Fixed cap bank during motor start

[MikiBg]

Hi all!
There is a 110kW motor in blower set. Manufacturer says that DOL motor start can damage the blower. Current design solution assumes star(wye)-delta starter, but there is a fixed capacitor bank connected to motor.
I'm afraid that capacitor inrush current/over voltage during motor start could neutralize the effect of star connection and damage the blower.
Is it a practice to leave cap bank connected during star-delta start, or control scheme should be design in the way that cap bank is connected to motor only after start and with delta connected windings?
Thanks.

 

[Masbibe]

MikiBG,

This is my opinion.
Inrush current of capacitors dont go to motor.
There should be no overvoltage. Maybe voltage drop during start will be smaller but I dont expact overvoltage on motor terminals.
Motor is probably low voltage and not only one on MV-LV transformer ?
What concern me more is what will happen when you open star contactor. Motor should take energy from capacitors (if I understood your scheme well). But I dont know which will be frequency. I think this is no more 50Hz or 60Hz. I think that frequency now depends on capacitors and electrical parameters of motor. Then what will be when you close delta contactors. How much will be voltage of capacitors. You will have much bigger inrush current if voltage of capacitors is oposite with line voltage. Maybe you will damage capacitors. Also how much will be torque peak in this moment. I think that this is very hard to answer.

So my conclusion is there should be no problem when you close star contactor (start motor) problem could apear when you close delta contactor.

But it will be interesting to hear someone else opinion.


Pozdrav,
Milovan Milosevic.

Milovan Milosevic

 

[MikiBg]

Milovan,
Thank you for answer, I agree. Besides motor starting there will be at least three more transients: star contactor opening, delta contactor closing and opening. That complicates the thing, so exact analysis requires software modeling. Even if I do that, I won't be sure what results are telling me, because I don't know what peak torque can damage the blower, or what voltage peak can damage motor insulation or capacitors. That's why I rely more on other engineers experience.
There should be two LV 110kW motors with 40kVAr cap banks on the same bus.

 

[waross]

It depends on the connection. If the capacitors stay connected to the line during transition from start to delta you may be safe. You don't want the capacitors to be connected to the motor when it is transitioning from wye to delta.
Any induction motor may be subject to damage if the power is briefly interrupted while capacitors are connected to the motor terminals.
A closed transition may be more prone to damage than an open transition.
Milovan has the right idea.

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

 

[Masbibe]

I just want to correct myself. There will be no current in transition period (when both contactors star and delta are open) if capacitors are connected between main contactors and motor terminals U1,V1,W1 in parralel, and star and delta contactors are conected to terminals U2,V2,W2.
But MikiBG maybe you should post your scheme to be clear.


Milovan Milosevic

 

[MikiBg]

If the capacitors stay connected to the line during transition from start to delta you may be safe. You don't want the capacitors to be connected to the motor when it is transitioning from wye to delta.

So Bill, to fulfill both conditions, I have to connect caps upstream of main motor contactor and open this contactor during transition. Am I right? Maybe one condition is enough?
Milovan, there is not a scheme yet, I'm designing it. I'll take a standard star-delta scheme, as you described. The question is how to connect caps and create a control scheme (define when and how contactors open/close).