Capacitor power connections in Star-Delta starter 5

[panou]

Hello to all.
I have a star-delta starter for a 37kW motor(400V,50Hz) and i want to connect the 30kVar capacitors to the circuit. What is the best connection scheme that i should use ? I want also to use a capacitor contactor. Thank you very much for your time.

 

[laundry]

Hello panou
you can only connect your capacitor to the supply side of your starter,Also you will have to fit a circuit breaker.30kvar is high for a 37kw motor?

 

[electricpete]

laundry is absolutely correct. Switching those large caps with the motor creates big risk for overexcitation during motor coastdown. Why switch such large capacitance with the motor anyway?

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[edison123]

About 15 KVAR should do for P.F. correction.

 

[jraef]

Caps must be on the line side of any kind of starter system where there is a transition that may happen before the caps can discharge. That would include reversing, 2 speed, and all electro-mechanical reduced voltage starters such as Y-Delta, autotransformer, reactor and primary resistor starters. It is also best practice to have a separate contactor control the caps after the motor is at full speed so that you are not "floating" the caps on-line when that motor is not running. And yes, those caps are troo large for that motor. Overexcitation presents a lot of potential problems and should be avoided.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[panou]

First of all thanks for your response.
I have already purchased Siemens capacitor contactors and i am thinking to energize the capacitor when the D-contactor(triangle-full speed) is on. I will connect them after the fuse-switch disconnector instead of using dedicated fuses for the capacitors only. Is it right or not ?

 

[laundry]

Hi panou.
This is wrong you need your capacitor at the live side of your contactor. Say inbetween your isolator and common contactor,With fused or circuit breaker for protection.
the reasen for you putting the capacitor in the system is to give you bitter power factor.connecting your capacitor to the delta will nock your powerfactor to hell,I take it this is why you are fitting the capacitor in the first place.

Barry.

 

[jraef]

Panou,
It is good practice to fuse the capacitors separately so that the fuses are sized close enough to the capacitive current to do some good. By relying on the motor circuit fuses alone, the time it takes them to react and blow may be too long for the caps. That can lead to the caps popping, oozing electrolyte all over everything.

Laundry,
You jumped to a conclusion. He didn't say he was connecting it downstream from the delta contactor, he said he would have it come on after the delta contactor closes and connect them downstream of the fuses, which would be correct. Please read the posts more carfully so that your responses don't confuse the original poster.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[laundry]

HI jraef.
the capacitor is to be fitted to a star/delta starter,so you have to connect it to the supply and not come in when the delta contactor comes in,the all point is to correct your fower factor and reduce your current,that wey will not.

Barry.

 

[jraef]

Huh?
Laundry, no disrespect intended, but it appears as though perhaps English is not your first language and maybe we are having a simple communications issue here. Please let's not side track this thread any further.

When the delta contactor closes, the motor is essentially DOL at that point, which is when you want the PF correction capacitors on-line. The only issue here is that you do not want them on-line when the starter transitions from start to run.

 

[electricpete]

jraef - you are probably right. But please lighten up a little. The problem description is not clear to me and English is my native language.

Why do we have such a large set of caps being cycles on and off in conjunction with the motor? It is far more than magnetizing current of the motor. If it is just general power factor correction for the whole plant, why is there need to switch these caps in conjunction with this motor?

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[electricpete]

ok, I get the picture now it is plant-wide power factor correction. The caps are switched out during motor switching so they don't feed the current spike on transition.

Just from curiosity, would the caps be switched out any time any start/delta start motor from the same transformer is started?

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[jraef]

That’s a good question electricpete. It seems to me that any PFC caps on a circuit where that Y-D transition spike happens could exacerbate the problem. Don't know for sure though, not really my area. Power quality folks would be better at that. Maybe Marke will read this thread and comment. He's better at that than I am.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[Marke]

Hello panou et al

If you are intending to use 30KVAR for a 37KW motor, then you are seriously over correcting. Static correction should be limited to 80% of the magnetising current of the motor unless you are using a dedicated contactor for the capacitors. For a 37KW motor, I would expect the magnetising current to be in the order of 25 - 40% of the rated current of the motor, so I would expect that the correction should be in the order of 10-15KVAR.

I have seen many installations with static power factor correction applied to star delta starters and the capacitors switched by the main contactor. I suspect that the connection of the capacitors to the starter will reduce the transient supply impedance at the starter and would probably increase the magnitude of the current and torque transients that occur after the open transition. I also suspect that there may be problems if the capacitors are connected to the output side of the delta contactor as during the open transition, you have a current path through the motor winding, through the capacitor and if this was near full correction, there would be effectively a very low impedance from the series LC circuit and potentially a high voltage generated. Bearing in mind that the motor is not connected in delta under these conditions, the magnetising current would be 57% of the delta mag current and if the correction was 80% of the delta mag current, the resonat frequency would be below line frequency and this is not desirable. In fact IT IS DANGEROUS!!. This is only a problem if the capacitors are connected to the output of the delta contactor and only while both the delta and the star contactors are open.

I would recommend a separate contactor for controlling the capacitors. I would connect this to the input side of the delta contactor and I would close this contactor after the delat contactor is closed. You could use an auxiliary contact on the delta contactor to close the capacitor contactor.

Please check the required level of protection required for that motor. It is not appropriate to try to bulk correct the plant based on the state of that motor only unless there are a number of motors that all operate at exactly the same time.
Either use the appropriate amount of correction for each motor controlled in sympathy with each starter, or use a power factor control relay to control bulk capacitors.

Best regards,


Mark Empson

http://www.lmphotonics.com

 

[laundry]

Hi again jraef.
I am english I live in the uk.I also have BSc Hons.MSc. yes with the delta contactor in you are DOL if you look at it that way,But the question was the best way to connect the capacitor to correct his power factor, and this is it fit a slave contact off the common contactor.Take the three phases from the live side of the common contact through circuit breaker or fuses,Now connect the three phases to the slave contactor L1 L2 L3,and connect your capacitor to the other side.The reasen for this is that you want to correct the PF on start up and not just in run.

Hello panou. please read the above.Also if you can tel me what your PF is and what KW loading your system is using I will work out the correct Kvars for you.

Barry.

 

[panou]

Thank you all for your time and advise.
I am doing what most of you suggested. Connect the capacitors to the live part of the contactors (after the fuse disconnector) and give control voltage to the capacitor contactor from a auxiliary contact of the delta contactor.
laundry the motor is 0.79 pf (from the ratings plate of the motor) and i want to make this 0.98. The loading is 60Ampere. But the problem is that i want to use this spare capacitor bank of the 30 kVar.

 

[laundry]

Hello again panou.
To raise your PF from 0.79 to o.98 requires 21kvars.
TO give you 1.0 requires 28.49kvars,you are over correcting by 1.51 kvar with your 30kvar bank.This is nothing to be bothered about, in fact if you look at your meter when your motor is running you will notice it is virtually stopped.

Barry.

 

[Marke]

laundry, not sure where your calculations come from, but if the line current is 60A at 0.79 and you want to correct to 0.98, the correction you need is 18.8 KVAR. To 1.0 (not recommended), the correction is 25.5KVAR

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[laundry]

Hello Mark. my calculations are based on motor kw load
0.79 cos theta tan =0.77
0.98 cos theta tan =0.20
0.77-0.20=0.57x37kw=21.09.kvars.


Barry.