Three Phase Converted To Single Phase

[Uzzy]

Can anyone either point me in the direction of a suitable website or answer the following questions. I hopefully will be getting a 3 phase milling machine that has a .75hp motor on it at present but needs to be converted to single phase.
Question:-
1 What size motor would I need?
2. How do I convert it ? do I need to change all the wiring and switch gear? and if I do what cabling would I require.

[catgate]

You should be able to use the present motor by converting it from "star" to "delta" configuration. You then need to connect 230v "line" to one terminal, connect 230v "neutral" to another, and then also connect the 230v "line" to the third terminal but via a polopropylene run capacitor of about 30mfd (40mfd/hp) in the line. This should run your milling machine. However you might find it will start up better with an additional "start" capacitor (which can be an "electrolytic" type of 4 or 5 times the value of the run cap.) running in paralell with the run capacitor. Some sort of provision must be made for switching this capacitor out of circuit when the motor is up to speed. Failure to do this will inevitably render the motor NBG. One final point, do not run the motor at full load for long periods. Try to run at no more than 80%. (Oh, and if it revolves the wrong way for you, just swap any two of the three connections)

[ChrisCrosskey]

You might do better to leave the current motor and switchgear basically intact and get a 1HP rated inverter. I use one on the 3/4HP motor on my Myford and it is so smooth I can't believe I managed without it. Good ones will also let you vary the speed, I only bother engaging backgear for threading these days, otherwise I just slow the inverter..... Ebay is a good place to get them from, mine was secondhand but works fine and cost me 25UKP including postage.
chrisc

[Uzzy]

Further to my original email, my original 3 pahse motor is 3/4hp do I need a inverter with the same rating or would I get away with a lower rating?

[jackrae]

Answer to your question - it's unlikely that you will use the full power rating of your motor so you MIGHT get away with a lesser invertor. However 3/4 HP units (560w) cost not a lot more than a lesser rated unit. If you find a higher rated unit going cheap go for it.

However your main problem is that you can only generate 230 3phase using the conventional variable frequency invertor so you will need to ensure that you can modify the motor to 230V delta configuration.

Also, as it's currently a 3-phase machine, the control gear (contactor/starter etc) will most likely be transformed from 415v to some lower ("safer") voltage (usually 110 but sometimes 24)

If there are any autofeeds on it driven from the main motor - or by separate motors you will need to consider how these are tied into your new variable frequency supply.

Note that any transformers which may exist within the control scheme will NOT be capable of working with the variable frequency facility from your invertor, so if you wish to retain these you will need to feed then of the 230v mains (via a suitable voltage reduction transformer - if applicable)

As most who have used them will agree, invertors are the way to go - once tried, never again avoided.

jack