Soft Starters With Power Factor Correction

[Madala]

Hi my query is have 3 132Kw motors rated 210A that are started sequentially and would like to fit power factor correction to each. Would I have to utilise 1 main unit with a switching contactor to come on after all three have reached line speed. Each motor has its own soft starter with top of ramp contactor.

 

Thanks Madala

[jraef]

As long as the capacitors are wired upstream of the soft starters you should not have any problems. I would not use one large capacitor however. If one motor must be shut down for any reason, you would be over correcting. If however all 3 motors must run together, so you will shut down the other 2 if one fails, then this would be OK.

[Madala]

As long as the capacitors are wired upstream of the soft starters you should not have any problems. I would not use one large capacitor however. If one motor must be shut down for any reason, you would be over correcting. If however all 3 motors must run together, so you will shut down the other 2 if one fails, then this would be OK.

 

Thanks Jraef, but could not this affect the soft starter in next sequence as capacitors are suposed to be in line out not line in. All motors must run together if one fails all must be shut down. I was thinking of each motor having own set of capacitors and only when all three top contactors come in then all three banks would come into operation. would this not be better.

 

Madala

[jraef]

The biggest danger of capacitors in a soft starter circuit comes from the caps being downstream of the SCRs. The charging current of the caps is seen by the SCRs as a short circuit; too rapid of a rate of rise, which may cause self commutation. Conversely the rapid switching of the SCRs into the caps may cause them to overheat rapidly.

 

When located on the line side of the SCRs the chief potential problem is one of interaction (resonance) with the line side harmonics created by the SCR firing, but that is so short lived that is isn't worth worrying about IMHO. The other potential problem as described by Marke in his paper (Power Factor Correction capacitors paper) is that line transients can/will be amplified by the capacitors and may damage the SCRs if they are not firing. So in your case, having one set of caps on-line before you start the next motor does pose a potential problem IF there there is another line transient at that time, but my experience has been that this is only a problem on very weak lines (low fault current) where the supply is very inductive to begin with. In stiff supplies the likelihood of transients is much lower and it would only be a problem if the transient occurred under those specific circumstances.

 

All that said, there is certainly nothing wrong with waiting until they are all at speed. If it worries you, do that.

[jOmega]

JR:

 

...and what happens when the pf caps go live ..... the Soft Starter doesn't blink or hic-cup or anything ?

Just keeps chugging along as if no transient occurred on the mains .... Life is good ...etc ....

 

jΩ

[marke]

Power factor correction can be achieved either using static correction, where each motor is corrected and the capacitor is controlled by the starter, or bulk correction where the correction is controlled by an automatic controller.

Where you have three motors only, it makes sense to use static correction.

As Jraef has stated, the capacitors must be connected upstream of the SCRs. Connecting on the output of the soft starter will causeSCR failure and possibly capacitor failure.

The capacitors must not be connected until the soft starter is up to full voltage. Most modern soft starters have a relay output that can be used to control a capacitor contactor.

I would suggest one capacitor contactor per soft starter controlling the correct capacitance for that motor. This will ensure optimum correction.

Controlling all the capacitance from one contactor with three times the capaciatnce will result in no correction untill all motors are operating, and will result in a much higher switching transient when that contactor closes.

You will still need the same amount of correction, so theonly saving in using the three starters to control one bank of capacitors, is that you save two contactors, but replace one with one three times the rating.

 

You must be very careful not to apply the capacitors when the motor is not running. Closing capacitors on to an unloaded supply can cause very high voltage transients and resonance.

 

I would use one Capacitor contactor per starter and correct each motor individually.

 

Best regards,

[Madala]

Power factor correction can be achieved either using static correction, where each motor is corrected and the capacitor is controlled by the starter, or bulk correction where the correction is controlled by an automatic controller.

Where you have three motors only, it makes sense to use static correction.

As Jraef has stated, the capacitors must be connected upstream of the SCRs. Connecting on the output of the soft starter will causeSCR failure and possibly capacitor failure.

The capacitors must not be connected until the soft starter is up to full voltage. Most modern soft starters have a relay output that can be used to control a capacitor contactor.

I would suggest one capacitor contactor per soft starter controlling the correct capacitance for that motor. This will ensure optimum correction.

