Induction motors soft starter
Induction motors soft starter
(OP)
I want to know if there is any effect of capacitor banks for power factor correction on soft start of induction motors.
The capacitor bank is automatic directly connected to the busbar where several electrical motors (for pumping system) are directly connected.
Is there any technical regulation for this situation?
Thanks.
Dr. H
The capacitor bank is automatic directly connected to the busbar where several electrical motors (for pumping system) are directly connected.
Is there any technical regulation for this situation?
Thanks.
Dr. H





RE: Induction motors soft starter
But in general, I'm not a fan of bulk correction. Too much chance of over correcting (ending up with a leading PF). The best method is to have capacitors sized for each individual motor and use a separate capcacitor contactor that switches PFC caps on after the soft starter is done ramping.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Induction motors soft starter
>For a group of motors that generally run together, use a capacitor large enough to correct the group. The capacitor contactor may be arranged to close when the largest motor or more than one motor is started.
>Correct motor groups that run 24/7 before motor groups that run fewer hours per week.
>Over correct each group to compensate for motors that are not in a group.
>Add as much capacity as possible at the lightest load and leave it on 24/7. This may be just enough to offset the magnetizing current of the main transformer.
>There are more tricks that I remember when faced with the actual job.
>>Part of the art of power factor correction is a familiarity with the plant or factory process that you are correcting.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Induction motors soft starter
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Induction motors soft starter
Capacitor banks small in size compared to the incoming power capacity typically have a high enough resonance frequency that this problem does not occur. It should be checked though.
RE: Induction motors soft starter
Technically, compensation of each load separately is always the best. Compare it with centralized/bulk compensation is matter of B/C analysis.
RE: Induction motors soft starter
I take it you are referring to the caps being on the line side? Because on the load side of the RVSS, the cap charging looks like a short circuit to the soft starter as well and I have seen where it's usually a race to see if the caps swell and fail or an SCR shorts first.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Induction motors soft starter
RE: Induction motors soft starter
Even if your reasoning is correct, in principle, the two cases are orders of magnitude separated in severity and are not even thought of as similar phenomena.
Capacitors at the output of a soft starter is as bad as capacitors at the output of a VFD.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Induction motors soft starter
RE: Induction motors soft starter
If you connect any capacitor worthy of the name to such an inverter, the charging/discharging of the capacitor will be too much for the inverter and it will shut off immediately and show a message like "Short circuit", "Overcurrent" or even "Gate drive insufficient" on the display. It has nothing to do with life of the capacitor. Only with how VFDs work.
Same thing with SS, only that the voltage edges usually are at a lower frequency. The thyristors in an SS are not designed to charge/discharge capacitors. It is not only the thermal load that is shooting through the ceiling, there is also the di/dt to consider. It will be very high if you connect capactors to the SS output.
There is actually one inverter that can accept capacitors on the output. And that is because it outputs a pure sine wave to the motor. Google "nfo sinus".
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Induction motors soft starter
I read a NFO sinus brochure. Try them in practice? If those guys overcome PWM and solve EMI problems, that is VFD reinvention then. What are the prices, comparing with SE Altivar in example? Is the 15kW the largest unit, we use much bigger ones?
RE: Induction motors soft starter
Yes, the origional post asked about a bulk capacitor bank connected to the bussbar - likely the MCC bussbar or something similar. MikiBg seems to have hijacked the thread discussing SS and VFD output connections.
RE: Induction motors soft starter