Capacitor assisted starting

[jraef]

I have on several occasions heard of large medium voltage motors using "capacitor assisted starting" as a way of soft starting. I can find no direct information on how this works or why one would choose this method, but I can find several esoteric references to the fact that it was used quite often in the past. I am interested in reading any experiences in using it and what the pitfalls might be.

The specific project I was asked to look at was a 2400HP 4160V starter system. We bid on a solid state starter, someone else bid on a VFD (as a soft starter), and the successful bidder used the "capacitor assisted starter", which the engineer approved as acceptable. Any thoughts? That which does not kill me, makes me stronger... and pissed off!

 

[electricpete]

I think the cap is switched in during the starting period to help supply the inductive (var) load of the starting motor.

Since the vars are not drawn from the system, the voltage will not drop as much (or might even increase).

T&D engineers are used to the idea of switching in a cap where var load is high to boost the voltage.

 

[electricpete]

I hope I'm not beating a dead horse, but here's one more comment:

reduced-voltage soft-start methods attempt to reduce the starting kva.

the cap-start would not attempt to reduce the starting kva of the motor, but only to supply most of it from a different source (the cap).

With all that said, I haven't actually ever seen it done. I don't know if the cap would be switched in at the exact same time as motor. It seems like you would have to be careful to switch it out before motor gets up to speed to avoid overexciting/overvoltage. I don't have any links or practical info on implementation.

 

[cuky2000]

See if the enclose reference could help this discussion:

http://www.hampton-power.com/Hampto...6b6100175b5d/$FILE/Motor Starting Methods.pdf

http://www.ab.com/drives/energy_savings/Application Profiles-Whitepapers/Petrochem Pumping.pdf

http://www.myronzuckerinc.com/docs/CAPACITALK 106.pdf

http://www1.ad.siemens.de/ld/softstarter/daten/simostart-mv-e.pdf

http://www.ab.com/mvb/literature/1560D-1.0-JAN99.pdf

 

[jraef]

Thanks, cuky2000, for the links. The one from Hampton Power (an ABB application paper) was usefull in it's description of how capacitor assisted starting works. Electricpete, thanks for your response too, you were right. It just provides for reducing the strain on the power grid, not the mechanical stress. You were also right in that the caps must be quickly switched out to avoid overexcitation.

I'm still interested in any experiences, i.e. does this cause long term degradation of the winding insulation? It seems as though it would. "If it were not for electricity, we'd all be watching TV in the dark!" ...George Goebell.

 

[jbartos]

Suggestions:
1. Check with firms that are offering it, namely:

http://www.gptechnologies.com/html/services.html

2. Take a course

http://www.jaewoo.co.kr/Src/home/EDSA/edsa_01.htm

that includes "capacitor assisted" and "capacitor/reactor assisted" motor starts.
3. Look into software, e.g.

http://www.cyme.com/psafmotorstart.shtml