[AMPS] 3-phase fan motor - help please

[Richard Carroll]

Hi Chris,
    I have a booklet that describes this in some detail. There are
three ways to do it, tho only one that would apply to your case. It is
quite simple requiring only 2 capacitors, one a 'start' cap and the
other a 'run' cap, and a starting switch in series with the 'start'
cap.
   The 120VAC power is applied across 2 legs with the capacitors in
parallel from the third leg to one of the other legs. The start switch
in series with the start cap. One applies power, presses the 'start'
switch and holds it in until RPM is builds up, then releases the
switch.  It also says for automatic starting you can use a Normally
Closed relay, with the coil in parallel with the run cap, and the
contacts and start cap in series, (also across the run cap). When
voltage builds up across the run cap and that phase winding, the relay
functions, opening the start cap. If there is insufficient delay to
allow RPM buildup, place a resistor in series with the relay coil.
     It states that in reality with this method the motor is being run
more as a single phase than a three phase. Neverless this is the
method  recommended for use on motors for use on drill preses, lathes
and other machine tools where the motor is subjected to nameplate
maximums for short periods of time only. It goes on to say that this
method should NOT be used for motors that must be run at maximum load
for long periods since the motor winding attached to the capacitor is
subjected to unbalanced curents and prolonged running undre heavy load
will cause it to overheat.
Presumably your blower motor is rated for something more than routinely
required.
    It says that a run capacitor much larger than usual  run capacitors
is needed in this hookup, ususlly an oil capacitor (to handle the heat)
of about 28 MFD per horsepower is used.  In addition an electrolytic
capacitor of about 60 MFD per horsepower is required for starting.
   While these figures are for larger motors, this should give you a
starting point to work with. The other two methods involve using an
autotransformer as well as 2 capacitors , and running several 3 phase
motors simultaneously on a single phase. I don't imagine these would
apply to your case. Good luck.

73, Dick W0EX

Chris W Tran wrote:

> Monday 24th August 1998
>
> Hello all
>
> I'm building a new HF amp. and the electric motor on the
> blower/fan I have needs a 3-phase supply (it is about
> a 1/3 HP motor)
>
> Can anyone tell me how I can run this motor off the AC
> supply in my house - I have 2 phases available and I was
> thinking of connecting the 2 phases, then a capacitor (or
> inductor) to the 3rd phase connection on the motor.
> Would this work ??
>
> I know that 'converters' are available (I guess some sort
> of 3-phase oscillator powered by a single-phase supply)
> Where could I find a circuit to build one of these if my
> 2-phase supply does not work ?
>
> If all else fails, I'll have to replace the 3-phase motor
> with a single-phase one, but this is difficult mechanically.
>
> Thanks for any advice.
>
> 73     Chris Tran  GM3WOJ
>
> Please reply to <chris.tran@zetnet.co.uk)
>
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