Single phase induction motor start up problem

[OldWood]

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 diagnosis? 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

 

[9fingers]

If there was genuinely no other load on the 32A circuit, I can't see a 650watt motor even allowing for start up surges ever tripping a fully functioning breaker.
These combined RCD MCB devices are the work of the devil when it comes to diagnosing faults
Do a proper earth leakage test on the machine first or if you do not have the equipment to do this, temporarily remove the earth lead in the plug, connect to the 32a circuit and without touching the metalwork of the machine, switch on and off a few times and see if it still trips.

If the nuisance tripping stops then you possibly have a earth leakage fault on the machine. This can be confirmed if you can measure a significant AC current between mains earth and the body of the machine when it is running and the circuit trips once the motor has started.
If the tripping continues, then you might have a motor starting fault. This all assumes that the RCBO is working properly and has no other partial earth leaks making it 'trip happy' anyway.
Warnings!
Do remember to re-fit the earth lead
Don't let other effing safety types see you doing this test
I did not suggest this test - OK? :lol:

WARNING TO OTHER READERS. ONLY DO THIS SORT OF TEST IF YOU UNDERSTAND WHAT THE CONSEQUENCES MIGHT BE! I am only suggesting this to the OP as he has some electrical understanding



Bob

 

[OldWood]

Hi Bob - I was hoping you would be around somewhere ! I was thinking of PM'ing you but experiences of this type are well worth sharing.

Many thanks for the final comment !! "Electrical understanding", as all of us who have *some*, can get you into trouble on occasions. :shock: (title of this Smilie is 'Shocked' - must use it more often in electrical posts !! :lol: )

Perhaps I didn't make it fully clear in the post that I had taken the machine, fortunately on castors, to two other sources of mains power, both on different C32 MCB's and with separate RCD's, and on both the MCB tripped rather than the RCD. So in my mind it's clearly a current overload problem.

I've now got it in pieces and am satisfied that this is an electrical problem rather than an overload due to a mechanical one. I've investigated the centrifugal switch and it seems fine - at least there's no obvious fault with it and it moves freely. On the basis that the start capacitor is the cheap option, I'm currently blaming it.

I'm now back home with the start capacitor in front of me and could get a replacement off Ebay though I will need to do a double check on the dimensions as the case on the motor doesn't have much spare space. One thing that does puzzle me is that the capacitor (200mfd) is 125vac - odd. I'm wondering if I would be better to get a 250vac one and figure out a way of mounting it.

Another thought did cross my mind and that is what trips first if there is a hard ground fault, the RCD or the MCB ? My logic says that the RCD will have a designed in 30msec delay, whereas if the current is high enough the magnetic part of the MCB will fire first.

Rob

Rob

 

[9fingers]

Hi Rob,

Apologies for the confusion on my part. 125Vac are sometimes found esp in Chinese motors. They have one end of the starter winding returned to a tap on the run winding. So during starting the run winding acts as an auto transformer dropping the voltage seen by the capacitor and enabling the start winding to be smaller than a 240Vac one. To achieve the required phase shift with a smaller winding, the capacitor has to be higher in value but cost of this is offset by the lower operating voltage and often by using a AC electrolytic type of capacitor. It all shaves off a bit more cost but generally less reliable. I have this arrangement on my table saw and whilst mine is fine, a forum friend Philly (he of Philly planes fame) has had a failure and brought me the motor to look at and the capacitor was virtually open circuit.

Buying a 125v replacement will likely be a lot cheaper and it is the luck of the draw how long it lasts.

hth

Bob

 

[OldWood]

Thanks again for your help, Bob - that explains the voltage rating, and agrees with Ebay in that there is a 2:1 price difference. Having said, if I can get the higher voltage one fitted I think I will go with that - I could use the excuse that it isn't my money but for all the effort of dismantlement, etc. I would like to think that it would be worth it. Sod's Law of course will no doubt prove that it wasn't the capacitor.

