Typical difference between RTD and average winding temperature
Typical difference between RTD and average winding temperature
(OP)
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?
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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RE: Typical difference between RTD and average winding temperature
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.
RE: Typical difference between RTD and average winding temperature
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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RE: Typical difference between RTD and average winding temperature
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RE: Typical difference between RTD and average winding temperature
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
RE: Typical difference between RTD and average winding temperature
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RE: Typical difference between RTD and average winding temperature
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.
RE: Typical difference between RTD and average winding temperature
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.
RE: Typical difference between RTD and average winding temperature
The machine is 4kv, Open WP-2.
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RE: Typical difference between RTD and average winding temperature
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RE: Typical difference between RTD and average winding temperature
RE: Typical difference between RTD and average winding temperature
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 ?
RE: Typical difference between RTD and average winding temperature
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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RE: Typical difference between RTD and average winding temperature
Just remember that you can just as easily cook the lamination varnish and when you get that the typical motor is ruined.
RE: Typical difference between RTD and average winding temperature
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.
RE: Typical difference between RTD and average winding temperature
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RE: Typical difference between RTD and average winding temperature
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.
RE: Typical difference between RTD and average winding temperature
RE: Typical difference between RTD and average winding temperature