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Single phase induction motor starting problem



 
 
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  #1  
Old May 17th 11, 02:52 PM posted to uk.d-i-y
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Posts: 1,616
Default Single phase induction motor starting problem

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
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  #2  
Old May 17th 11, 03:42 PM posted to uk.d-i-y
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Posts: 17,440
Default Single phase induction motor starting problem

robgraham wrote:
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


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


and its a shot to not very much cost wise to see..
  #3  
Old May 17th 11, 10:23 PM posted to uk.d-i-y
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Posts: 565
Default Single phase induction motor starting problem


"robgraham" wrote in message
...
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.




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.


  #4  
Old May 17th 11, 10:42 PM posted to uk.d-i-y
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Posts: 1,616
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
  #5  
Old May 18th 11, 12:20 PM posted to uk.d-i-y
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Posts: 1,616
Default Single phase induction motor starting problem

On May 17, 2:52*pm, robgraham wrote:
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


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
  #6  
Old May 18th 11, 12:49 PM posted to uk.d-i-y
external usenet poster
 
Posts: 5,146
Default Single phase induction motor starting problem

On May 18, 12:20*pm, robgraham wrote:

Further guidance now sought - I've got the capacitor off, but am
puzzled by its working voltage of 125vac (capacitance of 200mfd). *


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?
  #7  
Old May 18th 11, 05:48 PM posted to uk.d-i-y
external usenet poster
 
Posts: 1,616
Default Single phase induction motor starting problem

On May 18, 12:49*pm, Andy Dingley wrote:
On May 18, 12:20*pm, robgraham wrote:

Further guidance now sought - I've got the capacitor off, but am
puzzled by its working voltage of 125vac (capacitance of 200mfd). *


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?


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
  #8  
Old May 21st 11, 08:26 AM posted to uk.d-i-y
external usenet poster
 
Posts: 1,616
Default Single phase induction motor starting problem

On May 18, 5:48*pm, robgraham wrote:
On May 18, 12:49*pm, Andy Dingley wrote:









On May 18, 12:20*pm, robgraham wrote:


Further guidance now sought - I've got the capacitor off, but am
puzzled by its working voltage of 125vac (capacitance of 200mfd). *


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?


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


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
 




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