# Repair 3phase electric motor(Unisaw)



## razinman (Oct 4, 2014)

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


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## CluelessNewB (Oct 4, 2014)

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.


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## JimDawson (Oct 4, 2014)

CluelessNewB said:


> Q1) Have you ever run this motor before?
> 
> Q2) Are you sure it is wired for correct voltage?   Maybe wired 440 with 220 applied?
> 
> ...



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.


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## Ulma Doctor (Oct 4, 2014)

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)


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## razinman (Oct 8, 2014)

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


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## CluelessNewB (Oct 8, 2014)

I'm still betting on a bad connection or bad switch/starter contact.  Motors windings don't just go bad sitting around.


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## Ulma Doctor (Oct 8, 2014)

razinman said:


> 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
> ...




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.


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## razinman (Oct 8, 2014)

Clueless, 

                     Thanks I going to give it a try !

                Kerry


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## razinman (Oct 8, 2014)

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


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## Ulma Doctor (Oct 8, 2014)

no need to thank me ... we haven't got it runnin' yet
you can thank me after we find your problem.


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## razinman (Oct 9, 2014)

Hi Mike, 
                 I finally got the motor out of the cabinet, all the wires are paired and numbered and went to check your e-mail 
       with instructions of how to check the motor with the ohm meter and it got lost. Could you please re-send the instructions
       of checking the lines with the ohm meter.

                  Thanks...............Kerry


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## Ulma Doctor (Oct 9, 2014)

the site had a crash and we lost some postings, an unfortunate casualty of technology.
i'll see if i can replicate what i said on the lost post.

with the ohm meter set on RX1 or the lowest setting 
put 1 meter lead on phase 1 or 7,1 pairings and the other on 8,2 pairing or phase 2, record the reading. there should be low resistance- way below 20 ohms
put 1 meter lead on phase 2 or the 8,2 pairing and the other on the 9,3 pairings or phase 3 record the reading, this resistance should be below 20 ohms
put 1 meter lead on phase 3 or the 9,3 pairing , the other lead to phase 1 or the 7,1 pairing, record the reading, it should also be below 20 ohms.
put 1 meter lead on the 4,5,6 pairing, the other on phase 1 or 7,1 record
put 1 lead on 4,5,6 and the other on phase 2 or 8,2 pairing and record
put 1 lead on 4,5,6 , the other on phase 3 or the 9,3 pairing. record.
 all the 4,5,6 to phase X should also have less than 20 ohms

you can also test all pairings to the grounded motor case or frame . there should be no continuity between any phases and the motor case or ground.

lmk if this helps out or makes no sense


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## razinman (Oct 10, 2014)

Hi Mike, 

                  Thanks for the outline, I'm going to do it tomorrow and let you know the results.

                     Regards...............Kerry


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## razinman (Oct 11, 2014)

Hi Mike, 

            Here are the results [my multi-meter had Blk(COM) for Ground and Red( lead)] so I tested it both ways
            Phase 1 (7-1) + phase 2 (8-2). Blk on phase 1 + red on phase 2 =  2 ohms, Red on phase 1 + blk on phase 2=3 ohms
            Phase 2(8-2) + phase 3 (9-3).  Blk on phase 2 + red on phase 3 =  5 ohms, Red on phase 2 + blk on phase 3= 2 ohms
            Phase 3 (9-3) + phase 1 (7-1). Blk on phase 3 + red on phase 1 =  3 ohms, Red on phase 3 + blk on phase 1 = 7 ohms

            Common leads (4-5-6) + Phase 1(7-1). Blk on phase 1+ red on Common lds= 2ohms,  Red on phase 1 + Blk on common lds = 3 ohms
            Common leads (4-5-6) + phase 2(8-2). Blk on phase 2 + red on common lds= 2  ohms, Red on phase 2 + Blk on common lds = 3 ohms
            Common leads(4-5-6) +  Phase 3(9-3). Blk on phase 3 + red on commons lds = 2 ohms, Red on phase 3 + Blk on common lds = 3 ohms
             All below 20 ohms as you predicted , the readings jumped around in the beginning but finally reduced to the measurements above.


         I tested in it with both leads( Red and Blck on the mult-meter )so that there would be no ambiguities. What is the next step that I perform at this point.

                Regards.................Kerry


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## Ulma Doctor (Oct 11, 2014)

just an FYI Kerry, the OHM meter should read the same regardless of polarity when testing an electric motor. 
another way of saying it is that the readings should not differ if you start red to black or black to red, it's the same reading. so no big deal either way. just shifting a lead can cause fluctuation in your reading. as long as the reading is stable when you take the reading you are ok.
when testing rectifiers and diodes that's when polarity will make a difference because current flow is blocked in one direction due to the nature of it's construction.

the motor, by your testing results, appears to be good winding wise.
one other test you can perform would be phase to ground test... simply test each wire pairing to ground i.e. 
7,1 to ground
8,2 to ground
9,3 to ground
4,5,6 to ground
if there is no continuity on any of these , the motor is not shorted to ground.

if there is no short to ground i would be very suspect of any control component that is switching the single phase 240 volts single phase.


dumb question here... does the motor turn by hand???


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## razinman (Oct 11, 2014)

Mike, 

               Yes, it does turn by hand(very easily), another dumb question to test phase to ground Do I use the Ohm meter test as I did before?

                    Regards.............Kerry


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## JimDawson (Oct 11, 2014)

Yes, one lead to the motor frame, and one on the wire.  Check all 9 wires to ground.


