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PF correction capacitors in the front end of a VFD
2

PF correction capacitors in the front end of a VFD

PF correction capacitors in the front end of a VFD

(OP)
In my motor repair shop, we test-run a wide variety of LV and MV motors on no-load (open shaft). The present method is voltage control (direct voltage control for LV motors and a step-up trafo for MV motors) via a stepless autotransformer with PF correction caps across the auto transformer secondary to keep the current drawn from utility to a minimum.

The drawback of this method is the sacrifice of starting torque which results in some motors not starting at all (at least not without a high inrush current) or some of them not being accelerated beyond a certain speed.

So, I plan to use a VFD (415 V, 500 amps rated) to improve upon the torque to get the motor up to its rated speed. Some of these motors have a no-load current as high as 500 amps on the 415 V side. So I plan to use cap banks on the front end of the VFD so that the primary current drawn from the utility is still kept low.

So, what do the drives gurus think about this ?  

Muthu
www.edison.co.in

RE: PF correction capacitors in the front end of a VFD

(OP)
The proposed set-up

For LV motors - Utility supply feeding both the capacitor bank (intially set to match motor no-load current) and the VFD front end simulatneously. Then ramp up the speed slowly over 2-3 minutes.

For MV motors - Same as above except that the VFD output will now feed the input of a step-up trafo (415V/3.3-6.6 KV), whose output will feed the MV motor.

Muthu
www.edison.co.in

RE: PF correction capacitors in the front end of a VFD

Done that. But no PFC capacitors.

The problem is that your motors are the ones with the bad PF The drive actually has a very good displacement power factor, close to unity for the fundamental. But it usually has a very bad P/S power factor (distortion PF).

There are CFDs with a so-called Active Front End (AFE) that betters the distortion PF by pulling a clean sine wave from the grid. They cost more, but may be the way out for you if your grid is weak.

I know a few winder shops that have installed diesel generator sets to be able to test run large motors without having to pay big bucks for the utility to keep power, that they use very sporadically, available for them.  

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: PF correction capacitors in the front end of a VFD

Typo: CFD shall read VFD.
 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: PF correction capacitors in the front end of a VFD

A generator is my thought also. You may consider connecting the motor before starting the gen-set and ramping up with a fairly constant V/Hz ratio.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: PF correction capacitors in the front end of a VFD

I wouldn't recommend having PF correction capacitors on the same supply as the VFD - in your case 415V. Put them on the HV side and/or via an isolating transformer.
I have had cases where the PF capacitors were on the same bus bar as the VFDs and the VFD dc link capacitors failed. This was solved when the PF capacitors were switched off and subsequently moved.

RE: PF correction capacitors in the front end of a VFD

(OP)
Thanks Gunnar. AFE VFD's will be way out of my budget. When you say that with AFE, PF is closer to unity, do you mean that the current on the VFD front end drawn from will be around 50 amps even the motor no-load current is 500 amps ? Because, right now, I am able to keep my utility input current to about 50 amps max through my variac+700 KVAR capacitor method.

Bill - Yes, I use 1500 RPM diesels but the minimum cranking speed is 1000 RPM so using it as a ASD seems to be ruled out.

Drivesrock - Suppose I feed the VFD directly from the utility bus and I feed the caps via a continuously variable auto (variac), would that constitute an isolated feed ?

Muthu
www.edison.co.in

RE: PF correction capacitors in the front end of a VFD

Edison123,

I would suggest to take a look at the generator reactive capabity curve to make sure the generator will suppot the start-up of VFD. We sucessfully started VFD-Compressors by using one generator. With the existenece of the capacitance in front of VFD,  you should size the generator properly.  

RE: PF correction capacitors in the front end of a VFD

The VFD will only draw the required real power from the line. The real power is the power you will be using to actually turn the motor, plus supply the VFD losses and motor losses. Power factor capacitors will gain you absolutely nothing in this situation.

If the power system can not handle the VFD input current distortion - you have too much voltage distortion - then look at methods to mitigate the current distortion. Typically, a line reactor will drop the THDi from around 100% to around 30%. Then, you're looking at a more complex harmonic filter (look at a Mirus Lineator) or use a 12-pulse or 18-pulse rectifier.
 

RE: PF correction capacitors in the front end of a VFD

(OP)
Thanks Lionel for that definitive "VFD will only draw the required real power from the line". Voltage distortion is not a much of a problem for me. The utility supply is strong but I get penalized heavily for pf < 0.95. Hence my need to keep the input current very low.

Another question - how will the VFD handle a continuously low pf motor load since the motors will be running continuously only on open shaft for 8 to 12 hours ?

I am looking for a good reliable VFD vendors in India. I'll post here the specs once to I get them for your analysis.

Muthu
www.edison.co.in

RE: PF correction capacitors in the front end of a VFD

It won't make any difference to the VFD if the motor is loaded or not.

I'm not sure what manufacturers are represented in your part of the world. Maybe look at Yaskawa or LS.
 

RE: PF correction capacitors in the front end of a VFD

(OP)
Thanks Lionel. We have Schneider, AB, ABB, Siemens et. al. here.

Muthu
www.edison.co.in

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