Motor Winding Temperature Measurement

[Sparky2003]

Can anyone explain in Detail the process for measuring motor winding tempertaure using the change in resistance method. I believe there is a formula to calculate same - hence if someone can provide and explain the formula and its parameters and also the process for taking readings.
Many Thanks in Advance for your help on this.

 

[jbartos]

Eng-tip:
A linear dependence of resistance of material (e.g. copper CU) on a temperature variable is often described by:
R2=R1[1+a(t2-t1)]...Eq.1
where
R1 is initial resistance @ temperature t1 in ohms
R2 is final resistance & temperature t2 in ohms
t1 is initial temperature in degrees dependent on dimension of a, e.g. °C
t2 is final temperature in degrees dependent on dimension of a, e.g. °C
a is a coefficient describing the material resistance increase per degree, e.g. °C
Now, t2 is sought by Eq.1 while t1 and R1 has to be known and measured first at certain ambient temperature, e.g. ta=20°C. R2 is measured at constant ambient temperature ta=const. e.g. 20°C
Eq. 1 may be modified from linear to nonlinear for more accurate measurement by adding extra terms.
A level of detail of the response is requested.

 

[edison123]

Sparky,

R2 = R1*(234.5+T2)/(234.5+T1)for copper winding.

R1 = Resistance of the winding at a temperature of T1 deg C
T2 = Temperature in deg C for which resistance R2 is sought to be known

The above temp is only the average temp of the winding and not the hot spot temp, which may be several degress higher. Hence, it is advisable to place RTD's around the winding to measure hot spot temperatures which determine the maximum operable winding temp.

A rough thumb rule for copper is 4% resistance increase for every ten degress C of temp rise.

Note : Motor windings have typically low resistances and hence accurate micro-ohmmeters should be used for correct resistance measurements.

 

[aolalde]

Select and tag a pair line to line winding terminals and measure resistance at ambient temperature (R1, T1).

Run your motor under desired load until surface temperature gets steady.

Stop your motor and measure resistance of the same two tagged terminals before 30 seconds (R2).

Calculate average winding temperature with formula given by Edison123.
T2= R2/R1 *(234.5 + T1) – 234.5

See detail procedure in IEEE Std 112 part 8.

 

[RUSO]

One way I do it is buy burying a J type thermal couple in the winding of a motor. If you have the equipment, this works okay. But the resistance method is the best. It gives the average heat of the coil. Where as the thermal couple kinda gives a localized temperature response.