Using soft-starter for motors with PFCC 1

[harris1975]

I have a couple of questions regarding use of soft-starters for large motors having PFC capacitors.

1) What is the typical harmonic spectrum for a commercial three-phase Full-wave (scr-scr) soft starter?

2) Are their any known issues with using electronic soft-starters for AC motors using static PF correction capacitors? Can they damage each other if some precautions are not taken?

 

[Skogsgurra]

Yes. A soft starter (SCR or VFD type) certainly doesn't want to see an HF short circuit, which the capacitors are. Connect the PFCs upstream of the soft-starter.

Regarding the harmonic spectrum. It changes throughout the starting, but it surely is very rich in harmonics. And they reach well into the MHz region.

 

[Marke]

Hello harris1975

1) While there are harmonics up to relatively high orders, the majority of the harmonic energy is concentrated in the 3rd, 5th 7th harmonics due to the highly inductive nature of the motor during start.

2) Power factor correction capacitors must not be connected to the output of a soft starter. Doing so will cause SCR failure.

Connect the capacitors via a contactor to the soft starter input once the motor has reached full speed, and disconnect before a ramp down if soft stop is employed.
Most soft starters have a "top of ramp" output that can be used for controlling a bypass contactor. Use this output to control the PFC contactor.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[harris1975]

Thanks Marke and skogsquarra for this invaluable information. But, it has resulted in an additional concern.

The situation is that there are about 5 motors (physically in proximity - same bus) with RVAT starters and PFCC. The idea is to replace RVAT starters with soft starters.

My concern is that even if PFC capacitor is switched at the "top of ramp" output, the capacitors of other running nearby motors will see the starting harmonics. IS my concern valid and if yes, is their any fix?

 

[DickDV]

I don't think there will be a problem especially if the softstarters have bypass contacts that close at the end of the accel ramp.

The high harmonics are present for only a short time so I wouldn't expect any problems.

 

[Skogsgurra]

I second that.

 

[MVRockwell]

harris1975,

If you're thinking of replacing autotransformer starters with a solid-state starter, you'll need to determine why an autotransformer was used/needed. The reason I mention this is that for voltage drop/sag concerns an autotransformer will always be better than a SCR based controller (due to the transformer action of an autotransformer). However solid-state starters are much more flexible and easier to change settings.

 

[jraef]

Resonance in connected caps from ramping of nearby soft starters is a concern in theory, but only if the supply is unusually weak. A stiff system will keep it to such low levels that it need not be a concern. Even the harmonics from adjacent ramping soft starters is still of such short duration that the likelyhood of causing any damage is miniscule. It is more likely that other switching transients on the common bus, such as grid switches or across-the-line motors turning on, will be amplified by the connected caps and create spikes, and that situation exists no matter what the starting method is. If you are really concerned about it, put some extra distance between the devices so that the resistance is greater between the caps, or add some inductors (reactors) in front of the caps. I personally wouldn't bother with it though.

"Venditori de oleum-vipera non vigere excordis populi"