VFD & PFC

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ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
I haven't had to visit this issue for quite some time and Yes, it is more than I know.
H =SQRT((C37/D37))

When looking for the harmonic frequency of a ___KVAR capacitor what short circuit value should I use? The value @ the transformer or the calculated one near the load?
HzKVAscKVAR
H (HARMONIC FREQ)70.715,000.001.00
50.005,000.002.00
40.825,000.003.00
35.365,000.004.00

or

H (HARMONIC FREQ)127.9816,378.411.00
90.4916,378.412.00
73.8916,378.413.00
63.9916,378.414.00




.
 
I'm not quite sure what you are asking. I wouldn't use a PFC capacitor with a VFD. The VFDs are non-linear but not phase displaced. The linear elements are 5th, 7th, 11th, 13th etc
 
I am not using a capacitor with a VFD, but I do need them on the other motors.

What I have had issues with in the past, IIRC, was a harmonic generated between the VFD and the typical motor PFC. The PFC would fail, sometimes rather spectacularly even when located some distance away.

Engineering from the Capacitor mfg told me the issue was with having VFDs. even though they were a small portion of the motor load.
 
ptonsparky said:
I am not using a capacitor with a VFD, but I do need them on the other motors.

What I have had issues with in the past, IIRC, was a harmonic generated between the VFD and the typical motor PFC. The PFC would fail, sometimes rather spectacularly even when located some distance away.

Engineering from the Capacitor mfg told me the issue was with having VFDs. even though they were a small portion of the motor load.
Click to expand...
A OK. We used de-tuning reactors on our Static Kramer drives. We tuned them to 177Hz.
 
In your table are you showing the output frequency of the VFD?
I believe your failures of PFC capacitors could be from a resonance of these capacitors with the inductance of the motors which is driven by currents at harmonics of 60 Hz. The rectifier at the front end of the VFD will produce such harmonic currents. There will be a strong harmonic at 120Hz with a full wave bridge rectifier in addition to others.

I think it's less likely that the problem harmonics are related to the VFD output frequency because the DC bus between the input and output provides some level of isolation (although not full galvanic isolation).

Are the PFC capacitors connected directly to the motors, or is there a bank of capacitors for all motors?

The value of the capacitors might be adjusted to move the resonant frequency so that it's further removed from a harmonic.
 
The problem from what I understand is any harmonic that may be generate by the VFD on the incoming lines. Harmonic distortion?
So... if I had a harmonic of 128 hz that would give me problems with a 1 kvar.

We had an issue with a 20 kvar for a 60 HP motor that failed a couple times. It has not been replaced since. Right or wrong, using that logic, the 20 would have had issues with a 29.hz harmonic.

Using the 16.378 KVA short circuit capacity at the transformer.
Changing to 5 KVA to that available at the load, would change the harmonic. I think.

I know it is well above my pay grade, but I am curious.

They doo offer hardened PFC for a premium.
 
ptonsparky said:
Yes, I may have found the harmonic frequency of a 20 kvar but 29 Hz won't be found on the line side of the VFD when my system is 60.

How do you even model for the KVAsc? Just tune them all to 177Hz?
Click to expand...
The harmonics are all reactive and 5th upwards up. We used rated current sized for the VFD drive and its harmonic content.
 
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