From Delta to Wye, concerns. 1

I am running a 15 Hp motor rewound from 480 only to run on 230 delta off of a roto phase. As we are installing 120/208 wye, we would like to reconfigure this motor for the new service type. Is this a problem that can be addresed by reconnecting the motor field and stator leads? As the leg voltages are different on delta than "Y" (no high leg) I presume the excisting connections wouldnt be right. Would anyone care to comment on this issue. Hoping the motor won't have to be rewound for a second time. Only wye type supply is available in our area. If a rewind is a must, would a transformer on one leg for boosting puposes be a cheaper option. Thanks in advance for your thoughts.

 

[resqcapt19]

Most motors wound for use on 230 volts can also be used on 208 volt systems. You should check with your re-winder. The motor doesn't care if it is connected to a delta or wye supply. The delta supply would be 230 volts phase to phase and the wye is 208 volts phase to phase. The motor also doesn't care about the high leg on the delta system as that high voltage is a phase to ground voltage and the motor only sees phase to phase voltage. If the re-winder says the motor can't be used on 208 you can have it rewound again or use a buck/boost transformer to step the voltage up from 208 to 230.
Most 480 volt motors are dual voltage 480/240. Did you really have it re-wound or just reconnected at the motor terminal box?
Don(resqcapt19)

 

When the machine was purchased used 5 years ago the name plate indicated that it was for use on 480 50Hz only. The machine dealer had it sent out for rewinding to work on 230, the plate on the unit now indicates 9 lead 230-460 delta. As an aside, it sure does have awfull amperage imballance, maybe because of the roto phase. I would have to recheck but I belive it was around 38-38-12 amps.

 

[busbar]

Treat the source and load as 3ø 3-wire, and the differentiation of delta and wye is not of concern. Compare the cost of rewind materials and labor to that of an open-delta-connected ‘buck/boost’ transformer pair. A guide on voltage matching with buck/boost transformers is at

http://www.electro-mechanical.com/7700-94.pdf

 

[SteveKW]

If you have been running all this time on an add a phase with that kind of imbalance you can be sure the motor is not loaded down and she will run just fine on 208!

 

Thanks for the replies everyone, the phase to phase view makes sense, I guess I should have have thought about it a little befor asking. As always, thanks guys for your time and effort, I always enjoy and learn alot from the reading on this forum.

 

[busbar]

Good point, SteveKW. Watch the phase currents while operating the '230V' motor on 208V. If they don't exceed the nameplate values for 230V operation, you're in tall cotton. Verify that motor overload relays are correct for '230V' operation. Also, see that associated control-power voltage is OK (via transformer taps or starter-coil voltage).

 

[buzzp]

Those current readings means the motor is running at a 59% current unbalance(NEMA method). That is beyond the 50% unbalance for defining a single phase condition. If the 38 amps is not above the service factor amps of the motor then your alright but the motor will be drawing more current to do the same amount of work because of the negative phase sequence field acting to oppose the rotation of the motor. This also means power factor will fall(more money here to if you have pf penalties-maynot be significant since motor is only 15HP). Likely the imbalance is because of the rotophase. There are methods for determining if the unbalanced currents are because of the incoming power or down stream of the power supply(i can send an article I wrote that describes this if you give me your email). Worth noting: if the voltage unbalance is 2% then the current unbalance can be as high as 2-10 times that, so worst case around 20% current imbalance for 2% voltage imbalance. I could go on and on but I wont. I build motor protection so if there is something I can recommend to help you out then let me know. Thanks Buzzp