Help with SP motor please

radiohead · Jul 14, 2020

I'd really appreciate some help with a single phase motor driving a mixing machine. Firstly let me state now I have little experience of motors but am trying to help out a client. The mixer mechanics were fully serviced by others with new bearings and runs smoothly. Before being serviced the 125 mf cap had burnt out, and after the service I was asked to replace the cap like for like to get it running. It burnt out again. A second new cap is clearly going the same way.
Question is are the caps wired correctly,(it's an old motor and has clearly been messed with) and if so what other reason might be the cause?
I have included a pic of the motor plate and a diagram of how the 80 and 125mf caps are currently connected.
Thanks in advance for any assistance.

westward10 · Jul 14, 2020

From the sizes that is the start capacitor and one of the most common causes of failure is the start sequence not disconnecting quick enough over a period of time.

radiohead · Jul 14, 2020

Ok, I attempted to work out how the existing connections were configured but the connection box was a rats nest. So I disconnected the whole lot, traced the start and run windings along with the centrifugal switch and reconnected, which was scary with my limited motor experience. It now starts and runs without the start cap cooking. So it must have been wrongly connected, I wonder whether the start cap was not connected through the centrifugal switch? Would that cause the cap to cook?

westward10 · Jul 14, 2020

Start caps are different to run caps in their construction. They obviously had it connected incorrectly so well done for correcting it. Leave a start cap connected for too long and it won't survive, they are not continuous duty.

pc1966 · Jul 14, 2020

Just as westward10 said - start capacitors are not rated for continuous use.

Generally motors caps in the 2uF or so range will be plastic film and low loss, they can run with a few amps all the time no problem.

But caps in the 10uF+ range are almost certainly non-polarised electrolytic types. They usually have a limited running current as their internal impedance can be in the fractional ohm range so they get hot with a few amps running for any length of time. When hot the lifetime can be in the 1000 hours or less region, that is much the same as Ye Olde Filament Lamp!