Calculating motor winding temperature 4

[charlierod]

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

 

[edison123]

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.

 

[nbucska]

Use the coil resistance to measure the temperature

<nbucska@pcperipherals.com>

 

[OperaHouse]

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.

 

[aolalde]

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.

 

[charlierod]

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?

 

[nbucska]

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

<nbucska@pcperipherals.com>

 

[edison123]

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.

 

[blacksea]

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).

 

[charlierod]

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

 

[edison123]

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.

 

[nbucska]

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>

 

[nbucska]

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>

 

[charlierod]

nbucska:

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

 

[nbucska]

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>

 

[jbartos]

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

 

[charlierod]

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.

 

[electricpete]

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

http://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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[busbar]

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

 

[electricpete]

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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