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