Motor Temperature Rise/Service Factor, Effects on life expectancy 1

[PQ]

Hi guys,


We purchase a Altas Copco compressor coupled to a Siemens brand motor with nameplate data, 750 kw, 460 volt delta, 60 hz, 0.88pf, nominal efficiency 95.8% Full load Amphere(FLA)= 1100 amps, Service Factor(SF)= 1.15, SFA= 1300 Amps, RPM= 1788, Insulation Class F, Temperature rise 40 deg C

The Motor actual load or running current (continuous)
L1= 1268.2Amps,L2= 1262.3Amps,L3= 1294Amps, max recorded winding temperature = 105 deg C

Motor operates above full load current (per nameplate
data) but still within the service factor value.

We would like to seek an insight on what whould be the effect if we allow the motor to run on this condition. Is this a normal practise?

 

[uno]

Hi

Since you have already purchased the motor , it would not matter if you run it continuously within the service factor value , and within temperature limits of class F insulation since the motor with the specified service factor is designed to be operated continuosly at that value.

If it were in the tendering stage I would have asked for a continuous load to remain within the nameplate reading of full load amps since I would have liked to keep the margin provided by the service factor.

 

[electricpete]

Although loaded above FLA, your average winding temperature seems relatively low, even below that of Class B insulation (allowing for hot spot).

Perhaps this was specified as Class F insulation with a Class B rise.

Based on that margin I agree with uno. If you have 6 rtd's available I would double-check to make sure all 6 are close.

It is often assumed that a 10C increase in winding temp results in a doubling of the aging rate. The standards allow motor at SF to run 10C higher than motor at Full load.



=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[PQ]

electricpete, do you think a 90 to 105 degC winding temp is normal? It is already near to class F limits. I believe that the normal temp is around 70 to 85deg C. How can I check that the motor is designed for a SF continuous basis not for temporary overload?"

 

[aolalde]

PQ

To evaluate correctly, ambient and total winding temperatures are required.

The class F insulation could work at up to 155°C, but with 20,000 hrs B10 of life expectancy. Theoretically life will duplicate for each 10°C step down of that limit temperature.

for 105 °C total winding temperature and allowing 10°C for hot spots, Tmax. = 115°C
Delta = 155 – 115 = 40 °C; life expectancy = 20k*2^4 = 320k hrs. If the reported winding temperature is accurate, that motor winding will have a normal life.

By the way, you stated as nameplate data “Temperature Rise 40°C”, that figure is not normal. Is that correct or 40°C are for max ambient temperature?

 

[edison123]

For class F m/c with class B temp rise, max winding temp, as measured by winding RTD's, is 120 deg C. If you recorded temp as per winding RTD's, you are well within this limit. No need to worry about reduced life or damage to winding insulation.

40 deg C you mentioned is the maximum allowable temperature of the inlet cooling medium. Lower this temp, better is the cooling of the m/c.