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4

3 Phase Motor Troubleshooting?

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randyjaco

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#1
I just picked up an old Yates American jointer. When I cut on the motor, it buzzes and turns slowly. Being a 3 phase there are no condensers or start switches to check. The fly in the ointment is that this is a direct drive machine. The cutting head and rotor are one piece, just swapping motors is not an option. Everything turns very easily, no bad bearings or obstructions.
Any thoughts on what might be wrong?

Thanks
Randy
 

dieselshadow

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#3
After making sure you have proper power, with the power disconnected, ohm out the leads to each other. 1-2, 2-3, and 1-3. They should read similar and fairly low. Then ohm out each leg to the motor case and see if you read anything at all. If you read anything, that's a bum motor but it usually trips the breaker.

More than likely you are single phasing the motor by not having all three legs at the proper voltage and correct phasing.
 

jim18655

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#4
Assuming you have a magnetic starter, disconnect the motor leads and check voltage at the T leads of the contactor to make sure all contacts are closing. Any chance it's wired for high voltage?
 

whitmore

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#5
I just picked up an old Yates American jointer. When I cut on the motor, it buzzes and turns slowly. [three phase motor[ Everything turns very easily, no bad bearings or obstructions.
Now is the time to look for sawdust in the switch or contactor. Burnishing the contacts
will usually help. You just have to find them (all of them) first. The buzzing means (usually) a
phase has gone open, so an exploratory tightening of every electrical connection would be
in order also.
 

Ulma Doctor

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#6
Hi Randy,
being a saw, i'd be suspect of the main switch.
to test, disconnect power, get an ohm meter and inspect the power cord and plug connections
take a phase to phase reading at the cord to see if there is a short
take a phase to ground reading for each phase and see if there is grounded out wiring
remove the either incoming power cord or the motor cord from the switch, so that you can test the switch.

testing the switch,
most saw switches are linear in operation. the top 2 terminals are one pole, the next 2 terminals down are pole 2, the third set of terminals are the pole 3
with the sitch on the off position test the 2 top terminal together the meter should read open, then flick the switch and retest- the meter should read low resistance
repeat for each subsequent pole, noting any variations
reinstall wiring and tighten the incoming power connections if these tests are sufficient.

a visual inspection of the motor and associated connections are important, if no obvious faults are seen you can test the 3 phase motor.

testing the motor,
test pole to pole with an ohm meter, then test the last pole to the first pole- there should be resistance measured on both poles in respect to the first.
test pole to ground of the motor frame to each pole, there should be no continuity in this check.

should there be an open in the winding of any of the 3 motor poles, the motor will need to be repaired or rewound

reconnect motor power cord to switch and tighten connections if the tests showed no faults


i wish you the very best of luck! :)
 
Last edited:

killswitch505

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#7
Sounds a lot like the motor is wired for 480 and your sending it 208. If that's the case it an easy fix follow the legend on the motor name plate you'll need to also change that control transformer.
 

randyjaco

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#8
I played with it today, still no improvement.
Here's the motor plate data
Line start induction motor
Volts 220
Amps 3
HP 1
Phase 3
Cycles 60
RPM 3600
Serial 276247
Frame 512
Style C
Type CT
Temp 40
Time cont

I have 3 good legs from the wall.
I rehabbed the switch. No change. I then bypassed the switch. Still humming with slow rotation.
I only have 3 wires coming from the motor. To get to the others I am going to have to disassemble the whole cutter head. I was hoping to avoid that. Any tests that I can do with those 3 wires, prior to disassembling?

Interesting situation : at the wall I got 120 vac on each leg and 240 combining them. On the switch I got 121vac on each leg and from 260 to 290 combining the legs. How does one get more voltage at the machine than the wall?

Thanks
Randy
 

Technical Ted

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#9
A lot of times when a 3 phase motor is single phasing (missing one leg) it will just sit there and hum, but my gut tells me you are missing a phase somewhere down the line....

Good luck,
Ted
 

Technical Ted

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#10
Please forgive me if I’m being Captain Obvious on this... I have no idea what your level of experience is and I just thought I would offer this to help you troubleshoot your circuit. If this is basic stuff to you, maybe someone else can benefit from it.

In the attached picture, I show a partial 3-phase circuit for demonstration. The source is at top. A,B & C are terminal connections. Please note that a wire or a component (contact, connections, whatever) on the middle phase from the source is broken, so no current flow through it from the middle leg of the source to the B terminal. The L is a coil, or maybe some other component like a motor, relay, transformer, etc.. Something that current will flow through.

When testing for proper 3-phase voltage on the terminals A, B & C the measurements can be misleading if not properly applied. Measuring to ground, you will see 220v on all three points A, B & C. Now, there is a broken connection on the middle leg, but you will see a voltage there because it is being back fed through the coil from the C terminal. This could make you think that you have 220v on all three legs when, in fact, you only have 2 legs and are seeing leg C twice.

Further, measuring from A to B you will see 440v, again B is being back fed. Measuring A to C you will see 440v and this is correct because you have both of these legs. Now, checking from B to C you will see zero volts and this will show you are in fact missing the middle leg B.

I hope this makes sense... and I’m not insulting anyone’s intelligence.

Good luck,
Ted
3-phase diagram.jpg
 

gzoerner

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#11
Randy,

From your description, it sounds like you don't have 3-phase power going to the motor. The clue is that you are measuring 120V across some of the wires. You should be seeing 240V between all of the wires coming from the wall. What you are describing is what you would get from standard 240V single phase with a neutral.

I don't know if you are in an industrial setting, but 3-phase power in a residence would be very unusual.

