Twin capacitor single phase motor, running but is it ok??

[lchris21]

hi all,
had to swap the motor over on my small lathe in the garage on the weekend.
had an old motor, which looked original finally die on me.
anyway, have had a spare brand new motor for it for a good few years, which I eventually found and fitted.
the lathe has a forward and reverse switch, with 2 x pair of cables coming out to connect to the motor.
the new motor has 2 x pair of terminals for CW or CCW rotation. Simples.......
I connect the pairs as indicated on the terminal box of the new motor, from the switch, but after a few minutes running one of the capcitors is getting hot, I mean starting to smoke
I have removed this capcitor and the motor runs fine, and reverses, no overheating and plenty of torque, no problem.............
so what's happened? do I need to replace the capacitor?
I'm no spark, but from what I've googled, there's a start and run system?? and one cuts out when it is running? how does this happen? theres no mechanical cut out I can see, unless its buried inside the motor frame??

any pointers? should I repalce the capacitor or risk damaging the motor, blowing it up?

cheers

 

[MattF]

There's usually a form of mechanical switch inside the unit for the start cap. It generally, on any motor I personally can recall seeing, consists of some form of weight(s) which breaks the connection due to centrifugal force.

 

[rosinante]

Centrifugal switch

H If the motor has a centrifugal switch you will usually hear a click as it makes on rundown,hth
Chris

 

[KemppiFrog]

H If the motor has a centrifugal switch you will usually hear a click as it makes on rundown,hth
Chris

Yes, but both my motors that use centrifugal start switches don't have capacitors. So??

 

[eil]

Any sort of getting hot and starting to smoke is not good on any electric motor, If the motor runs ok without this capacitor then why would the manufacturers put it there in the first place I wonder. Do you have any pics of your set up.

 

[the repair man]

sounds like caps are in wrong order, ie starting on the run cap and running on the starter cap, starter cap will smoke after 2 seconds use.
If it has a centrifugal switch, you can normally stick something in the back end of motor to push/pull it and make sure its free.

 

[the repair man]

Yes, but both my motors that use centrifugal start switches don't have capacitors. So??

That would only be the case with a 3 phase motor or a motor designed for an external speed controller, and the caps would be in the controller.
Or praps you have a missing cap!

 

[shenion]

That would only be the case with a 3 phase motor or a motor designed for an external speed controller, and the caps would be in the controller.
Or praps you have a missing cap!

Not so, there are universal motors (wound rotor) and resistive split-phase motors that do not have caps.

My lathe had a resistive split-phase motor; no caps. There quite common on older lathes and appliances.

 

[the repair man]

Not so, there are universal motors (wound rotor) and resistive split-phase motors that do not have caps.

My lathe had a resistive split-phase motor; no caps. There quite common on older lathes and appliances.

So what does the centrifuge switch?

Never mind, will look it up, I'm being lazzy

 

[shenion]

The centrifugal switch controls the start winding.

 

[the repair man]

The centrifugal switch controls the start winding.

Yeah just read it, remember them now..sorry

 

[lchris21]

got the capacitors, both of them sitting in front of me.
one is 200uF which on the motor connection diagram is designated "C" that was ok.
the one in question is a 40uF which on the motor is deignated as "Cst" (i'm assuming that would be the start one??)
an't see / hear any switching mechanism??
will get the capacitors checked and revisit the motor....

 

[Exuptoy]

The switching will probably be taken care of by the centrifugal switch once up to speed.

 

[rtbcomp]

If you can't hear the switch clicking it sounds like it may be jammed closed, thus keeping the start capacitor in circuit all the time and causing the start cap. to overheat, although that doesn't explain why the motor starts OK with the cap. removed.

 

[lchris21]

will see if i can find a switch somewhere? assuming it'll be under the fan cowling somewhere

 

[rtbcomp]

They're usually inside the motor, most likely behind the endplate where the connections are.

 

[eil]

Your start cap is the 200 uf and the run is the 40 uf one, your run cap is the one thats fried. The motor could still run but would be drawing more current. If you had them wired on the wrong windings I would have expected the start cap to pop.

 

[lchris21]

Your start cap is the 200 uf and the run is the 40 uf one, your run cap is the one thats fried. The motor could still run but would be drawing more current. If you had them wired on the wrong windings I would have expected the start cap to pop.

think I may have fried it myself, cant even remember where I had the thing connected now, it was late and -3 in the garage, thats my excuse anyway........

the lathe has a fwd / reverse rotary switch (which I cannot find any info on) with one pair coming out of each side of the switch. same switch below in tiny picture...........
so based on the connection plate below, and assuming I can salvage/replace the toasted capacitor.

how do I connect the motor up?
cheers

 

[shenion]

4 wires from the switch. 2 are just on/off and go to U1 and U2.

The other 2 from the reversing switch pair go to Z2 and V1.

 

[eil]

The 200uf cap between V2 and Z1 and your 40uf between V1 and Z1

That switch looks like one off a Boxford.