3 phase motor running from single phase supply

Hi All

I've been reading up and googling this as I have a couple of new 3 phase motors I could use on projects that don't need variable speed. These motors are 1/2 HP running at 2850RPM. I have studied Jim Cox's workshop practice books on motors and it seems very straightforward. I'm thinking of using this circuit to drive the motor:

Acknowledging Jim Cox WPS book 16 for this circuit.

Does anyone have any experience, comments on running 3-phase like this? Thanks

Trev

Edited By Toolgrinding on 21/07/2014 21:08:32

Trev,just looking through the archive and see you had no replies, not much on technical electrics but this will push you to the top of the pile. And hopefully some one with the necessary will reply.

M<y understanding is that these circuits work, but experimentation with capacitor values may be needed to get sufficient torque without overheating. Obviously considerable care is required when altering mains circuits like this.

Neil

Someone told me to try this idea of using capacitors across the lives to get a small coolant pump motor running on a colchester lathe. I didn't do it but apparently it works ok for small motors.

I found the table in Jim's book to be a bit "off" and had to do as Neil says some experimentation with values to get the running voltage close to equal on all three legs.

In another book in the Workshop Practice series the author Graham Asterbury uses a better formula for calculating the required capacitance, I found it gave more accurate results than the tables in Jim Cox's books.

Paul

Warning always discharge the caps before working on your circuit.

Some will and some wont I have two 1/3 hp coolant pumps which are wired Delta but wont start or run on any capacitor setup yet will run quite happily on an Inverter...

My coolant pump which is labelled 240/440 volts runs fine on single phase 240v with a 12Uf capacitor in circuit all the time.

I have done this quite a few times. As stated, it the capacitor value you have to juggle around with.

You need a large capacitor value to get the motor up to speed, I use two distinctive groups ,in parallel for starting, then one group (the start group larger in value ) is disconnected, leaving the smaller group (the run group) in circuit. I use two contactors to do the switching. Pressing the start push, count five or pause brings in both contactors and connects both lots of capacitors to the mains and the motor, then release the push and the motor runs. The contactors are so wired as to give no volt release should there be a mains failure and there is a local stop push. Ex fluorescent light capacitors are OK for this application as are ex power factor correction capacitors. Please be extra cautious when handling capacitors of this quality, they can store charge of energy for a time if not fitted with a discharge resistor. I suggest you switch off, wait a while, then short out the terminal of the capacitors with an old screw driver before working on or connecting them. Belts and braces. For coolant pumps I found one small capacitor to be enough as it starts off load in effect. Actually it best to test all three phases with a volt meter and that way one can adjust the capacitor value to get near balance, also put your hand on the motor after its been running for a while, bearing in mind that modern motors run very hot. Ted