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## razinman (Oct 11, 2014)

What type of numbers am I looking for with the ohm meter, and if there are no readings then what?


Kerry


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## JimDawson (Oct 11, 2014)

razinman said:


> What type of numbers am I looking for with the ohm meter, and if there are no readings then what?
> 
> 
> Kerry



Then life is good.  That means that the windings are not shorted to ground.  A reading of 0 would be a bad thing.  No reading or infinite is good.


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## razinman (Oct 12, 2014)

Hi Jim, 

               I tested all the wires(9) to the motor frame, my multi-meter recorded a 1(this is NOT a reading)it stays on the 1 when I turn the dial on the multi-meter
to the ohm(resistance) position. 
              You and Mike(Ulma Doctor) have been most helpful, any thoughts what to do now ?

               Thanks............Kerry


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## Ulma Doctor (Oct 12, 2014)

Hi Kerry

i would really be interested in the RPC's output at this point. i think you mentioned it worked for other motors.

dumb question what is the RPC rated for or how many HP is the motor on the RPC?
if we are trying to start a 3hp 3phase motor with a 1.5 hp RPC we will have some problems for sure.

we have basically ruled out the unisaw motor fault, the windings are not open or highly resistant, there doesn't appear to be shorts to ground.

we are pointing to controls after we determine RPC output


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## Ulma Doctor (Oct 12, 2014)

i may be adding too much information here but you can secure the motor to a bench or a vise.
wrap a cord or small rope around the motor's output shaft.
hook single phase220/240v power to the motor Phase 1 (7,1) and phase 2 (8,2) then quickly pull the rope like a rope starter on your lawnmower.
the motor if it is good will run and will generate the third leg itself by induction. shut it down by unplugging power


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## razinman (Oct 12, 2014)

Hi Mike, 

                  I will try that, it's almost the same set up as my RPC. Answer to first question, I've always used the phase converter on the unisaw. The unisaw is
    a 3HP motor and so is the RPC.  This solution is no so SIMPLE.

            Regards..............Kerry


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## JimDawson (Oct 12, 2014)

I'm running out of ideas here.  Maybe start checking the phase to phase voltage in the entire system, starting at the RPC, and work your way through the entire system measuring at each connection point.  At some point you may have an open circuit.


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## Ulma Doctor (Oct 12, 2014)

without doing live testing here we are at an impasse .
we will have to determine whether the RPC is producing 3 phases, then the voltage between phases
before we can really go any further.

 any answer at this point is speculation


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## razinman (Oct 12, 2014)

The RPC is working , I've tried it on two other 3 Phase motors worked fine.  I value living, I have a very good knowledge of electronics and electricity. I've
always have done well, but it was working on not live current. I have some fear, but let me think about it and try a few more ideas I have in mind.

         Regards.............Kerry


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## Ulma Doctor (Oct 12, 2014)

do what feels comfortable to you. and don't feel the need to impress anyone.

that being said, i can assure you that it can be done with relative safety if you take a few precautions and not actually touch any bare conductors when you are hot.
your leads are insulated, and the wiring is insulated so it won't jump gaps at this voltage and try to bite an unsuspecting victim.
direct contact would be your greatest concern


any decision you make, 
i'll be happy to lend a hand or explain, my thimbleful of knowledge, to you .


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## razinman (Oct 13, 2014)

Hi Mike, 

                 I had another 3 phase switch which worked on another 3-phase motor I tried to test the  RPC, by chance I thought
 to connect the switch to the Unisaw motor in question -IT WORKED!!. I suspect it was either the external switch to the Unisaw
 motor or the electrical line/3-phase outlet that was non-functioning - Who would ever think?
             Once again thanks for your help- Guiding me through it all- I'll know for the NEXT time.

                     Regards................Kerry


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## Ulma Doctor (Oct 13, 2014)

razinman said:


> Hi Mike,
> 
> I had another 3 phase switch which worked on another 3-phase motor I tried to test the  RPC, by chance I thought
> to connect the switch to the Unisaw motor in question -IT WORKED!!. I suspect it was either the external switch to the Unisaw
> ...



hey Kerry,you are welcome.
anytime you need help shoot me a message, always happy to help out.


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## razinman (Oct 13, 2014)

Mike, 
            I appreciate that , I'm sure we'll "talk" again!

                     Regards...............Kerry


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## razinman (Oct 16, 2014)

Hey Mike, 

                     Finally got the motor back in the saw and making some sawdust again. The problem was when I wired  up the outlet to the RPC I mistaken the "Z" screw hole
  on the outlet for the GROUND . When I took it apart I made the switch to the correct ground "W" on the outlet, it all worked fine. Well the saw needed a cleaning and some
  lubrication anyway!

                 Regards.............Kerry


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## JimDawson (Oct 16, 2014)

OOPS.  We've all been there.  Good Job.:thumbsup:


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## Ulma Doctor (Oct 17, 2014)

I'm glad you got it all worked out Kerry.
i figured it was a simple mistake.

look at it this way,
 you now know more than 80% of people about 3 phase electric motors and RPC's)


don't be a stranger, you can send me a private message anytime.
mike)


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## razinman (Oct 18, 2014)

Hi Mike, 

            I will ,  don't I need to know your e-mail to send a private message or is there another way.
           Regards.............Kerry


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## Ulma Doctor (Oct 18, 2014)

you can send a private message on this forum or my regular email is 
ulmadoctor@yahoo.com

i'll send you a PM and maybe you can see how it works.
 if nothing else you can just hit the reply button to the PM i send you
mike


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