Motor Voltage Dip During Starting 1

[X49]

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%.

 

[ScottyUK]

12% dip at the genset should be fine if the set is the point of common coupling and assuming you don't have unusually high drops in the feeders to the other loads. If you're feeding lights off the same transformer as the motor then they will either dim or possibly go out if they're MH or sodium types. Check the restrike time - some need a while to cool before restrike which might be a safety problem.

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!

 

[rbulsara]

The motor or the generator itself would not care much about the 31% or even 35% voltage dip, as long as the starer coil can hold on. They both can recover on their own. The issue is the other loads connected and starter coils for other motors, if connected. Light flicker would just a nuisance.

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.

Rafiq Bulsara

http://www.srengineersct.com

 

[X49]

The reason I'm not using 480V for the motor is that it is existing. It is more cost effective to add a transformer than replace the existing electrical service, starters, and motors with 480V.

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.

 

[dpc]

Per NEMA standards, motor contactors are required to stay in down to 85% voltage, IIRC. Below that, there is no assurance. So contactor dropout/chatter may not be as unlikely as you might think with 23% dip (depending on location of contactor in relation to motor).

I wouldn't worry about fluorescent lamps. The problem is with HID lamps due to long time required to re-strike.

David Castor

http://www.cvoes.com

 

[waross]

If the pump is submersible you are stuck with it. If it is a base mounted, motor driven pump the motor may be reconnectable for 480 Volts. Much 240 Volt motor control gear is also rated for 480 Volts. The O/Ls must be adjusted or changed and you may have to add, change or reconnect a control transformer. This is not a big cost item if the starter is usable on 480 Volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[X49]

Bill, The pumps are base mounted and if the pumps are reconnectable for 480V this will likely be our best option. I will check this when I am on site. Is it possible to confirm this based on the motor nameplate?

 

[itsmoked]

Yes!

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

 

[waross]

What itsmoked said.

Bill
--------------------
"Why not the best?"
Jimmy Carter