PF capacitors and VFDs

[cvirgil467]

Harmonics from VFDs can damage pf capacitors.

Lets say i have a motor control center with a bunch of 3 phase motors. Some motors have VFDs while others do not. Since there is no neutral conductor installed to the motors, how do harmonics travel to the PF capacitor the capacitor and damage the capacitor? Assume PF capacitors are insatlled on the load side of the motor starter.

It seems to me that the PF capacitors will cause more problems than solve them when installed with individual motors or a group of motors with VFDs.

 

[pwhite]

Re: PF capacitors and VFDs

while i'm no expert, i called my loacal Ab rep and here's how i understand vfd's.

vfd's as you know change the frequency of the power being fed to the motor. all motors connected to vfd's should be rated for vfd use, otherwise, they will run hotter and the wiring insulation could fail.

an ac vfd has 2 main functions, #1 is to convert the incomming power (460 volts 3 phase for example) to dc. the second part of the vfd converts this dc power into a pulse width modulation waveform or pwm (dc power). this dc pulse width modulated waveform then powers the motor.

because this is dc power, part of this waveform is reflected back to the inverter. the amount of power reflected back to the inverter is dependent on factors such as motor horsepower, distance from vfd to motor, wire size, wiring insulation, and others.

all vfd manufacturers recommend a load reactor be placed between the vfd and motor. this is especially true when these variables come into play.

without this load reactor, the reflected waveform will travel through the vfd and into other powered items. please see your vfs manufacturer for recommendations.

 

[jtb]

Re: PF capacitors and VFDs

If you need a motor for a VFD system, the type is usually referred to as "Inverter rated". The main difference is the coil insulation, sleeves to prevent Corona discharge (PWM pulses are High Voltage spikes or square waves, modulated in time to produce a RMS equivalent to a sine wave at the frequency desired), and various techniques of winding.

 

[tom baker]

Re: PF capacitors and VFDs

A power factor capacator is not compatable with a VFD. If you don't know you have one and install a VFD, you will find out!
The issue of inverter duty rated motors is not related to a capacitor. Filters or chokes are often used on the load side of the VFD.

 

[cvirgil467]

Re: PF capacitors and VFDs

Tom,

Capacitors can be used with vfds and other high harmonic equipment. Capacitors are usually specified with iron core de-tuning reactors and filter reactors to mitigate the effects of harmonics when used with vfds. If not, the capcitors can be damaged by the harmonics

So far the all posts are informative but none have answered the question.

 

[harryg]

Re: PF capacitors and VFDs

cvirgil,
take a look at this

 

[cvirgil467]

Re: PF capacitors and VFDs

Thanks for the link.

In the past i have not had problems using pf capacitors with vfds but the article has some good points.

All of the literature i have on applying pf capacitors state that thye can be used with vfds when installed with iron core reactor and de-tuners to protect the capacitors against harmonics. The otherside of the coin is damaging the vfd due to capacitor switching transients.

 

[bwilson]

Re: PF capacitors and VFDs

sorry