Single-Phase Motor with Burning Capacitor

I doubt if this is the original capacitor that was fitted to the motor.

Capacitors are fitted to single phase motors for different reasons , but mainly to provide the starting torque and direction. With the capacitor fitted in series with the start winding it puts the start winding 90 degrees out of phase with the main winding providing the motor with a starting direction.

A singe phase induction motor without a starting winding will not run as it does not know which direction to turn.

It is common for the capacitor start winding to have a centrifugal switch which cuts out the starting winding and capacitor when the motor gets up to speed, thereby running on the main winding only. The overload is set for, and protecting the main winding and not the start winding.

If the motor has a centrifugal switch it may not be functioning correctly and keeping the start winding in circuit.

There are also capacitor start - capacitor run motors where the capacitor is kept in circuit all the time.

Without more information on the motor it is likely to be one of the following

Its a capacitor start - capacitor run and the switch is not cutting out due to being faulty or the motor not getting up to to speed during starting.

It is this wrong capacitor for the motor

There is a fault in its associated winding drawing excess current.

Note: There are two different types of motor capacitors, i.e. starting and running. The starting type can be electrolytic and are not designed to be energized continuously. (More capacity in a smaller can) The current to them is turned off by the starting switch after the motor comes up to speed.

Running capacitors (used on capacitor run motors) are different in manufacture and are designed for continuous operation. They are often oil filled paper or mylar insulated.

Most single phase motors use the capacitors only for starting. You can generally hear the switch click as it starts, however, It is not uncommon to have a single phase capacitor run motor. They are a little more efficient and have a better torque and power factor characteristic.

A couple of years ago we had a bad batch of motors used in pumps. The centrifugal switch was sticking and was eventally modified but caused us a lot of problems replacing dozens of motors. I would check this component first.

I think the motors were Compton Parkinson but may be wrong.

My memory may be faulty, but it seem to recall, that if the centrifugal switch does not open and cut out the starting winding (on capacitor start motors) the motor will not be allowed to come up to full speed. This would cause the motor to draw excessive amperage. Since the OL is not tripping, it would seem to me that the switch is operating as it should. If the motor is not coming up to speed relatively quickly (around 5 seconds or less) this could cause the start capacitor to fail. Low voltage (less than 10% of rated voltage) could cause slow starting and again the capacitor fails. If it is burning, man that thing must be in there a long time. Have you taken voltage readings at the pump when it is starting? That could be key. But if it is not coming up to speed, why isn't the OL tripping? I just read that capicitor manufacturers recommend no more than 20 three-second starts per hour or replace it with a higher voltage capacitor. Does your pump cycle a lot?

Almost certainly you have a centrifugal switch problem as most single phase motors ONLY use the cap for starting. You will need to place an AC ammeter in series with the cap to see if the current flows through the cap when the motor is running (once up to speed). It is usually out of circuit within 5 seconds at the most 10, if not sooner!

You could add a switch to remove the cap once running if cost is a problem, but do not forget to switch it back on before starting the next time!!!

Otherwise you will need to replace the whole motor as usually the switch is not easily accessible.....

Very few motors keep the cap in circuit (for extra torque) as a 3 phase motor does a much better job with regards to torque and is self starting....

The only other possibility is that the cap is being affected by high voltage transients for some other source, then I would recommend a cap with 1000V working, but of course the same value as before, it will be much larger and may be a problem to fit.....you did not mention your supply voltage......!!!

Another possibility is that the capacitor is a substitute polarised one, and not the un-polarised one required for this application.

Personal experience recalls a polarised electrolytic capacitor connected the wrong way round across a DC supply. The capacitor exploded, spraying the area with white fibrous strands fortunately without injury on that occasion.

It's time to check thoroughly before an injury occurs.

If it was a polarised cap, my experience is that they live about 1 second on an AC supply......that is not what the Blogger implied.....

• Another possibility is that the <...starter capacitor...> has become inadvertently connected to the main winding instead of the starter winding.
• Another possibility is that, if there is a starter switch, it didn't open after getting the motor away from rest.