Motor Current Normal but Winding Temp High

[sethar02]

Three induction motors with rating 6kV, 500kW, 60A, Class F. Each coupled with a Centrifugal Pump of Hydraulic Power 470kW. Motors are running at ambient temperature of 45 deg C. Each motor winding temperature reaches at 145 deg C but the current is 55A. Motor drive-end bearing temperature is 90 deg C at this time. Is this temperature rise due to motor construction (winding conductors), ambient temperature or any other factor? Also, is it feasible to run the motors continuously at 145 deg C, during summer? What will be long time effects?

 

[rbulsara]

There are certain expected hrs of operation (Life) at certain temperatures for different insulation classes. Refer to this for example ( just found by googleing). I do not take responsibility of its accuracy, but it has the crux of the message.

http://tristate.apogee.net/mnd/mfnrins.asp

For an authoritative answer you need to consult the motor manufacturer.

Rafiq Bulsara

http://www.srengineersct.com

 

[electricpete]

At 145C you have a 10C hot spot allowance / margin to the insulation rated temperature 155C.

Most people limit class F insulation to a class B rise but it is not striclty required. (what rise does the nameplate indicate?).

There is also a question whether it should get this hot given you are not fully loaded... slightly different question. Shot from left field - if you have voltage unbalance that can contribute to motor running hotter than it otherwise would.


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[electricpete]

Motor drive end bearing temperature is 90C - I assume that's on the housing. It is borderline unacceptable per IEEE841 specs. We have a few that run as hot (TEFC). If it is a 2-pole motor with 6313 or larger greased bearings, I think your bearing life might suffer a little.

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[electricpete]

Also should mention that bearing housing temperature is subject to short-term spikes for a few hours after greasing.

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[sethar02]

The motors are installed new about 4 months ago. Bearing grease has still not been changed. As per nameplate, Motor Type VSTC, FLA 60A, Pole 8, RPM 735, DE Bearing NU224M, Max. ambient 45C, Service factor 1.
Manufacture O&M Manual recommends :
"Embedded Temperature Detector for Winding at B class rise - Alarm 130C, Trip 140C"
In actual, Hi alarm 130C, Hi Hi alarm 140C.
Will there be any difference if low/high resistance copper coil for winding is used? What the expected winding temperature of normal motor in such conditions should be? What about the loading factor and efficiency of the motor?
I am thankful for expert opinions.

 

[rbulsara]

Mfr. recommendations sound reasonable for Class F insulation. But 145C motor winding temperature is not surprising in 45C ambient conditions. I have typically seen class H insulation in tropical locations where 45 C ambient is common and even reach 50C. Class F insulation is more common in moderate conditions.

Also I presume that calibration of the sensor/relay is checked and there is no reading error. If the temperature remains stabilized at 145C, I would venture to adjust the alarm settings to avoid false alarms, knowing full well that some day I will have to replace the motor insulation with Class H.

Or you can arrange for additional cooling if feasible, like an external fan, and see if it helps.

I am not sure where you are going with winding material type, loading and efficiency for this apparently normal situation.

Rafiq Bulsara

http://www.srengineersct.com

 

[jraef]

I'd double check your voltage imbalance. If memory serves me, every 1% voltage imbalance raises the winding temperature 10 degrees C.


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[edison123]

Questions

Is the motor open type or closed type ?

If closed type, air or water cooling (secondary cooling) ?

What type of RTD's are used for winding and bearings ?

Is the temperature scanner suitable for the type of RTD's ?

Muthu

http://www.edison.co.in

 

[sethar02]

The motor is closed type, air cooled. PT100 type RTDs are used for temperature sensing and connected with DCS system through transducer for recording. There is not any temperature reading error problem, because same RTD are installed on other motors and they give correct results. Also, the temperature readings of all these three motors are not coming normal at the same time.
I have checked and found that there is not any issue of voltage unbalance because 6000V are supplied through MR OLTC with MR AVR control on stepdown transformer.
The point is that if the temperature of motor windings is supposed to be 110C in such conditions and it is now running on 140C then how can i consider this high temperature as normal, although it is less than or equal to the standards recommended temperature. Does the ambient temperature is the only cause of increase of motor winding temperature because its current do not increase from the rated one?

 

[edison123]

Ambient is not the problem here since the other motors (identical ?) seem to be ok.

My checklist

1. Disconnect the RTD's from the DCS and measure the RTD resistances at the junction box. See if they correspond to the actual temp readings. If PT 100, every 10 deg C rise in temp above 0 deg C, PT 100 has app. 4 ohms rise. i.e. 30 deg C - 112 ohms, 40 deg C - 116 ohms, 50 deg C - 120 ohm etc.)

2. If RTD's are found reading the right temperatures, check the closed air cooling system,, especially see if the internal air is being blocked by lifting the heat exchanger.

Post a pic of your motor, if possible.

Muthu

http://www.edison.co.in