Motor Burnt out measures 370v when running

[doggone1]

I recently aquired a combination woodwork machine the previous owner said he changed a capacitor when i used the planer for about ten minutes the motor burnt out it has 3 3hp motors for different tasks
I think he may have replaced the run caacitor with one of 50uf instead of 40uf and i cant tell if it is a run or start capacitor also when i measure the voltage with the motor running it shows 370v could this be due to wrong capacitor being fitted thanks

 

[bernardgreen]

What does the rating plate on the motor say ?

Does it say 3 phase ( or 3ph ) and does it say frequency 50Hz

It is unusual for a 3 phase motor to have a start or run capacitor.

I assume you have a three phase supply if it is a three phase motor.

Or is it a three POLE motor.

Some 3 phase woodworking equipment is designed to run on 200 Hz supplies from a motor generator set. This means the motors can run at a much higher speed which results in cleaner cuts and smoother planing and sanding

 

[doggone1]

Sorry i should have said there are 3 3hp single phase motors 2,2kw

 

[bernardgreen]

That makes more sense.

Replacing a 40µF start capacitor with a 50µF capacitor should not cause any harm if [1] the motor runs up to speed quickly and [2] the speed switch operates and takes the capacitor out of circuit before the start winding can over heat.

Like wise if it is a run capacitor ( always in circuit ) increasing form 40µF to 50µF shouldn't do any harm.

What may have happened is that the replacement capacitor has failed, maybe due to not being the correct operating voltage and type.

Between which two points did you measure the 370 volts ? With 230v AC on one winding the other winding could have a higher voltage. Either by transformer action if not rotating or generator effect if rotating.

 

[bernardgreen]

duplicate removed

 

[doggone1]

Hi I measured the 370v on the run winding the start capacitors are only in circuit when the start switch on the machine is operated i think no centrifugal switch

If he has replaced the run capacitor with a start capacitor could this be the cause

 

[bernardgreen]

I have known motors to burn out if the start winding is continuously powered as the wire is thinner guage and not designed to be powered continuously. But not from having an extra 20% of capacitance in the circuit while starting.

Run and start capacitors are esentially identical in construction although run capacitors may be more robust to withstand continual use.

Test the capacitor to see if it is short circuit, that would damage the winding it is connected to.

Another thought. What supply is the machine connected to ? If the impedance of the supply is too high then the voltage drop between meter and motor may mean the motor is not receiving full voltage when starting and thus not reaching full speed as quickly as it should. During the acceleration it will be taking significantly more current and this could lead to over heating and failure of insulation in the windings.

 

[doggone1]

Ok I'll test it in the morning and report back thanks

 

[matt1e]

Like wise if it is a run capacitor ( always in circuit ) increasing form 40µF to 50µF shouldn't do any harm.

I don't agree Bernard
While I agree that when replacing a start cap the rating is not as critical (though the correct one should be used if poss)
the same can not be said when replacing a run capacitor
It needs to be the correct rating or the windings will overheat
which is probably what has happened to the Ops motor

Matt

 

[doggone1]

Thanks for the reply I have googled this quite a bit and it would seem that most people think a wrong sized run capacitor could cause the motor to overheat i have ordered a 40uf run capacitor today as this appears to be the correct size it has been hard to find out as the machine is 30 years old.
I took the motor for rewind and it is the start winding that has burned out checked to see if the start capacitors were taken out after the machine started and they appear to be removed after start up