Multiple VFDs

I recently bought a second Myford - a Super 7 this time - and originally intended to move the ML7 on but having access to two is proving very useful.

The original ML7 was converted to 3ph with an inverter which provides variable speed and I've bought a new motor for the S7 to do the same. Reading some technical detail about a Siemens Micromaster Inverter I see mention of restriction worded: "only one drive inverter is operated on one residual-current protective device".

Since my workshop is wired as a separate ring on one RCD on the distribution board (and there is no free space on the consumer unit) does this preclude me using a second VFD ?

Alternatively, can I use the existing VFD to feed both motors? The current VFD is rated at 1.5Kw. The motor on the ML7 is 0.37Kw and the new motor for the S7 is 0.75Kw?

Your collected wisdom would be most appreciated.

JG

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JG
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Hello, I design VFD's. The problem with using a VFD on an RCD protected circuit is that the EMC filter capacitor bleeds current to earth, in for the form of high frequency mush. If the bandwidth of the monitoring circuit of the RCD is high enough then it will see this bleed current and begin to give you nuisance trips at best, at worst this EMC mush will knacker the RCD.

On the drives I use to be involved with, if a customer was experiencing what you describe, then we would say that disconencting this capacitor between -ve DC link and earth would be ok, but that only applies to an industrial environment. Seeing as you are on a domestic supply, this would be highly inadvisable as there will be other equipment on your circuit (and potentially your near neighbours) that would take exception to this RF content being on the mains.

The advice that Siemens are describing seem to suggest that the normal EMC mush is not high enough to trip or damage an RCD.

Be aware also that the length of the motor cables can also play a large part in the amount of mush that will be transmitted, and also the PWM frequency and rate of change of the PWM voltages.

Actually a very very good way round this would be to use an isolating transformer in your lathes supply.

Is that OK with you?

Cheers,

Robert Wilson.

Reply to
Robert Wilson

from Robert Wilson

Thanks for your input Robert - most of which I understand :)

I'm not clear about what is involved in using an isolating transformer though.

Overnight I've thought about the problems of using one VFD (with remote control of Fwd, Rev & Speed) on two lathes and decided that it would be a total no-no!

So my current thoughts are to use the one VFD with remote console and connect whichever lathe I'm using at the time. I would be grateful for any comments with regard to this idea - particularly in respect of the different characteristics of the two motors (.37Kw & .75Kw).

JG

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JG

I've got an isolating (delta/star) transformer on the _output_ of the big old Danfoss inverter that I've been playing with... When I connected the output star point to earth, it tripped the inverter. Assuming I haven't got an insulation fault somewhere, it looks like I'm going to have to earth the star point via an inductor or resistor :-(. I was a bit surprised, because I had assumed that Nx3 harmonics would be minimised by design on inverters and I can't think of anything else that would cause the problem. More investigation is needed.

Input side is being fed by a single phase 240-350 step up transformer.

The good result from this particular setup is that the inverter seems to be large enough to cope with the startup currents of any of the motors I have, so I can use it for DOL starting. Until I eventually blow it up, this is going to save me a couple of thousand in payments to Central Networks for running a 3 phase supply to the house.

Mark Rand RTFM

Reply to
Mark Rand

Well if this inverter is a straight V to F converter, and not something fancy like closed/open loop vector, or rotor flux control which requires a re-tune, then it should be ok.

On the drives I have been involved in there was a second motor map that could be loaded by changing a parameter. Does this ring any bells with you?

Like I said if it's nothing fancy then the chances are that you'll be fine. The only current monitoring would be short circuit protection in this case rather than monitoring instantaneous currents.

Regards,

Robert.

Reply to
Robert Wilson

Hi, Why are you using a transformer out the output of the inverter? Do you need the motor volts bumping up?

What does the drive trip on when you do this? I doubt it's to do with harmonics though.

I would reduce the rate of acceleration on the drive in order to reduce the inrush to the motor. That is after what a drive is designed to do. This should not cause any long term damage as long as you are doing this from cold and the IGBT's or whatever they are using are operating within their Safe Operating Area (SOA).

Cheers,

Robert.

Reply to
Robert Wilson

Yes, it's to allow me to run 415V with the input voltage available to me with the transformers that I've got.

It's tripping on over current, so probably an insulation fault somewhere. Just haven't found the fault yet. There are no dead shorts involved, but I haven't yet scrounged the use of a megger to find exactly where the problem is. If it's not an insulation fault, then I must have allowed too much capacitance to creep in. I intend to add some series chokes when I make the installation permanent, to protect the inverter output stages.

The fault was before I had the load applied (ie, just the transformer plus an idler motor for a bit of inductance). I'm DOL starting the load motors off the inverter, so starting current is whatever the inverter gives.

I know that this is politically incorrect, and our friends on RCM think that it can't be done, but a 30A inverter seems to be able to supply the startup current of a 2hp motor without damage... not surprising since, even with the step up transformer, I should be running the inverter at no more than 60% of its rating. with a bad (7x FLC) motor.

Of course, it'd help if I could get a manual for the inverter, but it seems that Danfoss don't regard it as current any more (matches the machines I'm planning to drive with it:-)

Sorry to JG for stealing your thread

Regards Mark Rand

Reply to
Mark Rand

from Mark Rand

No problem Mark. Sharing information is what newsgroups are all about and if one question triggers a similar one - so be it.

JG (still looking for the manual on my VFD unit to check for multiple motor configurations)

Reply to
JG

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