What size cap' for 3ph motor

[Shox Dr]

I've got a hydraulic pump, it's 3ph 240v and only .75hp. So a capacitor should be able to run 1ph 240v

Soooo what size capacitor will I require.

 

[Arclikeharrypotter]

If memory serves correct it's 70× kw of motor. So 38.5 uf capacitor should be correct! But you may want to double check that

 

[Shox Dr]

I do have 2 37uf caps but they seems very a large [they were fitted to a 10hp static phase convertor motor].
I've also some 250w 3ph motors and they spec 2.5uf.

 

[Arclikeharrypotter]

Remember you are using the capacitor as the 3rd phase, different to a start capacitor.

 

[Shox Dr]

Remember you are using the capacitor as the 3rd phase, different to a start capacitor.

Isn't that what's its for. (I cant 'remember' anything I dont know)

 

[Dcal]

As Arclikeharrypotter says 38 will work. Anything from 30 to 38uf will be fine to the run third phase however you will probably need another larger capacitor in parallel during start up. This will need to be anything from twice to eight times the size of the run capacitor depending how much load you are starting against. Light load = times 2, full load from the start = times 8.
Remember you need to switch out the start capacitor once the motor is up to speed which can be achieved by using a motor starter but if the motor stalls it wont start up again and could burn out.
A better option, in my opinion, would be to use a single to three phase inverter which can be got for 50 to £75 second hand, even less if you are lucky.

 

[Dcal]

Also, that motor can run on 230 or 400 volts, make sure it's wired for 240v operation (i.e. wired in delta)

 

[MBB]

40uf per HP 240v,delta third of that if running on 415vstar . 3/4 hp 30uf

 

[Shox Dr]

Found a 35uf cap tonight, Test it tomorrow

 

[MBB]

You'll know if its the right size cap, if not the motor will rumble.

 

[Shox Dr]

Got it wired up, job stalled as I needed a hydraulic fitting. Try again tomorrow