Difficulties with motor overheating

[Heretic]

Hello everybody

I am seeking some advice for a problem I am having with a Motor Control circuit that we are trying to make compatible with 240V.

The problem is that the windings are getting hotter and tripping the overheat circuit earlier than the 110V we are modelling our design from.

The motor is a universal type, used in a food processing machine and is rated at about 1400 Watts, max 1600RPM. I am driving it through a MOC3023 Non-Zero Crossing optoisolator which in turn triggers two BTA225 triacs (not SCRs for various reasons) with diodes at the triac gates so each one triggers on alternating half cycles, to distribute the heat between them.

I have a snubber circuit across the triacs consisting of a 0.47uF 630V capacitor in series with a 330R resistor. The MOC3023 does not have any independent snubbing circuit.

My questions are these:
1) Could the overheating problem relate to the values in the snubber circuit?
2) I am also using a common mode choke and X2 capacitor at the mains input to provide EMC suppression. Could this be causing harmonics that are creating losses in the windings?
3) Are there any other reasons that could be causing this overheating?

Any replies would be extremely appreciated. I am under time constraints to get this working efficiently.

Thanks very much

 

[Marke]

Hello Heretic

I take it from this that you are controlling each half of the AC waveform independantly. The action of the triacs is to introduce harmonics into the load. This will result in changes in the losses and characteristics. Have you checked to ensure that you have an effective RMS voltage applied to the load that is equal to 110 Volts? Have you ensured that this voltage stays constant, independant of load and motor speed? You may find that the voltage is changing as the motor changes in speed and that this is causing your problem.
Best regards, Mark Empson

http://www.lmphotonics.com

 

[don01]

heretic,
marke is pointing to the problem so I will not add much to it(can't?). You are chopping the ac wave form and this will cause heat rise and noise in a universal motor (single phase I assume) as Marke has highlighted your rms may not be true as well.

We get lumbered with a lot of gear direct from Japan (head office) and it's all 100v / 220 v 3phase. (we're 240V 415 3 phase here) I've now found the best way is to cosy up wih our favourite motor rewinder and get him to supply tranformers to suit and boy does it make life lot easier. (and the cost isn't that bad once you work it out)

Regards
Don

 

[Heretic]

Thank you both for replying so promptly. I appreciate the time spent to add your comments.

I perhaps did not explain sufficiently that the original design is a 110V winding motor and the one I am trying to achieve compatibility with is 240V. I am trying to make the same controller circuit robust and flexible enough to be used on 110V with a 110V motor, or 240V with a 240V motor.

My concern was that the harmonics you mentioned as a byproduct of chopping the sine wave to get variable speed, are being lost in heat in the windings due to poor snubber design on my part, or some resonance in the filter components.

Is it likely in your experience that a snubber improperly designed would cause poor performance? On an oscilloscope I have noticed some voltage overshoot / ringing(?) at about the voltage zero cross point. I am not sure how tidy this signal should look.

I also read an article stating that the triac optocoupler should be snubbed independently of the triac itself. I am not sure how this would affect my temperature problem.

Once again, any comments would be much appreciated.

 

[cbarn24050]

Hi, the reason is that you are measuring your voltage with an ordinary voltmeter, these are calibrated to a sine wave and will read low. Get yourself a true rms meter.

 

[jbartos]

Suggestion: Normally, this kind of projects is completed by a whole electrical department, depending on the level of detail/accuracy. There is supposed to be performed computer modeling of the Motor Controls. The modeling should include the thermal analysis/calculations.
The voltage overshoots (ringing) of oscillatory nature is caused by fast switching, IGBT devices.