# Rewiring an electric motor to run with an capacitor



## GoceKU (Mar 27, 2018)

I own an old wood planer with an 2,9KW 380v 3 phase motor, but i have only single phase power in the garage where it is and i'm getting tired of dragging wires around to power it, i've rewired smaller motors up to 0,5kw using a capacitor to run on 220v single phase, now i don't know what size capacitor do i need for 2,9kw (4Hp) i have 125-160 mF 220v one but don't know is it enough, as for rewire it i have this diagram , now is wire like the left side for 380v and i'll swich it like the right for 220v.


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## benmychree (Mar 27, 2018)

The easy way around it is to buy a static phase converter; either way you will not get full power out of the motor, but the phase converter will automatically start the motor.


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## markba633csi (Mar 27, 2018)

Goce I would guess about 375 to 475 microfarad to start a motor that large- possibly even more.  The larger the cap the faster it will spin up to speed-up to a point
Mark


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## Ulma Doctor (Mar 28, 2018)

I start 5 hp motors with 250 Uf capacitors all the time in the unloaded condition.
30 to 50 Uf per Hp is usually sufficient to start most motors
you'll want to put the capacitor on a momentary switch so the capacitor doesn't blow up from being in the running circuit
here is how i do it...

https://www.hobby-machinist.com/threads/rpc-simple-design-unbalanced.12712/
this design will work with 50Hz supply too


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## markba633csi (Mar 28, 2018)

Sorry Goce I was thinking single phase- Mike is correct about 200-250 should work,  try the one you have first it may be enough
Mark


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## amuller (Mar 28, 2018)

You will want an electrolytic start capacitor, which can only be energized temporarily to spin up the motor, and at least one oil filled (or other design) run capacitor to get at least a little out of the third leg.   Or this is how I'd think of it.


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## GoceKU (Mar 29, 2018)

I will look for a 200-250 mF capacitor for constant running, it sure will be nice to use this machine without the 2 hours prep before every use.


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## amuller (Mar 29, 2018)

I would not think you'd need a run capacitor that large.  For example, a lot of single phase two capacitor motors (capacitor start/capacitor run) might have a start capacitor in the 1-300 uF. range and a run capacitor more like 10-15 uF.  (The u is supposed to be the lower case Greek mu but I am too lazy to find that on the keyboard).


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## markba633csi (Mar 29, 2018)

I believe you want to switch the capacitor out after the motor gets up to speed, at that point you will be running your 3 phase motor on single phase at about 2/3 HP.
Mark
ps It would be a good idea to put the capacitor in some sort of enclosure so that if it suddenly decides to explode someday, spraying toxic bug-juice and possibly shrapnel in all directions, at least you will be protected.


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## Ulma Doctor (Mar 31, 2018)

GoceKU said:


> I will look for a 200-250 mF capacitor for constant running, it sure will be nice to use this machine without the 2 hours prep before every use.



DO NOT RUN THE MOTOR WITH A 200-250 Uf CAPACITOR IN THE RUN CIRCUIT- YOU WILL BLOW UP THE CAPACITOR
you will only need a 25 to35 Uf oil filled capacitor for the running circuit


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## GoceKU (Mar 31, 2018)

It's starting to sound more trouble than it's worth, i may be better off buying some more three phase cable and running 3 phase power to it.


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## Ulma Doctor (Mar 31, 2018)

if you have 3 phase available, that would be the easiest route 

but a relay could be easily employed to switch between run and start capacitor circuits


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## GoceKU (Mar 31, 2018)

I'm very lucky to live in a fairly new neighborhood with good power infrastructure 3 phase 380v.


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## amuller (Mar 31, 2018)

If I had 3 phase power available at reasonable cost I would definitely use it.

Otherwise, it would make sense to spend some time reading up on the whole issue of running 3 phase motors on single phase.


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