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Star or Delta

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Robert Atkinson 229/09/2022 17:20:17
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1891 forum posts
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Posted by Jelly on 29/09/2022 16:10:14:
Posted by John Haine on 29/09/2022 13:23:07:
Posted by SillyOldDuffer on 29/09/2022 10:38:37:

If I had both high and low voltage 3-phase I wouldn't bother changing motors from star or delta, I'd just plug them in to the appropriate supply.

Dave

What are these high and low voltage supplies If you mean 415 or 240 then they are the same supply!!! It just depends on how you measure them.

I mean, you do see both 240v per phase (commonly called 415V) and 400V per phase (commonly called 690V) three phase supplies in fairly common use...

I always felt that the line to line voltage of the "normal" three phase being similar sounding to the phase voltage of the the "added beans" supply, added an unfortunate element of confusion which wasn't really helpful and meant you had to be just a bit more specific in your language when talking about installations with both systems in them...

In any case I doubt OP would have a supply like that mind, and even if they did it would still be "Low Voltage" (i.e. 50V < P.D. < 1000V) just a non-standard one.

A VFD output voltage is always quoted PHASE to PHASE. This is because there is no neutral on the output.
Modern VFDs rectify the input voltage to DC and then use solid state switches (MOSFETs or IGBTs) to "chop it up at high frequency into pulses. The timing and width of the pulses are filtered (often just by the motor inductance) into low frequency sinewave outputs. The DC voltage needs to be similar to the peak output voltage. This is easy for 240V in and 240V out, you just rectify and smooth the input. For 240V in 415V out you have to boost the voltage. This can be done as part of the rectification (and power factor correction) circuit or high frequency franspormers on the pulses. Either way it adds components and cost.

Robert G8RPI.

Barry Smith 405/10/2022 20:59:49
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29 forum posts
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Hi many thanks for your detailed replies, I have gone down the route of using the 240v to 415v VFD/Inverter and running the motor as star. Interestingly the 215 to 415 inverter was of UK Amazon for £70 and is rated at 3HP. I notice the same model has gone up a lot, but for £70 I was prepared for it to break knowing that Amazon would step in and offer a refund.

That said its been working OK on the main 415v two speed motor that runs the std machine (an Aciera F3) on and off for the past two years. The only thing I typically changed in the programme setup is to increase the slow start and decrease the stop speed as this save blowing fuses and of course wired the remote up for control.

Cheers

Barry

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