Run Capacitor for 240v Motor

[SurreyHills]

I've been given an old 370w 240v motor that has been stored for a few years. On starting it up the the motor quickly got up to speed but then the capacitor started to smoke and the motor stopped pretty quickly. So I'm trying to work out what replacement I need - the current one is rated at 250v 15uF. Do I need to replace it with like-for-like or would 450v one work as they seem to be more common.

 

[9fingers]

You might find the centrifugal switch has stuck shut (if it has one) normally 250v capacitors are used for short times during starting alone.
450v ones are usually for continuous use.
All depends on the type of motor and whether your 15uF @250v was the correct original part or one substituted incorrectly.

Maybe post a photo including the rating plate?

Bob

 

[CHJ]

The higher the working voltage the better, but some caution is needed to decide what exactly caused the capacitor to fail.

If in doubt make sure you purchase a Run Rated component as opposed to start rated which are designed for a short cycle working.


You need to establish if the motor is a simple capacitor start/run motor where the capacitor stays in circuit all the time, or a motor fitted with a mechanical centrifugal switch that disconnects the start capacitor once running.

Yours is most likely the former judging by the size (15mfd) of the capacitor.

Scenarios:
The capacitor was defective because it had been in storage and unused for a long time.
The mechanical switch is stuck due to rust, dirt etc. and is causing the capacitor to overheat because it is not being disconnected.

See Bobs (9fingers) detailed information on Induction Motors


Edit: Bob beat me to it.

 

[SurreyHills]

Thanks for the replies.

Unfortunately there is no rating plate on the motor. Having rechecked it's a 275v start capacitor rated at 15uF manufactured by Hunts. I'll check with the chap who gave me the motor to see if he can remember whether the capacitor is the original.

 

[Digit]

As Bob says, that is most likely a start up capacitor, and if the motor is of an age it would have a mechanical starter switch, and they can often be heard to 'click' in and out.
Did you here any such?

Roy.

 

[9fingers]

1/2 HP (375w) is at the powwer level when there are all sorts of configurations of single phase motor. Older ones with switches and newer ones without, repusion start, the lot.
Mind you it seems more and more modern motors of quite high powers are capacitor run types. Someone bought one into the workshop just today for a repair and re-fitting of the magic smoke that kept leaking out and that was 3 hp and capacitor run/no starter switch.
This time it was the brake giving trouble and once I had the parts free and de-rusted - needing persuasion with a blow torch - it just wanted the brake gap setting and away it ran like a good-un.

Bob

 

[Digit]

If it is an old 'un Bob I'd assume a mechanical starter switch then play safe and look inside before there's an 'orrible smell!

Roy.

 

[SurreyHills]

Further to my post yesterday I have had the motor going today. Unlike yesterday when the capacitor smoked, got hot and then the motor stopped. Today the motor runs, albeit slowly, but it trips the cut-out on my extension cable. When plugged directly into the mains circuit the motor runs, still slowly, and the plug gets very hot.

Is it the capacitor or should I bin it?

 

[Digit]

Sounds like it might be stuck in 'Start' mode as opposed to 'Run'.
Which brings us back again to any starter switch, I would remove the end cover and check for a mechaical starter switch, if it has one it is probably stuck.

Roy.

 

[9fingers]

The capacitor is almost certainly bu88ered once is has let any smoke out but just fitting another without investigation is asking for more trouble.

Open up the motor and see if it has a centrifugal switch in it and make sure the contacts are in good nick and that it operates smoothly. DO NOT LUBRICATE it. This will only cause it to attract dirt in the future.

If it has a switch and that is working, disconnect the capacitor completely and using a piece of string wound round the shaft, start it spinning and then switch on the power. It should run at full speed. repeat with the string would the other way round. Again it should run at full speed but in the other direction. As it slows down listen for a click as the switch cuts in and possibly a rubbing noise as it comes to a stop. This is all good and the motor is possibly worth buying a new capacitor for (ebay possibly best source) a motor start capacitor should be ok to use.

If there is no switch, the running tests above should work but you need to buy a motor run capacitor rated at 400-450 volts - again ebay is the best source.

These should only cost you a few quid.

If the motor wont run at full speed when given persuasion then there could well be something else wrong. Unless there is something special about this motor that you need - like non standard shaft or special mounting then consign it to be a doorstop and buy a new one from ebay.

hth
Bob

 

[Digit]

And make sure you can let go of the string!

Roy.

 

[9fingers]

And make sure you can let go of the string!

Roy.

:lol: :lol:

Thats why I said get it spinning and then switch on - hopefully unless the OP is an octopus, he should not have a problem!

Bob

 

[Digit]

With the smaller stuff I always held them down with my foot then pulled, but i saw a sparks at Vauxhall motors years ago Bob who pulled and switched on.
The motor rotated on its feet towards him, the string tangled and gave him a very nasty cut across the palm of his hand.
Like you say, make sure the string is off the shaft before turning on, don't rely on some one else on the switch either

Roy.

 

[SurreyHills]

The string start works - in both directions. The motor runs very smoothly and at full speed, when switched off I can hear the centrifugal switch click off.

So does this mean I should get a start capacitor rated at 250v?

 

[9fingers]

Have you opened it up and made sure the contact is actually opening??

Without this check, it is potentially risky just fitting a new capacitor as you could kill it straight away.

If the switch is working electrically then yes a starting type( rated for very short term use only) is the correct type to use.
There will be no harm done using a run capacitor but it will be bigger and may not fit the enclosure (if there is one).

Bob

 

[Digit]

(if there is one).

I got caught out on that one a few weeks ago Bob. The capacitor on my metal lathe self destructed shortly after i purchased it. Rather than bother Axminster I used one from stock, which would not fit in the cover, so I strapped the new one into place.
Couple of weeks ago I was up to my navel in Aluminum shavings when I had a power cut. Yep! A shaving across the terminals!

Roy.

 

[9fingers]

Been there too Roy! I'm gradually moving over to TEFC motors. The lathe and the mill are 3 phase TEFC with inverter control.
The old Pollard drill motor is about 8' off the floor so not too much swarf at that height. It has a home brew rotary converter and the motor for that is on the floor under the bench but is also TEFC.
The power hacksaw still has an open motor but that motor is right on the back and lives under the bench so pretty swarf safe.

The wood workshop is less of an issue and I stick the airline into the motors now and then.

Cheers

Bob

 

[Digit]

I stick the airline into the motors now and then.

A greatly underated tool IMO Bob. I have three, one in th woodwork shop, a portable for use on cars and bikes and another that sits outside the utility room that my wife uses to clean vacuum cleaner filters etc.

Roy.

 

[9fingers]

I can only run to two Roy and one of those has a split hose in an almost inaccessible place behind racking in the garage.
Sorting out that is on the round-2-it list but a long way from the top as the other compressor is on wheels and gets carted around as needed for all sorts of clean up jobs including cleaning computers and their power supplies.

Bob

 

[SurreyHills]

Finally opened up the case and the conatct is opening up and the switch is an electrical one. As there is no enclosure I will go for the run type as it seems easier to source.