Motor Voltage Dip During Starting
Motor Voltage Dip During Starting
(OP)
I am adding a 30hp, 240V centrifugal pump load to a 150kW gas genset. The pumphouse will be at the end of a 1000ft 480V feeder, and also has some small lighting and heating loads. A 75kVA transformer will step the voltage down from 480V to 240V (3 phase).
I'm concerned that the voltage dip may be excessive when the pump starts due to the genset voltage dip, feeder voltage drop, and the voltage drop across the transformer winding.
Calculating the voltage dip based on rated motor inrush current, I get a dip of 31%. If I iterate my calculations to account for reduced current due to voltage dip, I get a dip of 23.5%. (12% genset dip, 5% cable and 6.5% transformer).
Is this an acceptable level of momentary voltage dip for the pump, motor contactor, and lighting loads? I could increase the size of cable and/or transformer to compensate but the costs will add up quick, and it may not be worth it just to prevent lighting flicker. Total steady-state voltage drop is well under 5%.
I'm concerned that the voltage dip may be excessive when the pump starts due to the genset voltage dip, feeder voltage drop, and the voltage drop across the transformer winding.
Calculating the voltage dip based on rated motor inrush current, I get a dip of 31%. If I iterate my calculations to account for reduced current due to voltage dip, I get a dip of 23.5%. (12% genset dip, 5% cable and 6.5% transformer).
Is this an acceptable level of momentary voltage dip for the pump, motor contactor, and lighting loads? I could increase the size of cable and/or transformer to compensate but the costs will add up quick, and it may not be worth it just to prevent lighting flicker. Total steady-state voltage drop is well under 5%.





RE: Motor Voltage Dip During Starting
Is there some reason why you are not feeding the motor at 480V? You eliminate the transformer drop straight away and can likely feed the balance of the loads from a much smaller transformer.
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If we learn from our mistakes I'm getting a great education!
RE: Motor Voltage Dip During Starting
Typically 30 HP motor on a 150 kW generator should not be an issue. Also a centrifugal pump would not have much sustained a starting dip, as it speeds up rather quickly. Make sure it starts unloaded (discharge closed and slowly opened).
Also see FAQ237-766: Generator Sizing for a Motor Load.
Rafiq Bulsara
http://www.srengineersct.com
RE: Motor Voltage Dip During Starting
My plan is to install the system as described and in the unlikely event that there are problems with coils or fluorescent lighting ballasts dropping out, I will try to replace them with units that tolerate more voltage drop. (Any idea how much dip fluorescent ballasts can handle?)
I guess the other solution would be soft starters, but there's significant cost there.
RE: Motor Voltage Dip During Starting
I wouldn't worry about fluorescent lamps. The problem is with HID lamps due to long time required to re-strike.
David Castor
www.cvoes.com
RE: Motor Voltage Dip During Starting
Bill
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"Why not the best?"
Jimmy Carter
RE: Motor Voltage Dip During Starting
RE: Motor Voltage Dip During Starting
A photo of the plate would help us quickly tell you. Though, it should, flat out, say 240/480 on it.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Motor Voltage Dip During Starting
Bill
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"Why not the best?"
Jimmy Carter