Eddy Current
Senior Member
What is used to control motor spikes/dips? Is this what capacitor banks are used for or motor control centers?
What is used to control motor spikes/dips?
- Thread starter Eddy Current
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petersonra
Senior Member
- Location
- Northern illinois
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- engineer
I am not real sure what you mean by "spikes" or "dips".
If you are referring to transient voltage conditions that are either higher or lower than nominal, these transients are not "controlled" so much as tolerated in most motor applications.
Capacitor banks on MCCs are for adjusting power factor and have little if any effect on transients.
iceworm
Curmudgeon still using printed IEEE Color Books
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- EE (Field - as little design as possible)
Pretty much agree with petersonra - capbanks won't help much. However, there are a few things you can do.
Move the motors over to their own transformer. The dips won't affect the rest of the system as much.
Put in larger transformers. Less impedance between you and the source reduce the dips
Use soft starts. Minimize the transient inrush/LRC on starting
Use VFDs. Again minimize transients on starting.
Something I have not tried and have not seen any papers: Some installations put pf correction caps on the load side of the motor contactor. The caps come on-line with the motor. The LRC/inrush is highly reactive. Could be the caps at the motor will trace vars and reduce the inrush/LRC. But I don't know that. Could be your place in history to do the research write the paper. But you couldn't call it the "Eddy Current Effect". Somebody already got that one.
ice
- Location
- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Too late, "Capacitor Assisted Starting" is as old as the hills and twice as dusty when it comes to very large AC induction motors. PFC caps don't really do much in that regard, the kVARs they supply are only intended to correct from around .8 to a .95 PF. When a motor starts, the PF is at about a .1 to .2, rising only slowly until you get above around 70% speed. So the little bit of kVARs from PFC caps is almost meaningless as far as assisting with starting. For CAS, the caps are much much larger, and because of that, they MUST be switched out of the system at 70-75% speed or they over excite the entire system and cause big problems. That's kind of why people don't use them any more. That method was displaced by the relative safety of using a solid state soft starter ages ago, although about once a year I get someone asking me to help them build it. The price is always higher than just getting a soft starter, but people THINK it will be cheaper because they don't think through all the other issues involved and what it takes to mitigate the risks.
Eddy Current,
Since you mention MCCs, we can assume you are asking about 3 phase induction motors, so what iceworm said is the right way to go about it; use soft starters or if changing the speed has other advantages, use VFDs. If you are having troubles with starting of large AC induction motors causing a significant voltage drip, I suggest however that someone should assess the situation as a whole before you start throwing money at it. Soft starters have a limitation on how much they can limit the starting current without stalling the motor. VFDs have almost no limit in that regard, but are significantly more expensive in higher HP sizes (above 10HP) and add a new set of issues to contend with if not planned correctly.
As to "spikes", petersonra is right, those are more "tolerated" than controlled, because they tend to come down the pipe from other sources. The way of dealing with them is called a "Surge Protection Device", or SPD.
ATSman
ATSman
- Location
- San Francisco Bay Area
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- Electrical Engineer/ Electrical Testing & Controls
What prompted you to ask this question?
Are you experiencing equipment damage? If so then the source of the disturbances has to be determined; internal to the plant or external from utility. Have you done any measuring or monitoring?
On new installations we are seeing more SPD's being installed on LV main switchboards to mitigate voltage transients coming from the line.
petersonra
Senior Member
- Location
- Northern illinois
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- engineer
That is where one would always put PFC capacitors if one was correcting PF one motor at a time. Was very common to see specs not real long ago that required it for motors above a certain size (sometimes as small as 10 HP). Don't see it real often at all these days.
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