Motor stand still heating 1

[WTRVS]

When a squirrel cage motor is used at locations with high humidity (outside or maritime environment), most of them have a standstill heater inside. However I've seen motors without a built-in standstill heating but they where heated by automatically applying 24V to 2 phases when the motor is stopped. That means that there is always a small current running through the windings that causes the motor to heat up slightly.

I was wondering if this method can affect (reduce) the life expectations of the motor by heating always the same winding? Or is this method completely harmless? I'm talking about motors < 20kW.

Thanks!
WVS

 

[electricpete]

I think most likely any reduction in life from thermal aging is negligible and in humid areas there probably will be improvement in life (including mechanical components).

Remember the Arhennius curve - aging rate doubles for every 10C change in temperature.
Let's say running temperature is 100C.
Let's say ambient temperature is 30C.
Let's say space heaters keep winding at 40C.

Then with space heaters on, the thermal aging rate is roughly 2^6 = 64 times less than while running.
With space heaters off (at ambient temperature), the thermal aging rate is roughly 2^7=128 lower than while running, but there are other aging mechanisms introduced by allowing moisture into the motor. I'll take the first option (space heaters) any day.

A small complication is that we typically deal with average winding temperatures. The space heater may create a little bit of a local hot spot. But copper is a good conductor so it is generally not severe. Placement and type of space heaters can be important in limiting local hotspots also.

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(2B)+(2B)' ?

 

[electricpete]

A small complication is that we typically deal with average winding temperatures. The space heater may create a little bit of a local hot spot. But copper is a good conductor so it is generally not severe. Placement and type of space heaters can be important in limiting local hotspots also.

This complication is not relevant to your heating method which uses the winding itself.

Some would argue this is overkill for small motor, but I think it will certainly help the reliability.

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(2B)+(2B)' ?

 

[WTRVS]

Thanks for your response, Pete. I just wasn't sure if it could reduce life of the winding insulation.

Kind regards

 

[powerjunx]

wrtvs,
> most marine application motors has been using winding heaters thru 2 phase with 12-15% of motor voltage with current not more than 50% of rated current as per on board applications which was approved by classification society (ex. ABS). i don't think i reduces the insulation since the winding temp is just below the rated temp.
> Perhaps mvp members had a good theorethical explanations about this issues since neither can't i find a reference except the OEM.



"..the more, the merrier" Genghis Khan

 

[electricpete]

I'm guessing 50% is on the high side.

The limiting current during standstill is of course much less than the current during run due to lack of cooling.

I guess the choice of current is limited by available voltage selection unless you have a dedicated transformer and adjust the taps to what you want.

Here's what the EASA Large Electric Motor handbook has to say.

The current required to maintain a 100 C rise in the windings is approximately twenty five 25% of FLA. The voltage required to achieve this amperage is a function of the motor's single phase impedance, and therefore, will vary from motor to motor. Generally speaking, however, this voltage will vary from 7 - 10% of nameplate volts.

Although they don't provide any discussion of whether this comes from empirical or theory (does anyone know?), we can roughly derive that same thumbrule (7-10% voltage to achieve 25% current) using simple theory:
The impedance of one leg of the wye is locked rotor impedance ZLR = 1/6
The impedance of two legs is twice that or 2*ZLR = 1/3 p.u.
We want I = 1/4 p.u.
Therefore
V = I * Z = I*2ZLR = (1/4)*(1/3) = 1/12 = 8.5%.
That is smack in the middle of the suggested band 7-10%.

There is at least one error in that thought process - locked rotor impedance includes saturation which is not present during trickle heating.

This assumes wye winding. Target voltage to achieve a given currrent (like 25%) would be lower with delta and would induced high current in one phase and lower in the other two.

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(2B)+(2B)' ?

 

[electricpete]

Correction

100 C rise

should've been:

10 C rise

The target winding temperature is 10C above ambient per some older IEEE standards.

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(2B)+(2B)' ?

 

[electricpete]

The impedance of one leg of the wye is locked rotor impedance ZLR = 1/6

Actually, I think I used that impedance incorrrectly.
1/6 is the typical ratio rated phase-to-phase voltage over locked rotor current.
But we are interested an impedance relating phase-to-neutral voltage to locked rotor current.
So the impedance we are interested in would be (1/6)/sqrt(3) and the logic above would lead to V = I*Z = (1/4)*(1/3)/sqrt(3)~5%.
Additionally the unusual flux pattern associated with applying single phase voltage accross two terminals of the wye makes it questionable to use any three-phase-derived locked rotor impedance to begin with. Empirical approach is probably better.


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(2B)+(2B)' ?

 

[powerjunx]

well pete had a point.. rule of thumb works always in the field!

Though i don't have EASA reference, a 7-10% nameplate voltage and 10 Deg C rise would mean operational (running condition) and equal or more than the ambient temperature (usually 45C)...

As my best guess, as referred to OEM, 12-15% nameplate voltage and below 50% FLA were acceptable consideration and had compensated ambient temp below zero while on board ship (especially ice class ships) or any ship sailing regions with below 20 deg C.


"..the more, the merrier" Genghis Khan