Controlling all the capacitance from one contactor with three times the capaciatnce will result in no correction untill all motors are operating, and will result in a much higher switching transient when that contactor closes.

You will still need the same amount of correction, so theonly saving in using the three starters to control one bank of capacitors, is that you save two contactors, but replace one with one three times the rating.

 

You must be very careful not to apply the capacitors when the motor is not running. Closing capacitors on to an unloaded supply can cause very high voltage transients and resonance.

 

I would use one Capacitor contactor per starter and correct each motor individually.

 

Best regards,

 

Thanks Marke, as stated before each soft starter has a top of ramp contactor which I considered using aux contacts as interlock so that only when all three of these are activated can the cap bank contactor be put on line. Also considering to add a 10 sec delay for this so that no feed back from contactors closing at that point.

 

Best regards

 

 

[marke]

Hello Madala

 

Why do you want to use one capacitor contactor and capacitor bank that comes in when all three starters are operating, rather than three capacitor contactors so that each starter is individually corrected?

I am curious as the normal way to apply static correction is to each starter. If you have two starters operatin, you have no correction.

 

Best regards,

[Madala]

Hello Madala

 

Why do you want to use one capacitor contactor and capacitor bank that comes in when all three starters are operating, rather than three capacitor contactors so that each starter is individually corrected?

I am curious as the normal way to apply static correction is to each starter. If you have two starters operatin, you have no correction.

 

Best regards,

 

Hello Marke,

 

No I am using either 3 banks or one bank dependant on cost. As stated all three motors are started sequentially and my worry is being feedback on to SCR's. If one motor stops the line has to stop so all three will be shut down. I install equipment all over the world so want to fit PFC units to all Some countries have bad voltage control so this will assist them especially in rural areas. I am open to any sugestion that you can put forward.

 

Best regards

[jraef]

I think that in your case it doesn't matter either way since the 3 motors operate as a single combined load.

[Madala]

Marke and Jraef

 

Thanks to both of you at least have peace of mind now and keep up the good work

 

God bless

 

Madala

[marke]

my worry is being feedback on to SCR's.

There should be no issues to the SCRs provided that the motor is running before the capacitors are connected. The main problem is when the capacitors are connected on a high impedance supply before the motor is started. The running motor provides an impedance across the supply and will dampen down any potential resonance issues.

If there was an issue due to closing the capacitor contactor, it would be worse with a single contactor than with the three separate contactors because the transient current will be three times as high.

 

Best regards,

[Madala]

There should be no issues to the SCRs provided that the motor is running before the capacitors are connected. The main problem is when the capacitors are connected on a high impedance supply before the motor is started. The running motor provides an impedance across the supply and will dampen down any potential resonance issues.

If there was an issue due to closing the capacitor contactor, it would be worse with a single contactor than with the three separate contactors because the transient current will be three times as high.

 

Best regards,

 

Thanks Marke will then use three PFC units so suggest that small delay before each comes in so Transient current is reduced then.

 

Best regards

[marke]

I have never used a delay to control the power factor contactor. I normally just use the "top of ramp" relay output to directly close the power factor contactor and have not had iny issues to date. If the soft starter manufacturer recommends a delay, then use it, otherwise, I do not believe that it is necessary.

 

Best regards,

[Madala]

I have never used a delay to control the power factor contactor. I normally just use the "top of ramp" relay output to directly close the power factor contactor and have not had iny issues to date. If the soft starter manufacturer recommends a delay, then use it, otherwise, I do not believe that it is necessary.

 

Best regards,

 

Ok thanks Keep well to you both

 

Best regards

 

[jraef]

I have never used a delay to control the power factor contactor. I normally just use the "top of ramp" relay output to directly close the power factor contactor and have not had iny issues to date. If the soft starter manufacturer recommends a delay, then use it, otherwise, I do not believe that it is necessary.

 

Best regards,

 

I agree, an additional delay is unnecessary, although a lot of engineers do it anyway. What I do is use an auxiliary contact of the bypass contactor to pull in the capacitor contactor. That way, you are ensured that the bypass is in the circuit before putting the capacitors on-line. Sometimes the end-of-ramp contact may close prematurely, but if the bypass is pulled in, it will not matter.