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How to work out which capacitor I require to run a three phase motor from a single phase supply.

Hi I have a three phase motor that I wish to run from a single phase supply.

Does anyone know what the formula is for working out the correct capacitance.

Thank you

Daniel

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0 Roy Bowdler over 4 years ago

Circa 30 years ago I was head of The Electrical Training Department for a region of the CEGB (became National Power). We conducted many “experiments” with AC & DC Motors for training purposes, including using some equipment expensively produced specifically for training purposes, plus motor and motor-generator sets salvaged or surplus. The first thing to point out, is that the whole area was purpose built, RCD and emergency stop button protected. Have you carried out a RAMS process, because if one of your apprentices came to harm then you will be skinned alive!

At that time, there seemed to be many surplus three-phase lathes and other machine tools available on the market and I was asked on several occasions how to run these three-phase equipped machines in a home garage on single phase. I won’t go over the ground already discussed, but a significant part of the issue was; what capacitors of suitable voltage rating were actually available? Naturally the person concerned didn’t want to spend money on new capacitors. Therefore in practice they often found it easier to source a salvaged single-phase motor and adapt the mechanical drive. By coincidence, I was clearing a shed last week and came across and old Washing Machine Motor, which went to my local recycling centre

As has already been mentioned, lash-ups like this will never produce a smooth running full power motor. Small variable frequency drives were also available then, but at prohibitive cost for the person concerned.

Just to reiterate – please take care with possible demo’s and experiments like this. In an era when there was a little more leeway, I had to be very careful ,although I didn’t issue a Sanction for Test to myself, because this was a carefully controlled “training environment”.
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0 Roy Bowdler over 4 years ago

Circa 30 years ago I was head of The Electrical Training Department for a region of the CEGB (became National Power). We conducted many “experiments” with AC & DC Motors for training purposes, including using some equipment expensively produced specifically for training purposes, plus motor and motor-generator sets salvaged or surplus. The first thing to point out, is that the whole area was purpose built, RCD and emergency stop button protected. Have you carried out a RAMS process, because if one of your apprentices came to harm then you will be skinned alive!

At that time, there seemed to be many surplus three-phase lathes and other machine tools available on the market and I was asked on several occasions how to run these three-phase equipped machines in a home garage on single phase. I won’t go over the ground already discussed, but a significant part of the issue was; what capacitors of suitable voltage rating were actually available? Naturally the person concerned didn’t want to spend money on new capacitors. Therefore in practice they often found it easier to source a salvaged single-phase motor and adapt the mechanical drive. By coincidence, I was clearing a shed last week and came across and old Washing Machine Motor, which went to my local recycling centre

As has already been mentioned, lash-ups like this will never produce a smooth running full power motor. Small variable frequency drives were also available then, but at prohibitive cost for the person concerned.

Just to reiterate – please take care with possible demo’s and experiments like this. In an era when there was a little more leeway, I had to be very careful ,although I didn’t issue a Sanction for Test to myself, because this was a carefully controlled “training environment”.
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How to work out which capacitor I require to run a three phase motor from a single phase supply.

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