Induction motors soft starter

[26021960]

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

 

[jraef]

No regulation, but soft starter manufacturers will tell you not to hook up capacitors on the LOAD side of the starter. Caps on the line side are OK in LV systems.

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
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

The science of power factor correction is to correct every motor. The art of power factor correction is to minimize cost by;
>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

 

[jraef]

There are more tricks that I remember when faced with the actual job.

Love that statement. it describes the way I go through life...


"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

 

[LionelHutz]

The soft-starter will produce moderate levels of current harmonics, mostly 5th and 7th. It is possible that these harmonics could excite a resonance or near resonance and trip-off or damage the capacitor bank.

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.

 

[MikiBg]

Resonance (parallel, or anti resonance here in Europe) is just one side of the problem. It is rarely dominant in LV installation because anti resonant frequency is adversly depended on kVA of bank installed and source impedance, so u need quite large capacitor bank, soft (bad) power grid and high harmonic order in load spectrum to have a real problem with resonance. Much more often, capacitor bank is agening rapidly because harmonics simply flow into it easier then into resistive load for instance. That's why I guess it's better to install the bank upstream of SS, but I don't know what is THD situation there (comparing with SS output). Anyone knows?
Technically, compensation of each load separately is always the best. Compare it with centralized/bulk compensation is matter of B/C analysis.

 

[jraef]

LionelHutz,
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
For the best use of Eng-Tips, please click here -> faq731-376

 

[MikiBg]

Aren't these things all related to the same phenomenon? More higher harmonics in SS output spectrum - capacitors downstream of SS closer to short circuit. Otherwise, we would have the same troubles supplying capacitors with pure power grid sinusoidal.

 

[Skogsgurra]

Miki - there's a HUGE difference in the output voltage from a soft starter and the harmonics contents in the grid supplying it.
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

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[MikiBg]

Gunnar, I understand there is a difference in severity and origin between grid and SS voltage harmonic distortion. I was thinking from capacitors point of view. If you feed them with distorted voltage, they will be a low impedance for higher harmonics and their insulator will suffer excessive stress, no matter where from u feed them, right? But, is this the only reason why capacitors at SS/VFD output should be avoided, or there is a something else too? Let's say there is a SS/VFD with THD=0 output, and we're not taking internal capacitors into account. Would then be a problem to connect/leave external capacitors at the SS/VFD output?

 

[Skogsgurra]

The ordinary VFD output is nothing but rectangular pulse-width modulated voltage pulses with a very short rise-time, usually in the 100 - 300 nanoseconds region.

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

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[MikiBg]

Think I know what u're talking about, Gunnar. I have worked with UPS before. They have IGBT at the output, but most of them can handle with limited crest factor (3 or 5) at the output. Above that, "current limiting" or "inverter overload" can occur. We had that kind of problems during startups and blade servers for instance, I guess power capacitors can cause similar problem. There was a type or two claiming unlimited crest factor supported, don't know does it work.
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?

 

[LionelHutz]

LionelHutz,
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.

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.

 

[MikiBg]

Hijacked??? I would say evolved... we're still talking about SS and pfc capacitors