single phase AC Motor burnt

Hi Everyone !!

I was witnessing somebody working on a single phase AC Motor. it had only one capacitor in it. So i assume it was either capacitor start induction motor or permanent split capacitor motor.

Now the problem in this motor was that when we switched it on, little smoke started coming out of it and it showed no sign of motion.

upon checking, one technitian found out that the capacitance value was 13.5 MFD. However it was marked as 20 mfd. In his opinion this lesser valued capacitor had caused the motor winding to be burnt up.

What had happened actually that the motor switch at first used to cause some problems in switching the motor on/off. After sometime, it stopped working at all. At this point of time, one technician removed the switch and made the contacts directly. For one time it worked OK and for the next time, this smoke started coming out of the motor.

Now, in my opinion, it is not the problem of the capacitor. because i think capacitor's low value will cause the motor to run slowly at first (pretty notably) and then at some point of time may cause the motor to overheat and burn. (not what i have described to you)

so please tell me whose deduction is right ???

Motor capacitors have often +/- 20% tolerance. 13.5 µF won't be far from specified value in this case. Starting torque will lower with the low capacitor nevertheless and can cause problems, depending on the (unsaid) motor application.

Motor windings burn for different reasons. Mechanical overload/stall is probably most common, a manufacturing defect like bad winding insulation should be considered, too.

I don't understand exactly the "on/off switching problem point". Are you talking about a defective switch? Or is it that the motor didn't spin-up correctly? A stalled motor can easily cause a burned winding, either if it's due to a mechanical defect or a too small motor capacitor.

capacitors? Shower pumps use single phase AC motors and they don't have capacitors with them, other than a few nF for HF filtering.

Capacitor start motors cannot require to have its start winding to be de-energized after a few moments. Otherwise they will burn.

There is a centrifugal switch which accomplishes this. If the switch is faulty, or if your motor does not reach proper speed, the motor may be damaged.