On the basis of crude estimates with the capacitor showing a nominal 16 ohm reactance and a guess at 4 for the coils, then one could say that something like an instant 12.5 amps at start up, but if the capacitor broke down this would be 60+ amps and hence the trips - gives me some justification for spending the club's money !! If I'm right then I'll get the pat on the head, if I'm wrong then £15 is small beer in comparison with a new motor or machine.

Rob

 

[9fingers]

Capacitors very rarely fail short circuit. The trip mechanism you are seeing is almost certainly the excessive current drawn by the run winding under stall/stationary rotor conditions.

Bob

 

[OldWood]

Capacitors very rarely fail short circuit. The trip mechanism you are seeing is almost certainly the excessive current drawn by the run winding under stall/stationary rotor conditions.

Bob

Any suggestions as to why after 3 years this is occurring ? The only possibilities that exist then is failing windings or mechanical overload, and I'm comfortable that it isn't the latter, though I could go the extra mile and disconnect all the oscillating gearbox from the motor shaft.

The option of OC capacitor I'm tending to discount as the motor starts perfectly well, and why should an OC cap. be random and cause motor overload ? Yes, it would mean the motor doesn't want to start because the field isn't rotating, but in fact even when it trips, the motor does the initial run up as if going to run properly and then dies after a couple of seconds - ie about 3 or 4 seconds total start to stop rotating.

Rob

 

[9fingers]

Have you measured the capacitance of the current one? if the total start to trip to stop cycle is only 4 seconds I would suggest that the motor is not reaching synchronous speed and so the current drawn is high. You could also have a defective run winding.

With fast reacting breakers, it is very difficult to see what is going on for sure. Can you run it up on a circuit with wired fuses and then measure running current and any undesirable temperature rise?

Bob

 

[OldWood]

Thanks for keeping going on this Bob - I tried measuring the capacitance with my 3 1/2 digit DVM, but it ran over scale. Its maximum is 200uF - I suppose that does indicate there is capacitance there at low power.

Doing current measurements, etc will require a 'bring home to the laboratory' so that I can have control properly on what's going on. For 'laboratory' read workshop - probably easiest anyway if I'm going to try another capacitor. That means clearing the bench of all it's shavings, etc. And then the next problem will be rigging my workshop supply so that its MCB's don't trip - interesting!

Rob

 

[9fingers]

Ok Understood Rob. keep me posted please - all info helps build the mental database!

Bob

 

[NetBlindPaul]

Rob,
The RCD section of the RCBO will not have a built in time delay unless it is an S type, which is very, very, unusual.
The RCD section will be instantaneous, it has to be as it is 30mA which is for additional protection normally.

BTW please just be careful these things can go wrong as easily as they can go right!

 

[OldWood]

Thanks Paul - warning is fully appreciated. I've brought the motor home so that I can do some measurements on it.

I did have an alarming phone discussion with Axminster's technical department this morning. The guy took the details, said he didn't know anything about this and put me on hold while he had a chat with the workshop. Seemingly this is a known, but rare, fault and that the Jet manufacturers themselves do not know the cause.

I'm leaving it up to the club to pursue this now with Axminster under the SOGA conditions as the machine is just 2 years old. I will try a replacement capacitor as something has clearly deteriorated within the last month or so - it blew its plug fuse about 4 weeks ago, but it was only a 5A one (and is now 10A); quite reasonable for that to fail, but it does raise the question at this point as to whether this was when the fault began to manifest itself.

Rob

 

[Eric The Viking]

Just a thought on testing the capacitor: can you put the possibly faulty one in series with a known-good one, the value of which you already know.

IIRC, caps in series have a net value of:

1 / (1/a+1/b)

Meaning you can measure it. But the multimeter measurements are rather crude compared to a proper capacitance bridge, and only at low voltage too - no help if it's voltage dependent.

Also the so-called AC electrolytics are actually two normal ones back-to-back. This is horrid, as electrolytics ideally need a very small polarising leakage current, to keep them well plated internally, and this can't work in that configuration. If it is the cap, I'd go for an ordinary one instead (polyester?) if you can.

Sounds like fun.

