Voltage Imbalance
Voltage Imbalance
(OP)
I've been having problems with voltage imbalance damaging motors at a remote facility where the utility power quality is very poor. We have two 30hp 600V motors that we run on soft starts until they started tripping on voltage imbalance too often. They were then bypassed, and the motors failed within a year or so. Loads are centrifugal pumps and the actual load is approximately 25hp.
The utility logged the voltage imbalance at the facility for two weeks. It ranges from 0.5% to 2.5% during a typical day. Half of the days it spikes up to 3.5% (for a short duration). Twice it reached almost 6% (again for a short duration).
I'm considering either using oversized (50-60hp) VFD's with the current 30hp motors (non-inverter duty rated), 40hp motors on 40hp soft starts with imbalance protection disabled, or 40hp motors on conventional starters (client would prefer this).
I've found this derating chart from a NEMA standard, but it only goes up to 5%.
http://ww w.engineer ingtoolbox .com/elect rical-moto r-voltage- imbalance- d_648.html
Do you think installing a 40hp motor on conventional starters would be reasonable?
The utility logged the voltage imbalance at the facility for two weeks. It ranges from 0.5% to 2.5% during a typical day. Half of the days it spikes up to 3.5% (for a short duration). Twice it reached almost 6% (again for a short duration).
I'm considering either using oversized (50-60hp) VFD's with the current 30hp motors (non-inverter duty rated), 40hp motors on 40hp soft starts with imbalance protection disabled, or 40hp motors on conventional starters (client would prefer this).
I've found this derating chart from a NEMA standard, but it only goes up to 5%.
http://ww
Do you think installing a 40hp motor on conventional starters would be reasonable?





RE: Voltage Imbalance
I don't think oversizing the motors is going to be a solution for that magnitude of voltage imbalance.
Is any of this due to the plant load itself being imbalanced?
I'd be leaning a little more on the utility. Unless the plant load itself is extremely unbalanced, the power coming from the utility does not appear to be meeting ANSI guidelines, assuming you are in ANSI-land.
David Castor
www.cvoes.com
RE: Voltage Imbalance
I've used VFD's to solve this problem in the past, but the client has had problems with these in the past (surges? undersized?) and prefers a more simple and robust solution given the remote location.
The voltage imbalance is definitely coming from the utility. It is a problem with other facilities in this area. The utility is in northern Canada. Although they've been helpful in diagnosing the problem, their power quality standards do not guarantee low voltage imbalance in remote areas.
Oversizing the motors for 5% imbalance would theoretically require a 33hp motor. I'm thinking that a 40hp motor would be satisfactory for the occasional dip to 6%, although the NEMA chart may only go up to 5% for a reason.
RE: Voltage Imbalance
In the old days of "U" frame motors we regularly started 250 HP motors across the line, two 250 HP motors started eight times a day across the line. If "U" frame motors are available they may be worth the extra cost. They are much more rugged than "T" frame motors.
Bill
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"Why not the best?"
Jimmy Carter
RE: Voltage Imbalance
NEMA MG-1 does say that operation of motors with greater than 5% imbalance is not recommended.
David Castor
www.cvoes.com
RE: Voltage Imbalance
Muthu
www.edison.co.in
RE: Voltage Imbalance
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