3 phase supply (again)

A funny thing happened just now, I've had to sit down to recover. I agreed with the whole of a post by SoD.

There is only a current surge when starting an induction motor direct offline because full voltage is applied with the motor not turning. A VFD should control the applied voltage to keep the current within the set parameters. There should be no need to select a VFD of four times the rating. A little over the motor rating may be helpful if using a speed/over-current tradeoff to overcome short terms overloads when running.

My understanding of digital phase converters is the same as SoD. They create two phases from the incoming supply and create a third phase using a half bridge and PWM drive, which is then filtered to provide a sinewave output.

I've got a couple of VFDs on my CNC mill, but for the rest of the workshop I solved the problem once and for all by installing a 3-phase supply. This is particularly useful as some of my machines have two speed motors, and star-delta switching in one case.

Andrew

Bound to happen sooner or later, like those monkeys eventually typing Shakespeare...

Dave

I thought I was having a bit of deja vu - this was all discussed before in this thread

**LINK**

It's unusual but I have to disagree with you a bit Andrew,
VFDs are intended to be permanently connected to a motor and control start-up currents when they are commanded to start the motor. Even if set to maximum speed at turn-on there is some ramp up. Some also increase the current limit during ramp up at maximum rate. If you turn the VFD on and then switch a load onto it the only protection is the over-current. This is a fault condition and exceeding it will trip the drive off-line. Thus it has to be set high and may also be part of the 4x oversize recommendation. Of course setting the current limit high means the motor is no longer protected against over-load.

Starting a smaller motor first has also been mentioned on this thread and even running an un-loaded motor permanently connected is suggested by some sources. Not very efficient.
I'm not making this up I've had a serious look at and experimented with using VFDs as 3 phase converters. Just the amount of filtering required to tame the emissions when running off load was a show stopper.

Robert G8RPI.

Sorry, Robert, you are talking nonsense. Your thinking is decades out of date.

When you start up your Bilgeport, DS&G, Lumsden grinder etc, off the VFD which is acting as a fixed three phase supply, then the motor has exactly the same protection as it does when running off the utility's three phase supply. The motor starter and overloads in the machine's control box are the same as they were when it left the factory.

L C Sine wave filters are simple and cheap. I use a commercial one and it works perfectly.

Mark,

Only if the switch gear has overload cutouts and they operate correctly with the waveforms from the VFD neither of which is a given.
The machine switchgear does not protect the drive.
The REO filters you linked to are quite nice but they are not low cost. Did you buy a new one for a hobby installation?
Additionally the VFD may not like having a resonant filter on it's output loaded or otherwise. The smaller REO filters likely to be used in a home workshop are resonant at about 2.5 kHz so the PWM frequency needs to be a fair bit higher than this e.g. >4 kHz. Newer drives are probably OK but be sure to check that your drive is suitable for use with a filter.
I'm not that out of date. Part of my day job today involved similar issues with a multi kilowatt 3 phase converter.....

Robert .

Edited By Robert Atkinson 2 on 14/01/2022 21:17:09

If the motors don't have overload protection, then they wouldn't have any protection when run from 3ph mains...

The drive protects the drive, that's why there is current sensing either on the three separate output stages or (on cheap stuff) the DC bus.

The REO filters are made specifically for use with VFDs, and I thought the price was reasonable when I bought it from them.

4kHz switching frequency is towards the bottom end for smaller VFDs, Even my old 23kVA Danfoss unit that I started with was happy at 8kHz. In the case of long cables from the VFD, the filter improves life for the VFD output stages as well as eliminating EMI and motor noise.

The drive self protection only works for the conditions the drive is designed for. For most VFDs this does not include connecting or disconnecting the load with the VFD output energised.
It might work but that does not mean it's right.

Well, I just guess that some of us can keep using the things and others can keep saying they don't work. Then everyone will be happy.

I'm against assumptions.

Best not to generalise: if the output of a VFD has to be switched, Read The Flipping Manual! All will be well if the maker says it's allowed, otherwise...

Within my limited experience, which is only of low-end VFDs, the Manuals all forbid switching the output. I'm fully aware better units are available but so far as I've researched, they're all pricey.

Dave

It might prove more economic to change some of the less demanding machines to single phase motors.

Seems better to keep them all three phase, as they were built. Once you've sorted out the 3ph supply, it's done for all the machines you've got or might get,

I know in my case changing the motors would involve 11 motors, sime of which have got rather specific mounting arrangements and are of sizes no-longer manufactured. The amount of cost and work involved would be far greater than that involved in getting a 'digital phase converter' or even getting Western Power Distriibution* to put in three phase from the street.

*Other distribution network operators are available.