lot of motor rewind shops making a lot of money rewinding motors...

Do you have readings at the time of tripping? I suspect there is voltage variation (voltage drop)leading to higher current draw, as already told by some members.