Typical difference between RTD and average winding temperature 4

[electricpete]

1 - What is typical rise of stator winding temperature by RTD above average winding temperature (by resistance method)?

2 - Your opinion on a specific case:

We have 800 hp 900rpm 60hz vertical squirrel cage motor. Class H insulation. FLA = 111A. Normal load in plant is 93A.

Motor ran hotter than sister motors in the plant. At 93A, RTD's indicated in the neighborhood 210F - 240F with ambient temp 70F - 100F. Sister motors ran in the neighborhood 170F - 200F by RTD with same ambient.

Motor was refurbished. Core loss found at 7 watts per pound. No hot-spots identified by infrared. No repair was performed (not practical without rewind). Motor stator and rotor were cleaned and dried and reassembled.

After reassembly (and a storage period), a shop load test was performed at two points: 93A (normal load) and 111A (FLA).

At 93A
Ambient = 82F
Average winding temp (by resistance) = 165F
Rise = 83F
RTD's indicate 187F - 200F (6 RTD's spread within this range)

At 111A
Ambient = 82F
Average winding temp = 189F
Rise = 107F (similar to original factory test data on sister motor)
RTD'S indicate 212 - 226F

This appears to indicate a slight improvment in temperature compared to in-plant operation (possibly due to cleaning), however still hotter than sister units.

Again - does the difference between RTD's and average winding shed any light on the situation? On the surface it appears that rise during load test is similar to sister motors but RTD's read higher so presumably a larger rise RTD above average for this motor. What would cause that?

Core loss of sisters is unknown. Presumably high core loss would contribute to high temperature but it doesn't seem to me like it woulc contribute to difference between RTD and average, does it?

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[laundry]

Hello electricpete
the only thing i can think of is that you have a core flux problem,or even a drop in voltage on one phase but is not enough to trip your o/L.even harmonics can cause overheating of motors, But you did say your sister motor runs ok,Are they fed from the same supply.

 

[electricpete]

Core flux problem? Maybe. We know we have abnormal core loss 7 watts per pound.

Power supply in plant is similar to sisters and very reliable. Different power supply used for shop test and again saw temperatures higher than sister.

One thing to point out is the standard temperature rating system. Class B for example 40C ambient, 80C rise (average temperature above ambient) and 130C insulation. 130C - (40+80) = 10C suggests 10C "hot spot allowance" or 10C difference between average winding temp and hot spot temp. Similar result for class F. Presumably RTD is between the two numbers. But an article in this month's Electrical Apparatus cautions that we should not interpret this as a hot spot alowance.

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[electricpete]

Also regarding unbalance, during shop load test I observed about 1% unbalance ([max-min]/avg) in voltage and 5% unbalance in current which I considered to be a normal result of the voltage imbalance at the shop.

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[laundry]

hello again electricpete
I still beleve that your problem is your core slots that are creating hot spots.All so if you have laminations that have been damaged it is likely that your rotor may be damaged.just cleaning these up does not alwayes work.some years ago I got into a rangle with demag over a compressor motor over heating and burning out,they clamed everything was wrong but them.It turnid out to be the core laminations after a core flux test they had to replace the motor

 

[electricpete]

I agree high core loss is a plausible explanation for high overall temperature. I do not understand any reason why high core loss would elevate the RTD reading but not the average temperature. As far as I can tell this motor is similar to sister motors in terms of average rise (based on original factory test) but higher in terms of RTD reading.

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[aolalde]

Hi Electripete.

For 800 HP, 900 rpm vertical motor I assume the enclosure is WPI or WPII.

Under normal operating conditions the Stator winding coil heads are heat sinks absorbing part of the core losses heat, which adds to the joule effect of the current circulating in the same winding. The cooling air circulating through the coil heads takes part of the stator losses, the remaining air goes trough the core stack air vents cooling the core stack too.

For the above reason the temperature of the coil embedded in the slots is expected to be around 10 °C above the coil average temperature.

When the core losses are above normal, an increase on the temperature differential between the core and the coil is required to conduct the extra heat generated in the core stack cooled by the same air flow.

Extreme problems can arise when the extra loss is generated on some located areas or hot spots. That condition could burn out the insulation and develop a catastrophic failure.

 

[acmotorengineer]

Electricpete



2. RTD vs Avg temperature difference is high with open motors since the principal heat transfer is thru the end turns and therefore have a significantly lower temperature than the slot where most RTD's are placed. The difference is closer with TEFC motor where the principal heat transfer is thru the frame and the end turn temperature is close to the in-slot temperature.

3. Placement of the RTD's has a significant impact on temperature measurement and is a function of the cooling pattern. TEFC motors you can place the RTD practically anywhere, (in the slots), and get a good reading. RTD's in the Open motors require a temperature distributation test with many thermocouples - I've used up to 100 thermouples.

4. You didn't mention voltage, but it looks like 4000 Volts. Note that these coils are taped and therefore there is a significant temperature differential across tape. So your differential is even more alarming. I hope the manufacturer used 220C wire.

5. Also: when taking the resistance measurement, did you extrapolate the resistance from the measurement time to the shut off time??? - not doing so will give you a significant error in measuring average temperature -- i.e. Newton's law of cooling.

 

[electricpete]

Thanks for the info. (I'm still reviewing your responses).

The machine is 4kv, Open WP-2.

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[electricpete]

The resistance for both tests was taken 20 seconds after the power was secured (I timed it myself). We did not trend resistance over time to extrapolate back to point of deenergization but I beleive the drop in 20 seconds is insignificant. What do you think?

