# Questions about a 3ph motor



## MDShunk (Jan 7, 2007)

So, did all three legs have power or not? Not sure why you speculate that you lost a leg, if you checked.


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## Innovative (Jan 26, 2010)

What did you determine to be the "right" tester?? First thing to do would be to verify that all three legs have the proper voltage........ A humming motor can mean lots of different things.


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## RIVETER (Sep 26, 2009)

Welcome to the forum.


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## varmit (Apr 19, 2009)

Could be several things:

You say that you spun the motor by hand? This would probably eliminate a mechanical bind.

To check for phase loss: Either turn off the motor disconnect or disconnect the motor and check each phase for proper voltage. This is to eliminate false readings caused be continuity through the motor.

At starter:
Check the overload block visually and with an ohmmeter. I have replaced a bucket full of O/L blocks in the last year. Check the contact resistance through the starter contactor. These could be deteriorated.

Motor checks:
With motor NOT ENERGIZED, Check each "T" lead to ground with a megger, if you have one. If you have no megger, use a DMM on ohms. If windings read to ground, motor is shorted- replace motor.
Disconnect all 9 "T" leads at peckerhead. There are six windings in the motor - assuming this is a standard single speed, 9 lead motor. One winding may have broken or burnt open.

The T lead combinations to check are: 

If motor is Wye wound- 7,8,& 9 will be common to each other. the other end of these three windings are connected together inside of the motor. 1 & 4 will be a winding. 2 & 5 will be a winding. 3 & 6 will be a winding.

If the motor is Delta wound- 1 will be common to 4 & 9, 2 will be common to 5 & 7, 3 will be common to 6 & 8. 

If the resistance varies more than a few percent on any of these winding combinations, replace the motor.

This list is not every possible problem, just the most common. Two years of motor age is really not relevant.

Good luck!


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## jwjrw (Jan 14, 2010)

Ive replaced one last week at an army reserve building last week that was humming. The squirrel cage would spin. I didn't take the motor apart to see what was wrong. It was the problem though.


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## micromind (Aug 11, 2007)

Varmits post is great, I'll add a bit though.

To know if a motor is wye or delta, look at the low voltage connection diagram.

If it's 9 leads and the low voltage connection is L1-1,7 L2-2,8 L3-3,9 and 4,5,6 are spliced together, then it's a wye.

If it's L1-1,6,7 L2-2,4,8 L3-3,5,9 then it's a delta.

Rob


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## John Valdes (May 17, 2007)

I will add something too. Resistance readings across contacts are not reliable. The motor must be under a load to really know for sure. If you suspect the OLR, replace it. Checking continuity is not enough. That goes for breakers, OLR's and contactors. Many times the resistance readings look good until a load is thrown to them. Just from personal experience.


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## Toronto Sparky (Apr 12, 2009)

I trust you checked voltage phase to phase? 
Even with a phase down all phases will read same voltage to ground.


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## RJS3rd (Sep 17, 2009)

Ok I found the problem. It was getting only 30-60 volts across one phase.

When I first climbed on the roof (multi level, needed 32' extension ladder and a 6' to climb 2 additional levels.) All I had was an Amp Probe tic tracer at the time (boss borrowed my multi and 'forgot' to tell me) which is why I didnt pick up on it at first.

They originally had it wired to a contactor which was controlled by a time clock, the contactor (I guess a basic motor start/stop kind as it had START and STOP buttons on the front) would only work when the time clock was ON. At some point they bypassed the timeclock and had the contactor on a 3ph single throw switch. Why is anybodys guess.

After 3 hours of trying to teach myself how a 20 yr old cutler hammer motor control was wired I finally found that X3 was getting less than normal voltage. When I bypassed the contactor and wired the LINE/LOAD straight through it worked.

BUT it took me awhile longer to learn it was working because of the background noise and the motor being 2 floors above me I couldnt hear the damn thing. I kept saying "I got solid 120v on ALL 3 phases...why wont it work?!" untill the maintenance guy came by, stuck his head over the grill and said "Great! You got it working!!".

I was like "Uh...YEPPERS GOT 'ER WORKING!!" 

Now I am going to replace the switch, eliminate the contactor and install a 4 pole, single throw time clock (as per the customer).

Pretty please tell me me and my supply guy were on the same page when I asked him for a 3 phase time clock...

As is obvious my edumacation is sorley lacking in certain areas of our field :laughing:




edit-the whole procedure took 3 hours, not the learnage of cutler hammer motor controls derp.


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## RJS3rd (Sep 17, 2009)

I bookmarked this thread for further reading of actual motor TSing :thumbsup:


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## John Valdes (May 17, 2007)

An amp clamp is much more reliable than bookmarking this thread. That's how you locate a high resistance connection. ZER0 current on a phase or motor coil means open or close to open phase.


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## varmit (Apr 19, 2009)

Why not use a normal, single pole time clock to control a starter? You SHOULD have some sort of motor overload protection. The overload block on a starter would provide for this. If no starter is used, you could install an IEC type motor protector ( basically a manual starter / adjustable breaker 
combination ).


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