3 Phase Motor Troubleshooting?

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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I stand corrected. On a delta system line voltage is the same as phase voltage.
 
It's probably four wire though so he will have 120v for auxiliary loads.
 
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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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
 
So you have single phase service?
 
GregC

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

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

Randy
 
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
 
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

IMG_20170721_164355.jpg

IMG_20170721_164303.jpg
 
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