3 phase supply (again)

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3 phase supply (again)

This topic has 32 replies, 13 voices, and was last updated 17 January 2022 at 17:14 by Mark Rand.

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14 January 2022 at 20:05 #580475
Mark Rand
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@markrand96270
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 Your thinking is decades out of date.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.
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14 January 2022 at 21:15 #580489
Robert Atkinson 2
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@robertatkinson2
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I'm not that out of date.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
14 January 2022 at 21:52 #580500
Mark Rand
Participant
@markrand96270
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.
15 January 2022 at 15:29 #580580
Robert Atkinson 2
Participant
@robertatkinson2
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.
15 January 2022 at 21:21 #580632
Mark Rand
Participant
@markrand96270
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.
16 January 2022 at 10:21 #580664
SillyOldDuffer
Moderator
@sillyoldduffer
Posted by Mark Rand on 15/01/2022 21:21:26:

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
17 January 2022 at 15:13 #580848
old mart
Participant
@oldmart
It might prove more economic to change some of the less demanding machines to single phase motors.
17 January 2022 at 17:14 #580892
Mark Rand
Participant
@markrand96270
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.
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