Capacitor problems, advice please.

[t8hants]

I have Compton Parkinson 1/3HP motor where the capacitor has clearly failed.
I can start the motor turning by hand in either direction, but once it is spinning the speed is a fraction of the expected 1425rpm.
Is this a symptom of the same failed capacitor which looks ancient, or is there something else amiss as well?
Thanks Gareth








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[minilathe22]

I wonder if you remove the capacitor temporarily, whether the motor now runs at full speed? (with the jump start of course). If not then it implies there is more than just the capacitor at fault.

 

[Racers]

The cap is essential to the motor running at the correct speed.

Pete

 

[artie]

I thought my saw was down on power and, on testing the cap found it to be 16 uf instead of the quoted 20 uf . The guy who tested it called it a run capacitor, I asked him since I had always thought it a start cap. He said it played a part in running also.
I replaced it with a new one and it made no difference.

 

[MusicMan]

Some motors have separate start and run caps, some have one that combines the functions. I recently had to replace one on a bandsaw. At first I could make it run by jump starting it, but the speed and power were low. After replacing the cap it worked as new. But make sure you get one to the original specification.

 

[t8hants]

Some motors have separate start and run caps, some have one that combines the functions. I recently had to replace one on a bandsaw. At first I could make it run by jump starting it, but the speed and power were low. After replacing the cap it worked as new. But make sure you get one to the original specification.

That is part of the problem, there is nothing on the prehistoric thing to indicate what values it made to.

 

[porker]

There are tables to determine the rough values. The tolerance of older caps was pretty poor anyway with +/-20% not being uncommon. I would pick one that is close with a voltage rating that exceeds the voltage you are using.

Take a look here to calc the values for your motor link

 

[EddyCurrent]

If the centrifugal switch contacts are not making correctly when the motor is at standstill then the capacitor will not be in circuit.
It could also be one of the windings in the motor, they have a start and run winding.
The capacitor puts the start winding out of phase with the run winding and that's what makes the motor start in the correct direction.
We used to learn the word CIVIL as an aide memoire.
When it's capacitance (C) the current (I) leads the voltage (V)
When it's inductive (L) the current (I) lags the voltage (V)

 

[johnbs]

I have Compton Parkinson 1/3HP motor where the capacitor has clearly failed.
I can start the motor turning by hand in either direction, but once it is spinning the speed is a fraction of the expected 1425rpm.
Is this a symptom of the same failed capacitor which looks ancient, or is there something else amiss as well?
Thanks Gareth





Can you post a photo of the name-plate please.
John



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[t8hants]

Will do later this afternoon, if that helps.

 

[t8hants]

This the label on the motor

 

[guineafowl21]

That plate unfortunately doesn’t help much. Are you sure the capacitor markings aren’t just painted over?

For less than a tenner, think this is worth a punt:
https://www.amazon.co.uk/First4spares-Universal-Capacitor-Microfarad-Connector/dp/B00I5JNFE6?th=1
Get the 50uF one.

Assuming the cap isn’t a combined start/run (more than two terminals), this should do the trick.

 

[t8hants]

Luckily for me its a two cable capacitor, with some very ropy wiring, so could well do with a change, thank you for the info'

 

[EddyCurrent]

If it's a run capacitor it should be rated at 400V AC, if a start then around 250V AC

These are the ones from my Startrite bandsaw
Motor
Brook Crompton
REF HQD562TFHPCNADZ
H5A16752
FLA: 6.6
Watts: 1100
RPM: 2850

 

[t8hants]

I have ordered the one suggested by Guineafowl21, which hopefully will prove correct, but there are no markings on the original at all.

 

[EddyCurrent]

Assuming it is the capacitor at fault, the one you ordered looks like it well might do the job.

 

[Ollie78]

A small note.
I had a problem with a machine not working, I thought I checked everything, twice. So had the motor checked, changed the capacitor.
Turned out that the wiring inside the plug was good enough to indicate 240v but was holding on by a thread inside so could not provide any actual power to start it up.

Ollie.

 

[t8hants]

A small note.
I had a problem with a machine not working, I thought I checked everything, twice. So had the motor checked, changed the capacitor.
Turned out that the wiring inside the plug was good enough to indicate 240v but was holding on by a thread inside so could not provide any actual power to start it up.

Ollie.

An interesting incident Ollie, the motor will need rewiring anyway, as if it starts to turn normally I want to install it in my ML8, since the original smoked itself.

 

[t8hants]

The capacitors as recommended above have come, somehow I ordered two. However they are marked 'run' and have four terminals. The motor now runs at full speed provided I kick it off by hand. While striping off the old tin can capacitor I found the word 'start', presumably that is the type required, with two terminals as there are only two wires coming out of the old one. There didn't seem to be a combination of the four terminals that would act as a start only mode, so I presume I have to get rid of these new ones?

 

[guineafowl21]

The capacitors as recommended above have come, somehow I ordered two. However they are marked 'run' and have four terminals. The motor now runs at full speed provided I kick it off by hand. While striping off the old tin can capacitor I found the word 'start', presumably that is the type required, with two terminals as there are only two wires coming out of the old one. There didn't seem to be a combination of the four terminals that would act as a start only mode, so I presume I have to get rid of these new ones?

As far as I could see from the listing, the four terminals are really just two terminals doubled up.

A run cap is the same as a start cap, but usually lower value and rated for continuous duty. In other words, you should be able to use a run cap as a start cap, as you are doing.

So the cap you have should be fine. The next thing to try is adding the second cap in parallel with the first, which will double the effective uF value. If all works, you can either leave it like that or exchange for a 100uF one.

If not, next stage is the centrifugal switch in the motor - can you hear it click as the motor comes to a stop after running at full speed? Pull the back off the motor and examine the switch mechanism, wires and contacts.