COMPRESSOR MOTOR ALARM 2

[EEENGRX]

A compressor with Motor.XXX, MULTILIn alarmed on high stator temperature, DCS recorded the Multilin alarm and also Low Lube oil pressure
even while the aux pump continues to run. what i know so far is as follows
highest temperature is 126 DEGC
Alarm trip point is 125 DEGC
Motor nameplate design temperature is 43DEGC
The ambient temp at the time was 108DEGC
Alarm would not reset because it was above Alarm set point
Next day alarms have been cleared and alarms were on three RTDs?

i am looking at verifying all set points on the relay and looking at the aux pump

what do you suggest be done as a proper procedure to correct this?

Thanks
'

 

[petronila]

Hi oosani,

You can start with 10 degrees below the insulation class temperature for the alarm and the temperature of the insulation class for setting the trip. That is if the motor is rated insulation Class F you should use 145°C for the alarm and 155°C for the trip. Once the motor is in operation you will start to "know" the motor and if for instance, the máximum full load temperature is 135°C you can reset the alarm to 140°C. Just note if you set the alarm for 145 °C and you still had problems then you should investigate for detecting issues (overloading, etc).

Best Regards

Petronila

 

[RRaghunath]

Did you investigate cause of 'Low Lube Oil Pressure'?
There lies the reason for high temperature of motor stator windings.
The temperature trip setting of 125degC seems OK for a motor with Class-B temperature rise (Class-F insulation but temperature rise limited to Class-B typically). You may like to verify the Factory test report for acceptable temperature rise at full load, to confirm.

 

[Gr8blu]

oosaniIf I read your opening post correctly, you have:
1) At least one temperature detector (presumably in the stator winding, although you don't specifically indicate that) reading 126 C. This is NOT the temperature of the hottest spot in the winding itself; depending on the winding type (form-wound vs random wound) there is a differential between where the detector can measure and the actual "hot spot" within the coil. The general consensus is that trip occurs at allowable temperature class rise level MINUS THE DIFFERNTIAL DUE TO COIL TYPE. Thus a form-wound Class F insulation would typical trip at (155 - 10) = 145 C. The alarm point would be 5 to 10 C lower than that.
2) You have a reasonable alarm point for a machine rated for NEMA Class B (80 C above ambient) rise condition when the ambient is above the "normal" 40 C.
3) You indicate the machine is designed to operate with a maximum ambient condition of 43 C, hence the statement in 2). Be aware that "ambient" refers to the temperature of the primary coolant (usually but not always air) entering the machine - it does not necessarily relate directly to the "room temperature". If there is no temperature sensor in the incoming coolant stream, you have no idea what the "ambient" temperature actually is.
4) You do not clearly indicate whether the low lube oil was an alarm or warning ... or a fault condition. Regardless which it is, it should register in the relay (or motor control) as "the machine no longer has permission to run". This is independent of having the lube oil pumps actually powered and operating. For example - the piping could be disconnected and the pumps are pouring oil onto the floor, rather than into the bearing.

Things to do:
A) Use an ohmmeter to measure the resistance of each temperature detector (disconnect from the terminal block in the main motor box and read from the actual detector leads, not through the terminal block or from the MCC). What you're looking for is correspondence between all detectors - if they are "good", they should all read within 0.2 C for a 100 ohm PT device. To know if your detector is reading correctly, you'll need an independent measurement of the (presumably coil) temperature - or wait until the machine has cooled back to room temperature. If the detectors exhibit a discrepancy, then it needs to be addressed by finding out what the REAL issue is.
B) Verify your lube system (mechanically) - and your sensors for it (electrically). Verify your logic is correct for when you lose a "run permissive" such as lube oil. Be aware there may be other "run permissives" as well - which should also be verified.
C) Figure out the root cause of the "high ambient". If the ambient exceeds the nameplate, it is also a case of "the machine no longer has permission to run". Verify your relay (or motor control) is working with the correct logic sequence.

Converting energy to motion for more than half a century

 

[edison123]

The ambient temp at the time was 108DEGC

Are you sure?

What is the temperature history of the motor? If you are seeing a steady or sudden increase, it could be overloading, choked filters, frequent starts, higher ambient/inlet air temperature etc.

To ensure a long life (about 20 years), class F 155 deg C machines are recommended to run at class B 120 deg C temperature. Operating at higher temperatures reduces the winding life significantly.



Muthu

http://www.edison.co.in

 

[EEENGRX]

Great responses! Yes 126degC is the highest temperature recorded. And Yes the lube oil low pressure alarm is also a warning alarm and not a trip