Multiple VFD's on Single Iso Transformer w/ GFCI
Multiple VFD's on Single Iso Transformer w/ GFCI
(OP)
Hello all,
The application consists of four 1/2HP 115V 1Φ In / 230V 3Φ Out VFD's powering 1/2HP gearmotors that drive simple conveyors. The conveyors are dual narrow urethane V-belts riding on steel guides carrying egg flats. The VFD's will be loaded at approximately 75%. The machine has ground fault protection for personnel (Class A device) due to machine operation in a washdown environment. Also, the machine requires CE rating and therefore has an EMI/RFI filter on the incoming line. The drives are tripping the GFCI. An Isolation Transformer is being proposed to prevent the GFCI from tripping.
Common mode chokes on the motor leads are being proposed in lieu of shielded cable. A toroidal ferrite core has been applied (5 turns) to the PE (ground) conductor feeding the VFD enclosure. The GFCI is upstream of the VFD cabinet at the power feed to the machine. The drives manufacturer representative recommended increasing the carrier frequency in an attempt to minimize the leakage current.
1) What are the design considerations when applying multiple VFD's on one isolation transformer?
2) Should the transformer be K rated and/or shielded?
3) What are the design considerations when applying VFD's with a GFCI?
4) Is it practical to utilize VFD's and GFCI's, or in other words, is there any hope of preventing the GFCI from tripping?
5) Any other comments on the application?
Thanks everyone for your time.
Amp
The application consists of four 1/2HP 115V 1Φ In / 230V 3Φ Out VFD's powering 1/2HP gearmotors that drive simple conveyors. The conveyors are dual narrow urethane V-belts riding on steel guides carrying egg flats. The VFD's will be loaded at approximately 75%. The machine has ground fault protection for personnel (Class A device) due to machine operation in a washdown environment. Also, the machine requires CE rating and therefore has an EMI/RFI filter on the incoming line. The drives are tripping the GFCI. An Isolation Transformer is being proposed to prevent the GFCI from tripping.
Common mode chokes on the motor leads are being proposed in lieu of shielded cable. A toroidal ferrite core has been applied (5 turns) to the PE (ground) conductor feeding the VFD enclosure. The GFCI is upstream of the VFD cabinet at the power feed to the machine. The drives manufacturer representative recommended increasing the carrier frequency in an attempt to minimize the leakage current.
1) What are the design considerations when applying multiple VFD's on one isolation transformer?
2) Should the transformer be K rated and/or shielded?
3) What are the design considerations when applying VFD's with a GFCI?
4) Is it practical to utilize VFD's and GFCI's, or in other words, is there any hope of preventing the GFCI from tripping?
5) Any other comments on the application?
Thanks everyone for your time.
Amp





RE: Multiple VFD's on Single Iso Transformer w/ GFCI
A CE rated VFD has an internal noise filter which will trip the GFCI. You can't help it.
Interesting suggestions tho. Increase the carrier frequency to reduce leakage to ground! Exactly the opposite is true.
RE: Multiple VFD's on Single Iso Transformer w/ GFCI
My thoughts exactly on the carrier frequency. However, a Fluke 360 Leakage Current Clamp was used to measure the current in the PE (ground) conductor at the different carrier frequencies. At the high carrier freq, the current in the PE (ground) conductor decreased! I don't quite understand this result as it is counter-intuitive to my understanding of common mode leakage current.
RE: Multiple VFD's on Single Iso Transformer w/ GFCI
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Is this in the US though and/or are you concerned with the NEC? If so, I don't believe GFCIs are required for hard-wired systems like conveyor motors, only if they are plugged into a socket with a portable cord.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Multiple VFD's on Single Iso Transformer w/ GFCI
This application is an 'as wet as wet can be' application. It utilizes a frequency inverter for the pumps(s) and can be connected to a grid with a standard RCD/GFCI.
Test protocol (pages 10-12): http://w
Same inverter built into the system as I have told about earlier. I have measured around 0.7 mA leakage current in an application with a 1.5 kW inverter.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Multiple VFD's on Single Iso Transformer w/ GFCI
If you must have an RCD (it's going to Europe - what's a GFCI?
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RE: Multiple VFD's on Single Iso Transformer w/ GFCI
I for one am confused.
115V input and Class A GFCI? That would be North America.
CE listing? That would not.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Multiple VFD's on Single Iso Transformer w/ GFCI
I find it interesting that RCD's in Europe trip at levels high enough to be fatal (30mA). Are these devices for personnel protection?
In the US, the machines are typically sourced by 120VAC (240VAC in Europe).
The NEC requires all 125VAC circuits in wet/damp locations in Agricultural buildings to be GFCI protected (Article 547). This is a washdown application.
RE: Multiple VFD's on Single Iso Transformer w/ GFCI
RE: Multiple VFD's on Single Iso Transformer w/ GFCI
On the 'fatal' comment, 30mA is at the bottom limit of respiratory paralysis and is below the 'let go' threshold. Kinda depends on the reference! I'm sure 30mA RCDs have saved a few lives compared to not having them.
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If we learn from our mistakes I'm getting a great education!
RE: Multiple VFD's on Single Iso Transformer w/ GFCI
amptramp; As for your Fluke. I'm not familiar with that one but it strikes me as it could easily not be seeing the leakage that's at a high frequency. It could well be rolling off faster than the leakage is increasing which would give you your unexpected results.
VFDs all have different amounts of leakage. I'd check around and find one with the lowest. I also have one next to me that is plugged into a domestic GFI outlet and never trips it. So I know it can be done. It even sez in the manual "Don't bother trying to run this VFD thru a GFI".
Using one GFI per VFD for VFDs as small as you're using would probably work fine.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Multiple VFD's on Single Iso Transformer w/ GFCI
This document is about European appliances and RCD trip levels: http:
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Multiple VFD's on Single Iso Transformer w/ GFCI
I think these "let-go" levels are the basis for the US 5 mA level for GFCI protection. But the 5 mA level can be a nuisance at times, that's for sure.
David Castor
www.cvoes.com
RE: Multiple VFD's on Single Iso Transformer w/ GFCI
My thought is that the European 230 V level makes sure that at least 30 mA are produced when you touch a live wire and that the 100 milliseconds breaking time demand is what saves lives.
When you have a lower voltage, around 110 V, then the current produced is less and the 'must operate' limit is set lower to make sure the GFCI operates reliably.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Multiple VFD's on Single Iso Transformer w/ GFCI
Also, another aspect of the machine installation is the hatchery often has GFCI (RCD) protection for the receptacle powering the machine, so it is not feasible to provide GFCI (RCD) protection for each individual VFD. I must attenuate/minimize the leakage (common mode) currents for 4 drives. Is anybody familiar with the different types of GFCI (RCD) devices, i.e., Type AC, A, and B. I believe the device jraef mentioned must be a Type B as it is VFD "rated".
Also, I found this link to an interesting publication on electric shock and protection means. There is a graph on page F2 showing body current level versus duration time and associated categories of "damage". It seems to be written from an IEC point of view.
http:
RE: Multiple VFD's on Single Iso Transformer w/ GFCI
This is important for RCD's looking at the opposite side of nuisance tripping; i.e. when they don't trip due to a waveform a typical type A RCD would not recognise.
If you have a fault where a distored waveform is evident, and the RCD does not pick this up then it is a bigger problem of not tripping than doing so.