Repair 3phase electric motor(Unisaw)

razinman

Active User
Registered
Joined
Feb 23, 2013
Messages
152
Hi All,

I had a Unisaw 3phase 3HP motor seat idle for sometime, I got my converter working BUT the electric motor(3Phase) to my tablesaw
just hums and doesn't "turnover".
Besides taking it to a repair shop, and clues as to how I can diagnosis the problem ?


Thanks ................KJ
 
Q1) Have you ever run this motor before?

Q2) Are you sure it is wired for correct voltage? Maybe wired 440 with 220 applied?

Q3) What type of "converter"?

Q4) Does it turn easy by hand?

Q5) Are you sure all the connections are good, if one of the 3 legs isn't connected it will do that.
 
Q1) Have you ever run this motor before?

Q2) Are you sure it is wired for correct voltage? Maybe wired 440 with 220 applied?

Q3) What type of "converter"?

Q4) Does it turn easy by hand?

Q5) Are you sure all the connections are good, if one of the 3 legs isn't connected it will do that.

And to add to the above

Q6) Has the converter been tested and is known to work properly

Q7) Did you check the overload relay in the saw motor starter?

These are the questions I would ask if I walked up to the machine to repair it. This will help us help you.
 
the three phase motor appears to be "single phasing", that is not all 3 phases are connected or not all 3 phases are powered.
i would examine all connections first of all...if the connections check out i'd start of something like this...

take necessary precautions to avoid being electrocuted, the test is on a running unit not for the faint of heart and only recommended to those who have a healthy respect for electriciy!!!!!

a simple test,
voltage test while hooked up to the Running RPC,
voltage test Phase 1 to Phase 2, record your reading.(red to black)
voltage test Phase 1 to Phase 3, record your reading.(red to white)
voltage test Phase 2 to Phase 3, record your reading. (black to white)

if the voltage readings aren't somewhere close to each other within about 10% you have a problem Huston!

you can also test for open windings or shorted windings with an ohm meter while the motor is disconnected from it's circuit and no power is present should you have any second thoughts about working with a live circuit.

i'd be happy to describe the process for troubleshooting a 3 phase motor if the situation requires deeper explanation.
lmk if you need help, always happy to lend a hand
mike:))
 
Hi Mike,

I haven't had time to do the ohm meter test(too much work and a really bad cold).If the ohm meter test is negative, what's my next step. From what I read the
only real big expense is a motor rewind($$$) it's not worth the expense. The motor hums when there is power to it, and tried turning the fan prior to turning on the switch
to the motor it turned very slowly (200-300 rpms) didn't leave it going that long for fear of burning out the motor.So my question is, how do you know when it's a REWIND
JOB?

Thanks again,
Kerry
 
I'm still betting on a bad connection or bad switch/starter contact. Motors windings don't just go bad sitting around.
 
Hi Mike,

I haven't had time to do the ohm meter test(too much work and a really bad cold).If the ohm meter test is negative, what's my next step. From what I read the
only real big expense is a motor rewind($$$) it's not worth the expense. The motor hums when there is power to it, and tried turning the fan prior to turning on the switch
to the motor it turned very slowly (200-300 rpms) didn't leave it going that long for fear of burning out the motor.So my question is, how do you know when it's a REWIND
JOB?

Thanks again,
Kerry


Hey Kerry,
i would be very suspect of a faulty connection somewhere as CluelessNewB has pointed out.
i would not rule out an open winding until i performed the resistance testing though.
the resistance testing would be the determining factor for whether you'd need to rewind
to be honest in my whole career(25 years) i have only sent 4 motors to be rewound and those motors were over 10 hp.
in my experience, bearings and controls definitely fail before motor windings especially on older motors.
usually with small motors if i can't easily do a repair job,the motor is replaced and the old motor gets recycled

if you have any picture you can post of the set up, it may help out to troubleshoot the situation.
 
Last edited:
Hi Mike,

I going to take off the motor tomorrow and do as you said with the ohm meter test.

Once again THANKS FOR YOUR CONCERN!!!!!!!

Regards.............Kerry
 
no need to thank me ... we haven't got it runnin' yet
you can thank me after we find your problem. :)
 
Back
Top