I need some help from those that know about 3 phase electric motors. I changed my knackered single phase 3hp compressor motor for a 5.5hp 3 phase so that I could run 2 pumps from the single motor, how ever the new motor did not come with run capacitor(s).
Now what I need to know is do I need a capacitor for every phase then join to the poles of the motor or is it a case of just putting one on a single phase to just kick start the motor??? Also what sort of rating would I need? any help would be great please before I either burn the motor out or burn the workshop down!

A capacitor is to start a single phase motor otherwise the rotor would just sit there. You don't need any capacitors on a 3 phase motor.

I currently have the motor wired through the start/stop pressure switch but I'm not sure how long the switch will last as it is. It some times trips the 20amp MCB's so I wondered if I needed capcaitor(s) but looks like I need a higher rated MCB.

Whats your supply, actual 3 phase or a phase convertor?

3 phase 3x 240v on a linked 16amp mcb I'll swap it out for 20amp and check that it's type C not A or B.

When you say 3 x 240v, do you mean its a proper 415v supply, or you have taken 3 x feeds off a single phase consumer unit? Your motor shouldn't draw more than 20 amp, but we use C or D at work to stop the motors tripping the breakers.

proper 3 phase supply 240v each phase rated at 30amps each to my unit, I think I'll check what type the MCB is currently in the board as it could be that causing me the problem. The motor is only running one pump for now but it will need to run two soon so I need it sorted.

Don't compressors usually employ a starter relay? If you've switched the motor from 1ph 3hp to 3 ph 5.5hp I think you will need the appropriate starter relay.

If you were running single phase you'll need to fit a 3 phase contactor and run the coil /o/load circuit through the pressure switch, on/off switch. You cant control it just by breaking one of the phases if you do the current load will go through the roof thats why its tripping the MCB and you run the risk of burning out the motor, whatever you do dont upgrade your MCB's as it sounds like you havent got an overload trip you do you will burn out your motor.
A 3 phase motor will draw less current than a single phase as you dont have the large start draw of a single phase.
A start capacitor gives a 90 degree (thereabouts) phase shift to kick the motor into life on a single phase high torque start motor a capacitor isnt used on a 3 phase motor thats why you have 3 phases.
If your stuck u2u me and I'll sketch you out a diagram of what you'll need.
Cheers,
Bob
[Edited on 23/1/14 by splitrivet]

[Edited on 23/1/14 by splitrivet]

Got plenty of 3 phase starter panels lying around if you want to do it properly as above. In simple terms, switch the panel on, pulls contacts in, starts motor. Bobs your uncle, fannys ya aunt.