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Calculating motor winding temperature
4

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

RE: Calculating motor winding temperature

You caanot estimate motor winding temp based on your parameters since the cooling of the motor will be different for each type of motor. Normally, embedded temp detectors (called RTD's/ETD's) are used to measure the working temp of the winding. Again these RTD's may not be located in the hottest spot, hence the maximum temp limits set by standards for such measurements. Refer any standard on motors.

RE: Calculating motor winding temperature

I use coil resistance to calculate the temperature of relays for UL.  Of ourse, you have to turn the motor off to do it. Forget the number to use for eah degree C but I remember Julius Sumner Miller (any fan club members out there)saying it approximates 1/273.  Hmmmmmmmmmm......why is that interesting.

RE: Calculating motor winding temperature

1.    Measure motor winding resistance between two leads at ambient temperature.  R1 (ohms)  @  T1 (°C)
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

(OP)
I have heard before of the approach that implies turning off the motor but the matter in this situation is that it's not feasible (production constraints). Also, the motors in which i want to calculate winding temperature are relatively small to install RTD's (overload protection based on motor current).

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

Charlie:
Unfortunatelly it works only with DC motor. Why don't you
just measure the case tenperature?

<nbucska@pcperipherals.com>

RE: Calculating motor winding temperature

In hydro generators, the rotor winding (carrying dc current) temperature is measured by dc resistance (V/I) during running. Is this what you had in mind ?

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

The checking of motor SOA, except measuring of winding temperature, need re-calculation of few motor constants and taking into account environment condition. I was in touch with R.H.Welch Jr., well known motor designed and expert on this motor thermal issue. You may contact him directly: (rhwelch@stthomas.edu).

RE: Calculating motor winding temperature

(OP)
Comments on nbucska and edison123:

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

charlie,

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

E123:
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

Giving it some more thought, you can measure the cold Rc
(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

(OP)
nbucska:

I don't understand your previous comment, could you please
make it clearer?

RE: Calculating motor winding temperature

If it is important to monitor the temp, you can develop a thermal model by spending a few days on running,stopping
and measuring.

After this you can monitor the current, VTG and load and calculate the temp.

<nbucska@pcperipherals.com>

RE: Calculating motor winding temperature

Suggestion: 25°C ambient is stated in IEEE Std 112-1984 section 5.1.2

RE: Calculating motor winding temperature

(OP)
nbucska:

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

Charlierod.  There are three good papers on motor thermal modeling at www.selinc.com.

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

RE: Calculating motor winding temperature

Thanks busbar. Those are the papers I was thinking of (that you showed me a long time ago).

One more:
http://www.selinc.com/techpprs/6164.pdf

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RE: Calculating motor winding temperature

(OP)
Thanks everybody for your help. Before reading your posts i've already obtained such on line papers which i have found really helpful. I still don't understand the model wich uses R1/Ro as a capacitance value. Can anyone explain it?

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

(OP)
I forgot to say, one of the papers mentioned above states protection based on RTDs is not reliable due to slow reponse. Can someone expalin this?

RE: Calculating motor winding temperature

While RTD's may be slow to respond to transient overcurrents (since they rely on transfer of heat from winding copper by conduction, which involves time factor), they are time tested and accepted winding protection as per all international standards.

RE: Calculating motor winding temperature

Comment on charlierod (Electrical) May 7, 2004 marked ///\\\
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

(OP)
jbartos:
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

Interesting way to measure resistance on DC motors.  My question, even if you turn off the power to the motor there can be a residual magnetic field from the saturated iron.  This would produce an emf in the rotating armature wires.  How can you measure resistance with this unknown emf in the coils?

RE: Calculating motor winding temperature

Bob - residual magnetism should not affect a dc measurement.

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RE: Calculating motor winding temperature

The temperature rise of a motor is a function of the generated losses, the motor mass and the heat removed by the cooling system; you need a model that includes these factors to calculate it.

RE: Calculating motor winding temperature

The output from the armature leads due to the residual magnetism is actually DC. The motor we use is a universal motor (AC changed to DC before I can get at it at the lead wires on the brushes).  

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

Bob  - It sounds like maybe you want to measure resistance while rotating? In that case you may see dc voltages as you say. I don't understand any reason for measuring resistance while rotating. While stationary there should be no reason for residual magnetism to interfere with dc resistance measurment.

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RE: Calculating motor winding temperature

Yes, I want to measure it while the rotor is rotating. The motor powers a winch with a limited duty cycle.  Some users overload it or run it too long.  My hope was to use the triac to turn off the power to the motor for a cycle and make the measurement, then power it back up again.  If done quick enough the user wouldn't notice the power being off for the 15 milliseconds or so.

RE: Calculating motor winding temperature

Hello BobM2

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

Depending on the need for a fast response to overheating, I would take another look at RTD's, thermistors or bimetallic switches which can be purchased to respond to the maximum temperature you want to winding to reach.  This temperature can be below the temperature rating of the insulation system to allow for a hot spot.

It seems like a much simpler solution than attempting to measure the resistance on a motor while running.

Ed

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