Single phase induction motor starting problem

[robgraham]

The wood working club has a bobbin sander made by Jet and bought from Axminster some 3 years ago. Recently it has been tripping the C32 RCBO on the ring it is on as it starts up. This is random. Many times it will work OK, but on the umpteenth time there's an initial spin of the motor and then the system trips - and of course if there's another machine running on the same ring, then the trips occur more frequently. You can then go for another series of starts before it trips again.

In order to eliminate the RCBO I wheeled the sander along to two other circuits, both on 32A MCB's (forgot to see if they were B or C but probably C in a machine environment), and both these circuits tripped similarly after a number of starts.

So it is the sander. I took it apart this morning thinking that it might be the oscillating mechanism needing maintenance, but that seems fine with plenty of grease and no dust.

So what's the collective's prognosis? The motor is single phase with an input power of 650W. Is it possible that the capacitor is on its way out - the hoped for option I suppose as that is cheap to replace ?

An interesting side effect of this ring circuit being on an RCBO is that I didn't know to begin with if this was an overload trip or a leakage trip - one down side of the RCD and MCB being combined.

Rob

[The Natural Philosopher]

Its very likely to be the capacitor - typical life 10-30 years.

and its a shot to not very much cost wise to see..

[Andrew Gabriel]

When it was working OK, how did the motor start? Up to speed in under half a second, or more like 3 or so seconds? (Trying to work out what sore of induction motor.) When it trips, has it made it up to full speed, or trips before it reaches full speed. Also, how long from switch-on to trip.

Could be the capacitor, the start winding switch (if it has one, hence question above), something locking the rotor (stiff bearings or some other mechanical failure).

Some trip the toggle half way for earth leakage, and all the way for overcurrent (or is it the otherway around, I forget). But for the ones which don't, this can be a bit of a pain.

[John Rumm]

Which suggests an overcurrent trip originally and not an earth leakage one.

Inrush current on induction motors is typically anything from five to nine times operating current. 650W - lets call it 1kw just in case its PF is on the low side, would draw say 4A, so to start it could take 20 -

36A. Neither should pose any problem on even a B32 breaker.

It could be the cap - depends on how it has died - normally if they just lose capacitance then it cases a failure to start and much humming - but I would not expect a breaker trip on that size motor.

[The Natural Philosopher]

could easily if its well loaded an the cap is not getting it up to speed fast enough. Inductions will start on less cap, but not as well.

[Rick Hughes]

90% possibility it's capacitor ... my cement mixer motor went the same way, I just went to a scrap yard and found one similar in Voltage & uF rating .... been working for last 11 years OK. eBay wasn't around then ... sure you would find one on line, but for things like this a local electrical rewind place usually has a scrap bin.

They are only used during start, but if they are gone .. or leaking they either won't self start or will trip the breaker.

[robgraham]

Thanks Andrew (and all others for your input) I reckon the starting load on the motor must be pretty light as it is just revolving a small diameter drum of sandpaper, so effectively zero inertia there, and a small gear box operating a mechanism raising a lowering the sandpaper drum - a little bit of inertia there but the gearing down is so large that again the inertia on the motor is small. Unless there's something in the gearbox I'm not seeing, it looked to me as if all the mechanical parts were fine. The start up under normal conditions is rapid - when it trips I reckon it doesn't make it to full speed, and the trip time is enough to make the motor revolve but run down almost immediately, say 3 seconds from switch on to stop. I had the system partially dismantled today - tomorrow I'll have a look and see if there's a centrifugal switch. I did work on a 3 hp capacitor start induction motor recently that didn't have switch. An interesting thought - if the fault was breakdown to ground, do I take it the C grade MCB would trip before the 30ms delayed RCB ? The regular feed is via an RCBO, but I tested it on two other circuits that are 32A MCB with a seperate RCD and it was the MCB that tripped.

Rob

[robgraham]

Further guidance now sought - I've got the capacitor off, but am puzzled by its working voltage of 125vac (capacitance of 200mfd). I can get one off Ebay that I think will fit the case, but wonder if I would be better to get a 250vac one and arrange a new fitting.

Comments please - and many thanks so far.

Rob

[Andy Dingley]

Jet tools - US spec electrics. We no longer get the toggle switch controls with no NVR, but we do still get vibration-sensitive magnetic latches that wear out in 6 months, motors that overheat at 50Hz and these capacitors that are under-specced for their actual use. Let alone the abyssmal standards of construction and reliable earth continuity on most US-built kit. A lot of US kit simply isn't safe on arrival

Working voltage on capacitors like this doesn't mean that they'll fail instantly at anything over this, it's more of a design working voltage to achieve an adequate life. As you've discovered, when used in Europe the result tends to be a functional unit on arrival, but with an inadequately short lifetime.

I suggest fixing it by replacing the capacitor - it's easy to find replacements from anyone who deals with power electrics or motors.

I'd also suggest a complaint to jet and to your retailer. This American electrical farce has gone on for far too long. China doesn't try to sell this sort of under-spec kit any more, why should the US get a free ride?

[robgraham]

Andy So far I'm impressed by the earthing on this machine and the quality of the build, NVR , etc. The reason that the capacitor is 125vac is, I'm told, nothing to do with US specced machines but is a cost cutting design where the start winding is tapped into the main winding such that the main winding acts as an autotransfomer and the start winding only sees 125v. The cost of making 125vac capacitors is considerably less than that of 250vac for the same capacitance (verified by the price per item on Ebay) and the greater capacitance required has a considerably less significance on the end price.

The problem may be the capacitor, but the large capacitance and the lower voltage does back up the info I am receiving. Nothing has convinced me as yet to be suspicious of the design or materials.

Rob

[robgraham]

A new idea has been put forward on this one and it would be interesting if anyone has any comments on it.

Bearing in mind that the starter winding is not a totally separate winding but tapped into the main winding auto-transformer wise, the possibility has been suggested that a back emf / induced voltage is appearing across the starter windings that is in the opposite phase to the mains voltage and therefore is adding to the voltage across the capacitor and leading to it's breakdown - the randomness of the motor current overload tripping could be explained by the point in the mains cycle that the machine is switched on.

Plausible ?

Rob