Calculating motor winding temperature
Calculating motor winding temperature
(OP)
I'm evaluating motors operating conditions in a factory
Does anyone know a method to calculate motor winding temperature (hot spot) based on motor operating current, voltage and ambient temperature?
If so, which motor parameters do i need? are those parameters easy to obtain from manufacturer, tests or nameplate data?
Thanks for your comments
Does anyone know a method to calculate motor winding temperature (hot spot) based on motor operating current, voltage and ambient temperature?
If so, which motor parameters do i need? are those parameters easy to obtain from manufacturer, tests or nameplate data?
Thanks for your comments





RE: Calculating motor winding temperature
RE: Calculating motor winding temperature
<nbucska@pcperipherals.com>
RE: Calculating motor winding temperature
RE: Calculating motor winding temperature
2. Run the motor with full load and allow steady temperature (4 to 6 Hrs).
3. Stop the motor and read resistance for the same pair of leads (into 60 seconds of power cut off). R2 (ohms)
4. Calculate average total temperature of the winding T2 (°C)
5. T2= R2/R1(234.5+T1)-234.5
6. Hot spots = T2+10 (°C)
If you are interested in details, this is based on IEEE 112 and NEMA MG1.
Double bridge instrument is recommended to get accurate resistance measurements.
RE: Calculating motor winding temperature
I know about an approach in which you calculate winding temperature rise based on the relation of copper to core losses and operating to rated current, Have you heard about it before?
RE: Calculating motor winding temperature
Unfortunatelly it works only with DC motor. Why don't you
just measure the case tenperature?
<nbucska@pcperipherals.com>
RE: Calculating motor winding temperature
This method works only with dc circuits (revolving field generator rotors, field winding of dc motors etc)wherein you need to know the dc resistance of the winding at another (normally room) temperature. Then, you can use aolalde's formula or use direct temp reading meters based on this formula and values.
RE: Calculating motor winding temperature
RE: Calculating motor winding temperature
The motors of concern are AC induction motors with nominal powers up to 200 HP, all i want is to compare data obtained from thermographies with those obtained from analytical calculations if possible. I though the approach i mentioned above was applicable to induction motors.
Can i obtain a good estimate just by measuring case temperature?
Again, thanks for your comments
RE: Calculating motor winding temperature
I am afraid I do not know any method to estimate based current/voltage/ambient temp for measuring winding temp of ac motors (I would love to know if one exists). Also, casing temp is not a reliable parameter for estimating actual winding temp. As a matter of fact, I have seen in a 1.6 MW VFD motor, the casing temp was only 50 degc and the winding RTD's showed around 140 deg C.
refer
Thread237-80813.
RE: Calculating motor winding temperature
the question is about motors up to 200Hp -- the smaller
motors have better heat xfer to case.
Well, case temp is better than guess...
<nbucska@pcperipherals.com>
RE: Calculating motor winding temperature
(c-coil) run, stop, measure. Repeat, calculate thermal model including load.
If load is variable, measure torque and current. Using model
calculate temp.
It won't protect from partial short.
<nbucska@pcperipherals.com>
RE: Calculating motor winding temperature
I don't understand your previous comment, could you please
make it clearer?
RE: Calculating motor winding temperature
and measuring.
After this you can monitor the current, VTG and load and calculate the temp.
<nbucska@pcperipherals.com>
RE: Calculating motor winding temperature
RE: Calculating motor winding temperature
I`ve been reading about induction motor thermal models, Can you recommend a good document (textbook or paper) on the subject?
It would be good to know the tests to perform in order to develop the model.
RE: Calculating motor winding temperature
1 - comparing motor thermal models
2 - motor analysis protection part 1.
3 - motor analysis protection part 2.
Let me know if you can't find them.
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RE: Calculating motor winding temperature
Stan Zocholl has some online papers..