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[aolalde]

You do not need extrapolation if the resistance was taken after 20 seconds of energy shutdown. IEEE Std-112 Test Procedures for IM allows up to 120 seconds time delay for machines above 200HP.

 

[edison123]

I would expect properly located RTD's temp always to be higher than the average temp (by resistance method) since the later is what it says - average. When you measure winding resistance, the temperature is different in different parts of the coil. While end windings will be cooler due to cooler cooling air and absence of core (hence loss watts) in the vicinity, the in-slot portion will have higher temps due to preheated cooling air (from end winding) and due to core loss. So the net effect is lower temp with lower resistance values.

As far comparing with sister units, two questions :

1. Are they identical and were they made in the same year ? I have seen OEM's trying to cut cost over the years for the same frame with lesser copper.

2. How their winding resistances compare at ambient temp ?

 

[electricpete]

Thanks aolalde. That helps close any doubt about our technique.

edison - Great question about the copper winding resistances. I don't have that data handy but it might shed some light and I will definitely try to find any winding resistnce test results we have on sister units. Although again I would expect that to affect both the average and the RTD.... seems the average is similar and the RTD is higher.

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[mc5w]

Also remember that when rewinding a motor you still have the same temperature rating varnish between the laminations, so you cannot really exceed that. In a TEFC motor the direction of heat dissipation is such that the windings can run a little hotter than the laminations, but not much. In an open dripproof motor the laminations will generally be hotter than the windings. However, I have taken apart some open motors including applicance motors that have passages in the rotor and a way for air to flow through the air gap rather easily.

Just remember that you can just as easily cook the lamination varnish and when you get that the typical motor is ruined.

 

[vanstoja]

I presume the spread in RTD temperatures you cite refer to six different measurements for 6 RTDs located at various positions throughout the windings where RTD locations include in-slot and endturn locations. Endturn RTD locations can yield substantially higher temperatures than average winding temperature in motors where endturn cooling is not employed and the endturns are entirely surrounded by air. If endturns are cooled by encapsulation in epoxy-like compounds or by attached metallic heat transfer devices, then it is possible that end turn RTD temperatures might even be lower than average winding temperatures. Whereever located in the windings, RTD temperature reading accuracy can sometimes be affected by the method of installation of the metal "spades" which, in my experience, are wrapped into the layered winding insulation during the final assembly of the windings. Also the relative heat transfer effectiveness of the RTD spade to the remotely located RTD tip sensor can cause variations in readout temperature from RTD to RTD. One of our suppliers ran extensive testing of RTD connection geometry effects and found large variations in transferred temperatures to be possible.
Generally, your differentials between average winding and RTD temperatures seem plausible where some RTD locations are in uncooled winding endturns. The motor to motor differences are indeed perplexing if the same amperage is being drawn by all motors. It might pay to ensure that the abnormal motor is not drawing higher current due to load differences perhaps associated with unequal hydraulic outputs among parallel pumpmotors. We had such a problem related to centrifugal pump inlet flow counterswirl to one of several in-parallel pumps.

 

[electricpete]

I think all 6 RTD's are located in the slot section between upper and lower coils. I am under the impression that is a standard location for all form-wound machines. (someone correct me if I'm wrong).

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[edison123]

Right Pete,

In form wound coils, the RTD's are located in the axial centre betweeen top and bottom layers. That is supposed to be the area where the cooling medium doesn't have access and hence the hottest spot.

If you suspect the core, you could run the two motors decoupled (with same voltage, frequency and ambient temp) and look at the difference in steady state temps.

 

[vanstoja]

My 7/17 remarks were based on experience with specialty pressurized water cooled canned motor pumps where rodlike RTD's with tip sensors are spring loaded into a single thick-walled thermowell whose inner wall is connected to the predetermined (by prototpe unit test with many thermocouples) winding hotspot called maximum winding surface temperature (MWST)by a termally conducting metal strap embedded just below the surface of the last insulation wrap. Heat losses through the length of the thermal conducting strap and the thermowell wall cause "indicated" MWST to be as much as 50 to 80F lower than the actual hotspot temperature. A calculated addition to indicated temperature had to made to arrive at an RTD temperature corresponding to the insulation temperature limit which was used as an alarm condition for winding temperature. The six or more thick tape-like RTDs (30 or 70 X 80mils)used now in conventional air-cooled motors slots or between coils have three electrical leads which presumambly do not have the thermal losses in canned motor RTDs and should yield reasonably accurate measurements of temperatures at the tape sites. It would seem, however, that contact pressure variations on the RTD tapes associated with winding assembly effects might lead to temperature variations from motor to motor that could have nothing to do with circuit electrical or magnetic field deviations. The maximum temperature rise of 107C is only 76% of the allowable 140C rise for Class H insulation and assuming delta T proportional to I^2R would allow a current increase from 111 amps of 114.4% to about 127 amps before insulation temperature limit is reached and insulation life is adversely affected

 

[vanstoja]

Scratch the numbers in the last line of my 7/19 post. The cited 107C rise is really 107F and is for the average conductor temperature not the insulation MWST which is 226F at 111 amps. The MWST rise is 226-82=144F=80C. This is 57% of the allowable Class H insulation temperature rise of 140C. For I^2R based insulation temperature rise, allowable current increase to Class H limit would be approximately 132% from 111 to 146.8 amps. Determination of maximum allowable current to reach insulation temperature limits is important to know when variable frequency power is introduced to increase output of motor-driven loads by speed control.