High no load current on Synchronous motor?

[TurbineGen]

We just recently test ran a 12,000 hp synchronous motor and I noticed the ammeter read 100 Amps with the motor decoupled and the motor synchronized. Is this normal? I was told this is normal, but to me it seems high even for a motor that size.

Here are the specs I could find on the unit:

Voltage: 13.8 kV
Rpm: 1800
Hp: 12,000

100 Amps at 13.8 kV is more than 1 MW. That seems excessive for friction, windage and magnetization. What would cause that kind of high current? Is it possible the motor never synchronized?

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[dpc]

100 Amps at 13.8 kV is 2.4 MVA, not MW.

What power factor is the motor running at? Synchronous motors can operate at leading pf, generating vars. This will cause the motor current to increase.

For a motor of this size, you should have a data sheet giving motor current over a range of loads.



"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[ScottyUK]

Probably reactive current caused by a field current which is either too high or too low. Have you got reactive power metering on this circuit? Check your field current regulator settings: you can adjust the field so the motor neither absorbs nor produces VARs.


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If we learn from our mistakes I'm getting a great education!

 

[TurbineGen]

Unfortunately the only indication I had was a single ammeter. I had no reactive power indication. Finding paperwork or datasheets in this plant is like trying to find a needle in a haystack.

Here was my observation: On startup the motor was loud and the Ammeter was pegged at 600 Amps. As the motor accelerated the Amperage dropped slowly to about 175 and remained there for about a second or two. The Amperage then began dropping again and stopped at 100 Amps. As it made this second drop, the noise subsided. So I figure the field was applied when the motor reached 175 Amps. Historically, they manually synchronized this motor by waiting until the current was below 200 Amps for a few seconds, then brought in the field.

So my question: is 100 Amps usual on a decoupled motor? Should I consider changing the settings on the regulator or leave it alone? This motor has been run for about a year with it's present setting and it has been reported that they push it to around 13,000 hp at times. I'm not sure how they know this.

Thanks for the help

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[waross]

I have found it to be a good policy to wait and see with old equipment that seems to be misapplied. There may be factors that even the operators are not aware of.
The field may be set to allow the motor to supply VARs to the system to improve the plant power factor. This may cause the current you are seeing.
You can expect the current to change as the supply voltage changes if there is no automatic control of the field.
For a disscusion of synchronous motor starting, see:
Synchronous Motor Starter
thread237-230415

Bill
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"Why not the best?"
Jimmy Carter

 

[oldfieldguy]

If you have a chance, vary the excitation and see if the current goes up or down. If you decrease the DC and the stator current goes down, you were over-excited. If a change in either direction causes the current to go up, then you were likely at or near unity power factor.

I've had clients who used a tapped resistor to set excitation. You may have something like this, in which case changing excitation will be a hassle.

I'd recommend you have metering for watts, VAR, power factor, etc. If you have a fairly modern motor protection relay, these parameters might be easily available in the metering data.

old field guy

 

[edison123]

Why do you think a 100 A no-load current on a 12,000 HP, 13.8 KV, 500 A (?) motor is too high ?

 

[LionelHutz]

What is the FLA of this motor????

That's a fairly vital piece of info when you're asking if a motor current reading seems normal or not. It's a lot easier to compare the current reading you have with the motor FLA when the FLA is provided.

I'd guess that 12,000hp @ 13.8kV is about 450A. So, 100A is fairly normal for an induction motor. However, as already posted, since this is a synchronous motor if you adjust the field current up or down then the no-load current will change.

When dealing with AC, current times voltage gives VA, not W. Most of that100A is not being used to overcome the losses.

 

[TurbineGen]

Thanks for the help. I just couldn't figure out how a 460 Amp motor would run 100A no load. I was thinking in MW, not MVA. I just checked the motor and found the excitation was running a leading power factor. They have it set at a fixed resistance and under load the motor returns to unity or at least close to it and it hasn't been adjusted since installation.

Thanks again for the help.


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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[oldfieldguy]

TG--

That's pretty much what I though you'd find. AT no load, you're over-excited and shipping VAR back into the system. When you start loading up, you'll get closer to unity power factor.

old field guy