Best of luck.
Glen Zoerner
Spicewood, TX
 

gregc

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#12
A few things seem odd
The motor was 3 A @ 220v from the plate. Normally this would be 220vac between each leg and neutral. This is 660 va per leg. Normally the name plate specs the output power including efficiency and power factor. Each leg contributes to the output power 3x660 = 1980 w. 1 hp = 735 W. So for the input power one should have a 2.7 hp motor.

With 60 HZ power 2 poles would produce 3600 rpm. This would be the right number for a synchronous motor but it is specified as an induction where the stator usually slips From the rotating field under load. Typically this is 2-5% or something like 3500 rpm

You should be measuring 220 vac between each leg and neutral. Between any 2 legs 380 vac as each leg is at 120 degrees from each other using a leg to netural reference.


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gzoerner

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#13
In a Delta (3 wire) 3 phase system, there is no neutral. All 3 wires are "hot". I'm guessing that Randy will need to get either a rotary phase converter or a VFD to generate the 3 phase his motor needs. He'll get the benefit of variable speed and soft start with a VFD.

Glen
 

Technical Ted

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#14
A few things seem odd
The motor was 3 A @ 220v from the plate. Normally this would be 220vac between each leg and neutral.
I must be misunderstanding something. I guess that happens during retirement! :)

If I was checking voltages on a 220vac 3-phase system, I would expect to see 110v to neutral/ground on each individual leg and 220vac across any two of the 3 legs.

Oh well,
Ted
 

Technical Ted

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#15
Randy, I assumed you had 3-phase power... is that not the case?

I guess I know how we spell assume! :)

Ted
 

ddickey

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#16
Phase to ground you'll have 120 v. Phase to phase 240v.
 

ddickey

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#18
So does the OP have a 3 phase service or is he using a phase converter?

I reread and he has three phase service.
 

markba633csi

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#19
Randy: with power off you can disconnect and measure those 3 wires from the motor with an ohmmeter on the lowest range (hopefully RX1) and measuring between any 2 wires you should get the same very low reading (1 or 2 ohms or so) plus or minus 5 percent. If not equal then the motor has one or more bad windings. Check also that the windings are not shorted to the case.
Mark S.
 

4gsr

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#20
If you have true Delta-Wye 3-phase coming into your shop, the high leg or "wild leg" should be giving you a erratic reading of about 177-190 volts to ground. If this is the case, get the power company out to check the pole fuses on their side of the meter. One of them may have gone bad. I had that to happen at the family homestead up in Conroe several years ago. Couldn't get any motor to fully start in the shop. Started checking line voltages and that's what I found. A call to the utility company and discussing with the service guy that came out, he traced it to the high leg fuse on the pole.

Last thing could be a clogged up motor from years of saw dust collecting. Seen motors on machine tools clogged up from years of crud build up not wanting to run right.

Edit: I meant to say, if you are not getting this erratic reading of 177-190 volts to ground, and it is closer to 100-120volts, get a hold of your power company. Sorry.
 
Last edited:

gregc

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#21
Phase to ground you'll have 120 v. Phase to phase 240v.
If you have A volts from any leg to neutral there will be A • sqrt(3) or approx A • 1.73 leg to leg from the trig identities as the legs are 120 degrees apart.

If it was a single phase system the above would be true with 2 legs 180 degrees apart with respect to neutral


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ddickey

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#22
I stand corrected. On a delta system line voltage is the same as phase voltage.
 

ddickey

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#23
It's probably four wire though so he will have 120v for auxiliary loads.
 

gregc

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#24
In a Delta (3 wire) 3 phase system, there is no neutral. All 3 wires are "hot". I'm guessing that Randy will need to get either a rotary phase converter or a VFD to generate the 3 phase his motor needs. He'll get the benefit of variable speed and soft start with a VFD.

Glen
Almost universally the source side is wye connected with the center of the wye connected to ground and used as a neutral and the load delta. A neutral with a ground is required by national electrical code for fault detection. In this case if there is neutral current one leg is shorted or 'leaking' to ground. The equipment is locally grounded and there is neutral current the machine is hot.

An alternative is to have a delta with one transformer winding having a center tap with the center tap tied to ground. This center tapped winding looks like the 2 legs in a single phase system. This connection can serve for both single phase and 3 phase source. However the legs are not balanced. Generally it is preferred to have different transformers for the single phase and 3 phase sub systems. One can also have multiple secondaries to provide effectively independent transformers.


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randyjaco

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#25
Hey guys, thanks for the replies. I am a magic smoke electrician, so I am not offended by going to the basics.

I am pretty sure I am getting power from each phase. I have both an RPC and several VFDs. These are successfully operating other motors. Both of these converters produced the same result.

Right now I am trying to get the motor apart (no small feat) to get to all the necessary wires.

Randy
 

gzoerner

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#27
GregC

Thanks for the explanation. Troubleshooting usually begins with the basics like: "is it plugged in?" Randy is well past that stage.

Glen
 

randyjaco

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#28
Yes, I have single phase service to the shop to which I have a RPC and 3 VFDs to service my 3phase motors.

Randy
 

Ulma Doctor

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#29
Randy, I have a question
is your RPC a commercially made unit?
if so it would explain why you are getting line to line readings that exceed the input voltage.
some commercially made units use large run capacitors or a lot of capacitance to balance the voltage between legs.
what you may be reading in your testing, is the balanced output of the RPC's capacitors in and idle condition.
when idling, the voltages can test higher than while under load
 

randyjaco

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#30
Yes, it was commercially made

Today I managed to get the motor out. To my dismay it's all sealed in some epoxy like material. All I have is 3 wires T1, T2 and T3. I got out the VOM.
There was no continuity between the T wires and the frame.
T1 to T2 = 6.4 olms
T1 to T3 = 6.3 olms
T2 to T3 = 6.4 olms

Does this indicate anything?

Randy IMG_20170721_164405.jpg
 

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