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Capacitor Sizing

09/06/2008 4:08 PM

I have a air compressor with a 2hp 3phase 400 volt motor.

What size Capacitor do I need to convert a 2hp.3 phase 400 volt motor to single phase 220 volt?

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#1

Re: Capacitors ?

09/06/2008 5:32 PM

YOU CAN'T--I am walking out the door and don't have time, but someone here will help you out with a good explanation soon.

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#2

Re: Capacitors ?

09/06/2008 6:40 PM

step 1) Remember who told you this would work.

step 2) NEVER ask them another question.

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Join Date: Jan 2007
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#3

Re: Capacitors ?

09/06/2008 11:27 PM

The two previous answers are both correct, but to help you solve your problem.

Since you have a 400V three-phase power supply you can connect ONE hot wire to the transformer neutral to get nominal 230 volts. Then you need to purchase a 230 volt single-phase motor. The motor will probably come with its own correctly sized capacitor.

In other words, capacitors are used for improving the efficiency or power factor of a component--not for converting between three-phase and single-phase.

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#5
In reply to #3

Re: Capacitors ?

09/07/2008 12:23 AM

A capacitor-start motor is designed to function on single-phase alternating current. It uses a capacitor to shift the phase of the power supplied to the "start" winding of the motor. The phase difference between the "start" winding and the "run" winding initiates rotation of the armature.

After this rotation has commenced, the momentum of the armature allows rotation to continue without a rotating field, and the "start" winding is disconnected. In most applications, this is accomplished with an internal centrifugal switch, but in hermetic compressors for refrigeration the "start" winding is disconnected with either a current relay or a voltage relay, depending on the prejudices of the designer.

A three-phase motor has a rotating magnetic field at all times. All the windings are continually energized, and contribute to the motor's ability to do useful work. That makes them much more efficient than single-phase motors. For a given supply voltage and power output, three-phase motors can be smaller and lighter than single-phase motors.

Although it is possible to cobble together an arrangement that allows a three-phase motor to run on a single-phase supply, it is not good practice, and the power available will be substantially less than the motor's rating. It would be much better to purchase a separate phase converter. Or, just to buy a motor that was designed to work on single-phase AC.

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#7
In reply to #5

Re: Capacitors ?

09/07/2008 12:35 AM

Hello bubbapebi

from me

Kind Regards....

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#6
In reply to #3

Re: Capacitors ?

09/07/2008 12:31 AM

Thanks Sparkstation, again.--

Here is an opinion of WHY:

Your only option for using this motor on a single phase service is an inverter or drive-

Poly(3)phase service is separated by 120º. 3, as in the 3 phases together total 360º. 360º in one revolution of anything. Three phase motors rotate one way or the other automatically--I haven't seen a capacitor on any, they don't need it.

Single phase is 180º apart and won't cause something to rotate--That's what capacitors or starting windings are used--A single phase motor that doesn't have a capacitor for starting or if it's starting winding or centrifugal switch has failed, can run by turning the motor while applying the power(this is a bad idea, but that is all it needs, is something to start it rotating one way or the other).

the top is 3 phase and the bottom is single.

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#8
In reply to #6

Re: Capacitors ?

09/07/2008 12:36 AM

Hello Kilgore Trout

from me

Kind Regards....

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#4

Re: Capacitors ?

09/06/2008 11:56 PM

Hello dustyles

You could use a 1-phase to 3-phase converter/transformer unit to obtain 3-phase 400 Volt supply to run that 3-phase 400 Volt motor from a single phase 230 Volt supply.

Refer: http://www.eurotech.co.nz/

However, it would be cheaper, unless the 3-phase 400 Volt motor is an integral part of the air compressor, to purchase a 2HP 230 volt 1-phase motor for the air compressor.

Kind Regards....

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#9

Re: Capacitor Sizing

09/07/2008 9:49 AM

You can't reduce a AC the way you do DC.A 400 volt AC motor can't be turned

into a 220 volt AC motor.

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#11
In reply to #9

Re: Capacitor Sizing

09/09/2008 8:33 AM

My Dear markalfredsteele,

It can be turned... as the capacity of the motor(2 HP) is the same.

don't pass on wrong information...........

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#10

Re: Capacitor Sizing

09/08/2008 1:35 AM

A wallet with enough capacity to hold the $ for a new motor and starter.

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#12

Re: Capacitor Sizing

08/18/2010 10:49 PM

i have a question for anyone who can help me on it ....i am currently in hvac school and work parttime in the field ...my question is , how can i find the capacitor size of a motor if the capacitor is rusted up and or rubbed off , there has to be a mathmatical way of finding it out , my instructor and the head instrucotr said " sell them a new motor and capacitor " that may be needed but i would stil like to know the formula , thanks for any knowledge on this

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Anonymous Poster (1); BruceFlorida (1); bubbapebi (1); Gadget Guy (1); GW (1); Kilgore Trout (2); markalfredsteele (1); Paras Desai (1); Sparkstation (3)

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