Motor Voltage

[Fernando]

I have a 100 HP motor (460V). The line to line voltages all read 500V, but the line to ground voltages do not match (230V, 240V & 430V). The motor is fed by 3-75kVA transformer bank (Delta - Delta). Why would all the line to line voltages read about the same, but the line to ground voltages do not match? Why would I read almost line to line voltage, when measuring line to ground (430V). How does the variation in line to ground voltages affect the motor operation? If a motor is rated for 460V per nameplate, is that a line to ground or line to line voltage? Any advice?

 

[al hildenbrand]

I have a 100 HP motor (460V). The line to line voltages all read 500V, but the line to ground voltages do not match (230V, 240V & 430V). The motor is fed by 3-75kVA transformer bank (Delta - Delta).

This sounds like a 4 - wire delta secondary.

Does one of the 75 kVA transformers have a ground and neutral connected to the secondary center tap?

Like a 120 / 240 V, 3 ?, 4 - wire delta, what you describe might be a 240 / 480 V, 3 ?, 4 - wire delta.

 

[infinity]

You can find a detailed answer to this question in the other post in which you asked about transformers. Since you have an ungrounded Delta the voltage to ground is floating. For your 3 phase line to line motor load you're not concerned with the voltage to ground. Typically a motor can run at about 10% of it rated voltage. As mentioned in your other post you should adjust you transformer taps to get the voltage closer to 460 volts.

Don did a good job of explaining this here:

http://www.mikeholt.com/code_forum/showthread.php?t=80320

 

[hillbilly]

"Typically a motor can run at about 10% of it rated voltage."


infinity, I know that this was a typo error, but I want to clarify for the reader.
I think you meant to say...
Typically a motor can run at a voltage that's within 10% of it's rated voltage.
steve

 

[winnie]

Motors are rated for their line to line voltage, since that is the voltage that is seen 'across the coils' to develop the necessary magnetic field. As far as properly generating the magnetic field is concerned, the line-ground voltage is irrelevant.

However the winding _insulation_ will see the line to ground voltage, and you may have problems if the line to ground voltage is excessive. While the magnetic field will be generated properly, and the motor will run normally for a time, eventually the insulation will punch through and there will be a ground fault in the motor.

This can be a problem in ungrounded electrical systems, where during some types of fault conditions the line-ground voltage can get pumped up to several times the line-line voltage, toasting motors in short order.

-Jon

 

[scott thompson]

Sounds ungrounded to me...

Sounds ungrounded to me...

Sounds like the rare (to me ...) center tap grounded 480VAC 3 Phase 3 Wire Delta.

No common neutral grounded conductor used for circuitry - center tap is grounded at the Transformer - and sometimes only at the Transformer... as in not brought to the Service (would be viewed as an Ungrounded System).

It's odd, but not impossible or unused!

If the system is in reality a _Completely Ungrounded Delta_ (no winding / tap is connected to Earth at any point), then the inconsistant L-G readings may be from unbalanced loading on the system.

Nevertheless, it appears the Meter is a High Input Impedance type, which would display voltage readings L-G on an Ungrounded circuit, whereas any Meter that loads the circuit up (low input Impedance) will not display voltage readings L-G on _totally_ ungrounded systems.

As far as the 500 VAC L-L readings go, that's most likely not what the Motor's Terminals will see - especially when the Motor is loaded down.
The "above 460 VAC" readings may be from Induction Motors running without loads, or simply from the entire system being lightly loaded, and the PoCo's Transformer has been set to give a little higher Secondary Voltage (aka: FCAN taps) - and the system is lightly loaded.

Scott

 

[don_resqcapt19]

Scott,

No common neutral grounded conductor used for circuitry - center tap is grounded at the Transformer - and sometimes only at the Transformer... as in not brought to the Service (would be viewed as an Ungrounded System).

What about 250.24(A)? This would be a grounded system.
Don