Andrew Johnston | 14/01/2022 10:34:06 |
![]() 7061 forum posts 719 photos | 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 |
SillyOldDuffer | 14/01/2022 10:48:49 |
10668 forum posts 2415 photos | Posted by Andrew Johnston on 14/01/2022 10:34:06:
A funny thing happened just now, I've had to sit down to recover. I agreed with the whole of a post by SoD. Bound to happen sooner or later, like those monkeys eventually typing Shakespeare... Dave
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John Haine | 14/01/2022 13:37:33 |
5563 forum posts 322 photos | I thought I was having a bit of deja vu - this was all discussed before in this thread |
Robert Atkinson 2 | 14/01/2022 16:54:13 |
![]() 1891 forum posts 37 photos | Posted by Andrew Johnston on 14/01/2022 10:34:06:
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 It's unusual but I have to disagree with you a bit Andrew, 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. Robert G8RPI. |
Mark Rand | 14/01/2022 20:05:18 |
1505 forum posts 56 photos | 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. |
Robert Atkinson 2 | 14/01/2022 21:15:05 |
![]() 1891 forum posts 37 photos | 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. Robert . Edited By Robert Atkinson 2 on 14/01/2022 21:17:09 |
Mark Rand | 14/01/2022 21:52:50 |
1505 forum posts 56 photos | 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.
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Robert Atkinson 2 | 15/01/2022 15:29:52 |
![]() 1891 forum posts 37 photos | 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. |
Mark Rand | 15/01/2022 21:21:26 |
1505 forum posts 56 photos | 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.
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SillyOldDuffer | 16/01/2022 10:21:01 |
10668 forum posts 2415 photos | 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
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old mart | 17/01/2022 15:13:16 |
4655 forum posts 304 photos | It might prove more economic to change some of the less demanding machines to single phase motors. |
Mark Rand | 17/01/2022 17:14:36 |
1505 forum posts 56 photos | 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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