single phase motor advice

[bergie]

a customer has a table saw with 3 kw 17 amp single phase motor , capacitor start with centrifugal switch (18 years old hardly used)
it worked all the years no problem but lately every time you start it ,it trips the 20 amp breaker.
the motor was tested at the armature winders. no problem. starts easy when the belt is off. i installed a 20 amp slow curve breaker . doesnt trip ,but the 3 phase 45 amp main switch of the house now trips ( with everything else off)
the capacitor was replaced. no difference.
i suggested a new 3 phase motor ,but the mounting bracket is non standard and will be a mission to mount.
then i suggested a ac drive but electro mechanica dont sell them for single phase . they say it messes with the capacitor frequency.
a soft start they sell,but they cant tell me if it will work or not.
i'm surprised it didnt trip before as start up current will be high.
the mechanicals are all fine. blade spins easy.
any advice?

[AndyD]

A single phase motor usually has a start current somewhere in the region of 5 x the run current and a C-curve mcb will instantaneous trip at 5-10 times the rated current (In). If the 45A house breaker is tripping this suggests the motor is drawing way more than 85A (ish) expected start current so it's almost certain there's a motor fault.

I wouldn't install a soft start or a VFD on a single phase motor, neither are suitable as far as I know plus by installing some kind of drive you're treating the symptoms and not the cause which is usually not advisable.

I assume the rewinders would have tested the integrity of both windings and you've replaced the cap so that really only leaves the centrifugal start switch. This would be my best guess where the problem might lie.

[Justloadit]

I assume the rewinders would have tested the integrity of both windings and you've replaced the cap so that really only leaves the centrifugal start switch. This would be my best guess where the problem might lie.

The rewinders should have tested the centrifugal switch.
The motor being 3KW should also have a Run capacitor, some where around 40 or 50uF, which will definitely reduce the running current.

Did anyone test the rotor?
You need a growler to be able to do this test, but if there is a mechanical break in the rotor near the ends, it will definitely screw up with the magnetics and cause the high current

[bergie]

thanks for the responses. it only has the one cap. cape armature winders say there is some new electronic thingie that replaced the centrifugal switch. maybe i'll try that.

[AndyD]

If you've got the motor on a bench then just to test you can disconnect the wiring from the centrifugal switch and manually switch the capacitor during start using a standard lever or toggle switch. This is how it was done in the good old days.

[mikilianis]

Hi bergie you mentioned that the 3 phase 45 amp main switch of the house now trips ( with everything else off) and the load is a single phase motor perhaps the 3 phase 45 amp
circuit breaker is unbalanced and therefore functioning as it should, try running the motor on a seperate supply or change the 3 phase 45 amp circuit breaker to a D curve to accomadate the starting current. How old is the installation/45A circuit breaker ?

[bergie]

hi mikilianis its about 18 years old. i thought of changing the 45 amp breaker to a d curve but its the main switch but maybe eskom starts tripping. i think it may be a 35 amp tp which is even worse.

[mikilianis]

Hi bergie how is that single phase motor problem that you had,progressing have you removed it from the location and tried to run it from a different supply? Every now and again I get one of these curveballs thrown at me if it is not a fluorescent fitting that is messing around, or an automated gate motor, pool pump turning in the wrong direction what I got yesterday was a request to check 3 single phase pedestal fans you know the type that you can buy at Game for R200 bucks, firstly the damn things are so compact one cannot test without disrupting the existing wiring, all the leads are o.k. the switches are working, there is a small starting capacitor but if that were faulty it would rotate if it was turned, the windings don’t appear to be damaged, I must say that being an industrial electrician I enjoy a challenge apart from dumping them in the bin what’s my next step any comments.

[AndyD]

Sometimes with very small motors like those in domestic fans they don't follow the rules. They often design them to basically run in constant stall condition and vary the speed by changing the capacitance.

[mikilianis]

How do they configure the windings

[AndyD]

Not sure to be honest, it's something I've never had cause to investigate and they're nt designed to be dismantled so I doubt you could get one rewound.

As far as I can tell by looking at them they're an external rotor configuration and must be between 10 pole and 16 pole (or maybe even more) winding arrangement going by their RPM capabilities. They run with massive amounts of 'slip' but being external rotor and in the airstream the high heat produced by running close to stall obviously isn't an issue. I'd imagine their PF is also appauling.

Edit.

I just reread the thread and I think I misunderstood the type of fan. I was referring to the ceiling fans with the 3 speed rotary control that mounts on the wall. I think you might be referring the the oscillating desk type fans which I think either use a shaded pole motor or maybe a standard induction motor with multiple winding for different speeds. Maybe you can attach a photo of the motor and its associated wiring.

[mikilianis]

Hi AndyD here are some images of the motor and switch

[AndyD]

That looks like a standard motor with three speed windings. If it's not running at all and there's nothing showing on a clamp meter if power is applied then either the windings are damaged and open circuit or there may be a non-resettable thermal fuse laced onto the windings for overheating protection. To find out you'll need to dismantle the motor. Winding damage should be visable and you'll usually smell it as well and you can test if this fuse or thermistor has blown by bypassing it. Replacements are available from RS Components or ACDC. Sometimes these thermal fuses are easily accessible but sometimes you've got to do some surgery on the windings to expose them.

If the fuse has no markings on it then I'd suggest the replacement should be around 120 degrees C, these small motors often run quite hot so I wouldn't go below 100C and with standard winding shellac I wouldn't go above 130C.

The most likely cause of the thermal fuse blowing would be stiff rotor bearings/bushes, dust filled vents at the rear of the plastic motor housing or maybe people hanging clothes or towels over the fan which restricted airflow through the motor or were too heavy for the fan to oscilate side to side.

[mikilianis]

Thanks AndyD there is no burnt insulation smell and the rotor spins freely I shall preform surgery ( I like that ) on the motor only problem is 9 times out of 10, once surgery has been preformed on motors of this size they tend to end up slightly distorted i.e.they are never the same again,although if I get it to work there will be one happier pensioner what with the weather we are having in Cape Town at the moment.

[AndyD]

Yeah, it's a bit of an unknown until you crack one open. You might be lucky and it's immediately visible but sometimes they bury it quite deep in the windings and varnish over it then truss it up like a Christmas turkey with straps and string.

If you've got 3 identical fans then I'd go in with a veiw to sacrificing one as school fees for the greater good of the remaining two. Sometimes the thermal fuse looks a bit like a small capicitor type thing with a roundish body and the two legs exiting on the same side and sometimes it's a device that is the same form factor as a standard resistor or diode with a longer shape and one leg exiting each end.

I can only wish you luck