Isolation Transformer for AFE drive
Isolation Transformer for AFE drive
(OP)
Is it necessary to have a drive isolation transformer for an active front end drive? There is no requirement to step down the voltage for the drive input.
The VSD would be LV (440V) and the power system is unearthed.
The VSD would be LV (440V) and the power system is unearthed.





RE: Isolation Transformer for AFE drive
RE: Isolation Transformer for AFE drive
The only problems I know about are that the insulation monitors (Bender and such devices) sometimes have a problem with the idling AFE, seems to be some kind of sampling effect that tricks the Bender device to believe there is a true resistance to ground. Loading the drive makes the problem go away. Which is unexpected, but can be explained.
The other problem is more severe. An AFE (or any PWM drive) will usually introduce high PWM common mode voltage residues on the ungrounded system. This sometimes causes problem in other systems connected to the same grid. DC motors, for example, are known to have heavy bearing EDM on such a grid.
But, as I said, many systems work quite well on ungrounded grids. There is an issue with EMC filters, though. Your supplier can guide you there.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Isolation Transformer for AFE drive
RE: Isolation Transformer for AFE drive
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If we learn from our mistakes I'm getting a great education!
RE: Isolation Transformer for AFE drive
So, depending on what mains supply you have and your AFE or drives type could define if you need an isolating transformer to either earth or not the neutral.
But then there is the issue of where you are applying the drive - industrial or domestic environment? In an industrial environment I believe you would not have to have an isolating transformer (unless your mains is neutral earthed but you AFE cannot have that supply or visa versa). In a domestic environment - I'm thinking of maybe a lift AFE VSD in a block of flats - then you may need an isolating transformer and/or PWM/EMC filters as mentioned above.
I am intrigued as to why you have a floating supply - and do you already have VSDs on that power system? And then of course the questions are why and what is the manufacturer of the AFE?
I would agree that VSDs work much more reliably on earthed neutral power systems or dc link neutral point earthed. The DC link voltage - in your 440V case approx 620Vdc across DC+ to DC- rails - will then be 310Vdc from either rail to earth and should always be so. This means that the DC capacitors and power transistors and other internal components should always be at the same potential to earth - ie not floating. It's the floating around voltage that these components could have to earth that can reduce their working lives or causes a failure in VSDs.
Drivesrock.
RE: Isolation Transformer for AFE drive
I have seen floating grids in a few places. Last week, in Norway, for instance. The whole country has an ungrounded three-phase system.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Isolation Transformer for AFE drive
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Isolation Transformer for AFE drive
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Isolation Transformer for AFE drive
Sorry, the application is a ship, hence the 3 phase input voltage is derived from an IT isolated neutral system, i.e. the generator neutral point is left floating.
(IT is european blurb for Insulated Terre or isolated from earth).
It is interesting to note the problems found with the Bender insulating monitoring equipment as this is equipment that we use on our switchgear for earth fault detection.