CAPACITOR CONNECTION TO VARIABLE-FREQUENCY DRIVE for Improving Power Factor.

In VFD, the system AC is first converted to DC and then the DC is converted to AC which is supplied to the Motor. The intension of using the capacitor is to improve your system power factor. So, you have to use it before VFD.

Note: the capacitor is also used in DC part of VFD to filter the DC to make it smooth but you mentioned the use of capacitor is for PF improvement.

Hope it helps.

- MS

Dear Mr.MSAMAD,

Thank you for your reply.

The capacitor addition - It is for improving the Power Factor.

At higher speed hence higher Frequency, the Inductive Effect is more

and hence I am of the opinion that in order to compensate for the

fall in Power Factor, the Capacitor to be added close to the input supply

to the Motor and in this case of VFD. it is after the DC to AC conversion

to a higher frequency goes to the motor.

DHAYANANDHAN,

INDIA.

Yes, I agree wholeheartedly. Do not connect capacitors at the motor side of VFD. It will only cause 'Overcurrent' trip on the short circuit protection of the inverter output stage (doesn't harm VFD as protection is 'instant' and switches OFF all devices). There is also no need to fit Power Factor Correction capacitors at the power input as these are only fitted to improve on out of phase currents (improve displacement)

The capacitors in the DC link accept the reactive power drawn by the motor (lower pf) so this is the reason that Displacement Power Factor is near unity at the power supply side (normally 0.97+).

If you add reactors to the DC link (DCReactor) and/or the AC input this will dramatically reduce RMS input current improving the fundamental power factor, reamaining harmonics are more costly to correct by passive or active input filters, as you indicate. The DC choke is the most cost effective measure to improve RMS input current most effectively.

Dear Friend,

Are these motors (5kW to 160kW and 375kW) connected in one (1) busbar as motor control center (MCC)?

If all these motors connected in one (1) busbar as MCC, then compute the total requirement of kVAR you need, then make or order one cubicle for your capacitor connected to the main busbar to compensate the whole motors' power factor.

Your power factor corrector (capacitor bank) should be self-healing type, complete with all protections and monitoring devices.

As other advices, never-never connect your capacitor downstream of vfd,

My above advice is being used for vfd with by-pass only. I'm sure it has a by-pass.

Eren

eren,

VFD with bypass??? Perhaps you are thinking of a soft start with bypass. Any bypass type of arrangement with a VFD would require break-before-make contacts and proper protection to ensure that the bypass and VFD are in phase with each other before operation. Furthermore, the sudden removal of the load from the VFD could damage the output section and leave a motor with a sudden and large mechanical and electrical shock-load. If there are such devices, I would like to know more.

--JMM

I work with inverters that have bypass prepared in control logic (just add contactors, sequencing done), in order to use VFD for starting and stopping the motor + Load.

Some inverters have output switching proof also to disconnect power with no damage. Reconnection entails matching the output to rotating motor speed or else plugging currents are demanded which cause tripping and potential damge to VFD. The removal of power usually has no impact on the motor but the reconnection could if done wrong.

If you have such applications and would like more information I could arrange to send or link.

Dear Mr. Malcolmk,

Thank you for your response and offering to send more link.

I am intrested to get more details from you and request you to send the links.

Thanks,

dhayanandhan.s

To: Mr. jmueler,

Why? Have you not designed this for the large compressors and pumps? Try to ask Designer and Supplier like Siemens and Allen Bradley and others manufacturers of MV VFD,,, maybe you may add this to your experience.

Eren

Dear Mr. EREN,

Thank you for your comments. All the drives which I have mentioned is in different locations and only 2 motors are in the same MCC. Hence, in my opinion, arriving at equivalent capacitors will not arise.

From this forum, I understand that capacitors should not be connected after VFD. My thinking is the motor winding heat while working is to be controlled, which may be due to less power factor as for as the motor is concerned.

I am extremly happy to see a large number of CR4 MEMBERS repliedFor my quiery.

THANKS TO CR4 FORUM ALSO.

DHAYANANDHAN.S,

INDIA.

Thank you both MalcolmK and enereduarte. My experience, though significant, is obviously dated now. Good to hear of continuing developments in the field.

--JMM

Power factor correction is usually done way upstream of any <...VARIABL FREQUENCY DRIVE...>, because its purpose is to better match the characteristics of the aggregated loads to the incoming supply and its feeder cables. The financial justification for doing it is based upon a penalty charge in the supply tariff. The technical reason for it is to make the supply cables run cooler by reducing the heating losses associated with current that is out-of-phase with the applied voltage.

Adding it downstream of a <...VARIABL FREQUENCY DRIVE...> is just plain nonsense.

If he did it 11years ago when this was posted, I'm sure he has suffered the consequences and learned his lesson by now...