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robgraham robgraham is offline
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Default Single phase induction motor starting problem

On May 17, 6:53*pm, (Andrew Gabriel)
wrote:
In article ,
* * * * robgraham writes:









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 ?


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).

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.


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.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]


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