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VFD to convert 240 single phase to 208 3 phase and some problems

5K views 21 replies 12 participants last post by  Almost Retired 
#1 ·
I am working on a project to power an environmental chamber that requires 208. We only have 240 single phase. The chamber is wired delta with fan motor on one phase, a chiller compressor on the second phase, and a resistive heater on the third phase. The loads are not balanced and the chamber specifies a maximum of 20A load on each phase. It is expected that there will be cases during operation where there will be no load current on multiple phases. We bought a 7.5 kW LSIS VFD , as well as some inductors and capacitors to use to filter out the VFD noise.

To check out the VFD and filter set up, we just connected the filter to the VFD with no load. Just after start up, the VFD trips at 1 Hz and 150A with either 120V or 240V input. We think the filter inductors are saturating and the transient current through the 2.2 uF capacitors is then getting very high.

Our inductors are toroid ferrite cores with 3 windings each 470 uH 25A rated. Each weighs about 0.4#. We put 6 ( about 2.4# total) in series between the CFD and the capacitors. The capacitors were wired from each phase to a common node and then across each phase with no difference in VFD tripping.

Most filter inductors you buy for this purpose are steel laminated e-cores with 3 windings. A 2400 uH 25A rated one of these bad boys weighs about 28#.


Could our ferrite inductors be saturating right away?

Ferrite saturates much easier than laminated steel, but we wanted to save weight and volume. We will connect the capacitors to the VFD output without the inductor string to see if the VFD still trips out. We also will measure the capacitor current to see if anything interesting is going on.

With no load, will we have to worry about very high voltages on that might cause damage?

The LC filter resonates at about 2 kHz and the VFD chops at around 4 kHz.

Will the VFD PWM frequency be constant so we don't have to worry about it running at the filter resonant frequency?
 
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#3 ·
I am working on a project to power an environmental chamber that requires 208. We only have 240 single phase. The chamber is wired delta with fan motor on one phase, a chiller compressor on the second phase, and a resistive heater on the third phase. The loads are not balanced and the chamber specifies a maximum of 20A load on each phase. It is expected that there will be cases during operation where there will be no load current on multiple phases. We bought a 7.5 kW LSIS VFD , as well as some inductors and capacitors to use to filter out the VFD noise.

To check out the VFD and filter set up, we just connected the filter to the VFD with no load. Just after start up, the VFD trips at 1 Hz and 150A with either 120V or 240V input. We think the filter inductors are saturating and the transient current through the 2.2 uF capacitors is then getting very high.

Our inductors are toroid ferrite cores with 3 windings each 470 uH 25A rated. Each weighs about 0.4#. We put 6 ( about 2.4# total) in series between the CFD and the capacitors. The capacitors were wired from each phase to a common node and then across each phase with no difference in VFD tripping.

Most filter inductors you buy for this purpose are steel laminated e-cores with 3 windings. A 2400 uH 25A rated one of these bad boys weighs about 28#.


Could our ferrite inductors be saturating right away?

Ferrite saturates much easier than laminated steel, but we wanted to save weight and volume. We will connect the capacitors to the VFD output without the inductor string to see if the VFD still trips out. We also will measure the capacitor current to see if anything interesting is going on.

With no load, will we have to worry about very high voltages on that might cause damage?

The LC filter resonates at about 2 kHz and the VFD chops at around 4 kHz.

Will the VFD PWM frequency be constant so we don't have to worry about it running at the filter resonant frequency?
Depends on the VFD as far as carrier frequency. Not sure which one you bought. Read the manual.

Ferrites have a band pass effect and effectively are limited to several tens of KHz to Megahertz ranges. Good for EMI but terrible for power filters. Yes you have higher mu but it’s very frequency dependent. Siliconized steel doesn’t have that problem. The thinner it gets the smaller the domains which helps reduce saturation but at higher expense and mu rarely gets over 100. If you truly want excessively high and high bandwidth then the best way is using nano crystalline cores which can get up to 30,000-80,000 with reasonably high bandwidth but it has serious saturation limits considering the extremely high mu. I’ve had to use huge stacks of the stuff in filters handling tens of Amps. So may want to do this in stages. Ferrites OK for say 50 KHz or higher. Iron cores good for low frequency but it’s hard.

AND do not for a minute discount the REAL inductor model. You can’t avoid the resistance. In conjunction with capacitors you can easily end up with effectively a dead short. Running a frequency sweep and measuring filter performance can fool you because magnetics are pretty nonlinear.
 
#15 ·
Thanks for all the information. It might be worthwhile looking into rewiring for three single phase loads if we can make the internal controller work.
Paul, I am leaning towards the idea that our small sized inductors are saturating. We should be able to see that happen while monitoring the capacitor current.
 
#16 ·
Your going have a TON a problems with the heaters!

I did a single phase to 3 phase wafer machine and the VFD just crapped. So I called some old work mates at Eaton and I installed a VFD isolation transformer, backwards. Raised the min HZ up to about 66 and the project was off and running.

Wafer machine was used by the nuns to make money. My boss a Catholic kept telling me we were all going to hell if the project failed. With some help from some real electrical engineers the application worked flawlessly. Except for the extra 3 grand for the transformer.

Using a drive to make 3 phase works fine on rotating equipment when size correctly.


You have single phase loads and the drive is not going to like them. At least my understanding is single phase loads.
 
#20 ·
Finally got back to this project. Using a Danfoss 130B2446 sine wave filter with the 2.2 uF capacitors worked very well. The VFD is quite happy driving the motor, compressor, resistive heater load we have in our equipment. The Danfoss inductor is heavier than the ferrites we started with and runs at 47C vs 90C for the ferrite inductors. So there is more core and the laminated steel is much harder to saturate than ferrite. The ferrite has no problem with low frequencies but it is easier to saturate which is what caused the problems we were having.

We also had the the SSRs driving the heater blow up because they had to deal with multiple zero crossings in the noisy poorly filter VFD output. With a good filter we longer kill SSRs!!


Thanks for all the advice, we learned a lot on the project and I would have no concern about driving an unbalanced load like this with a VFD.
 
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