I've had three Chinese induction motors fail over the last year or so (in the workshop context). Only small ones - 1kW or less, reasonably new, and with and without centrifugal switches. I did wonder if there was a defective batch of wire around about three years ago - possibly insulation breakdown being the cause. In all cases they ran hot then failed.

It's hardly statistically valid data though. ;-)

Cheers,

E.

 

[OldWood]

Thanks Eric - that's actually quite useful information. I hadn't thought of capacitors in series (equivalent to resistors in parallel). I'll need to see if I can find a suitably large value one to make it work - will dig into the electronic components junk box, but may need to 'charge' up any one I find as they are well old now.

There's no way I can go anything but electrolytic on this one as the capacitance required is 200uF to compensate for what I suspect is rather a small starter coil. Your statistics are useful as the club is looking into whether they could have a SOGA claim against Axminster, though there is no evidence of this motor running hot but then if the fault is in the start winding it would cause the trip before it got that far.

Rob

 

[Eric The Viking]

Sorry Rob, I wasn't awake this morning -- 200 uF is fairly, um, big in that context!

I still think you're better off with another type if possible - you might struggle to find an 'AC' electrolytic of that voltage rating too. The only ones I've come across have been in cheap loudspeaker crossovers, working at around 100V tops.

Regarding Axminster, I'd talk to them first. I've always found them extremely helpful and keen to sort out problems, and I doubt you'd need to 'get heavy' with them, especially if it's a manufacturing defect. I also know they have good bench test facilities, and will probably know the internal parameters of the motors on their kit, for example the DC resistances and tolerance range of the windings, etc.

Hope it gets easily sorted, anyway.

E.

 

[OldWood]

Hi Eric
You'll see 9Fingers Bob's explanation of the starting arrangement of this motor near the beginning of the thread.

I've had a suggestion of dust on the centrifugal switch contacts, but having suffered that for many years on another motor I doubt it is that unless this motor has significantly different characteristics. I had hoped to disprove that today, but didn't get round to it.

Bob discourages me from thinking about the capacitor short circuiting, but an idea came out of a chance meeting this evening which could explain a breakdown. There has to be a fault of some type as this machine has run for 2 years without a problem. Because of the complication of the tapped main winding, it was wondered if there is a back emf off that winding that has opposite polarity to the incoming mains and therefore exceeds the breakdown voltage of the capacitor - this could depend on where on the voltage cycle the power was switched on. It sounds sufficiently plausible that I am going to go ahead and buy in a 250vac capacitor and hope I can find some way of mechanically fitting it.

Rob

 

[Tusses]

I dont know if this will help or not, but ..

When I 1st started to get 'proper' machines , TS BS and planer, I would run them off an extension lead and normal 13a plugs. I found that this arrangement would often trip the main box on start up ,and sometimes blow the fuse in the plug too.

I made up a beefy lead with a 16a socket on and wired it into its own breaker on the box, and added 16a plugs to these machines.

I've not had the tripping problem since.

My theory was that the voltage drops through the extension lead and 13a plugs were causing the problem. Proper wiring has solved it. Any of you 'experts' reading know if I am right ?

So, have you had any changes to the wiring system around the time your tripping problem started ?

Rich

 

[9fingers]

Rich,
One poster here a few months back found that introducing an extension lead seemed like a method of preventing trips with his bandsaw.
However I have yet to find someone reporting a proper installation of 16amp sockets and suitable breakers (ideally type C if the earth impedance is good enough) ever giving problems.

Bob

 

[OldWood]

Interesting thought but somehow seems a bit of overkill for something with a 650W motor and driving a load with very little inertia.

Bob - any thoughts on the idea of odd voltages occurring ?

Rob

 

[9fingers]

I'd have thought that any such effects would be taken into account when designing the motor. There is no reason why the tap for the starter winding need be half way along the winding and the technique is widely used on imported motors.
You can soon see if your motor is wound this way by measuring the dc resistance between each pair of the three wires. The tap is normally buried in the windings. If you have four winding ends then it will be conventionally wound but then the capacitor is much larger than I would expect.
I have a clone of the small Jet sander so if time permits, I might have a look inside that to see what sort of motor arrangement is fitted.

Bob