Using Thermal Limit Curves to Define Thermal Models of Induction Motors
Induction Motors: Part I - Analysis
Induction Motors: Part II - Protection
RE: Calculating motor winding temperature
One more:
http://www.selinc.com/techpprs/6164.pdf
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RE: Calculating motor winding temperature
http://www.iprocessmart.com/leeson/leeson_motor_temperature_article.htm
for more info
RE: Calculating motor winding temperature
Also in the document "INDUCTION MOTORS: PARTI-Analysis", the authors made some aproximations that i cannot realize (Rs = Ro/5, Xs = X1/2 and the way they find the value for Xo). I think those estimations are valid for the motor they use in the calculations but are not standard values. What's your opinion?
I'm thinking about applying the resistance method but i got two problems. First, we got motors with nominal powers up to 200 HP and this implies the use of a special kind of ohmmeter (wich i'm afraid is not available at the company and they won't be willing to buy one until i show the method works at least for smaller motors). Second, motor terminals are not readily available so most of measurements will have to be performed in the motor control center (corrections to be made to account for feeder resistance). Any comments about this?, Does anyone have experience in applying the resistance method?
Again, thanks for your help. I find really interesting all these motor thermal issues.
RE: Calculating motor winding temperature
RE: Calculating motor winding temperature
RE: Calculating motor winding temperature
I'm thinking about applying the resistance method but i got two problems. First, we got motors with nominal powers up to 200 HP and this implies the use of a special kind of ohmmeter (wich i'm afraid is not available at the company and they won't be willing to buy one
///Normally, it is tacitly assumed that whoever has 200HP motor and associated hardware or production means will have a smaller amount of money reserved for the necessary maintenance, testing, measurements, etc.\\\
until i show the method works at least for smaller motors).
///The ohmmeter that is more accurate is called:
Digital Low Resistance Ohm (DLRO) meter. Visit
http://www.avointl.com/cae/products/ProductDetailsWithSubGroup.asp?ID=458&ProductSubGroupCode=P701&ModelFamily=DDLRO
for a sample.\\\
Second, motor terminals are not readily available so most of measurements will have to be performed in the motor control center (corrections to be made to account for feeder resistance). Any comments about this?, Does anyone have experience in applying the resistance method?
///Precise motor feeder leads impedance calculations are suggested, either by hand or by computer.\\\
RE: Calculating motor winding temperature
I totally agree with you but that's life, some human beings have a strange way of reasoning ...let's see what happens.
RE: Calculating motor winding temperature
RE: Calculating motor winding temperature
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RE: Calculating motor winding temperature
RE: Calculating motor winding temperature
I'd like to not use a model but actually measure the temperature if possible. The idea of measuring the resistance is still appealing.
RE: Calculating motor winding temperature
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RE: Calculating motor winding temperature
RE: Calculating motor winding temperature
Your idea of turning off the power to the motor for a cycle or so in order to make a measurement could result in major problems and damage.
Open transition switching causes very high current and torque transients at reconnection. This can break rotor bars, couplings and do a lot of damage to the driven load.
The problem is that while the motor is being driven, there is a rotating magnetic field in the stator which is where the torque is derived from. When the stator is disconnected, the motor behaves like a generator, generating voltage out its terminals. The voltage and frequency are dependent on the motor speed. When the motor is reconnected, the generated voltage will not be synchronised to the supply, so effectively, you can be closing on to a much higher voltage than is usual. The generated voltage drops away with time and this dcay dpends on the time constant of the rotor circuit, but typically, you need to disconnect the motor for more than 0.5 seconds to eliminate it.
The other problem that you would have, is that while the motor is spinning and the rotor field has not decayed, there will be voltage across the terminals preventing you from making DC measurments.
It sounds to me like the best solution is to have some form of thermal model calculating the effective temperature from the line current. This is done in many of the protection relays, and some of the better soft starters on the market today. This can be pretty good at providing the sort of protection that you need.
Why not invest in a small electronic protection relay that includes a thermal model. There must be provision to program into the relay, the rated current of the motor, and the motor start time constant. (Locked Rotor Time)
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Calculating motor winding temperature
It seems like a much simpler solution than attempting to measure the resistance on a motor while running.
Ed