DEC RP06 power

From: John Lawson <jpl15_at_panix.com>
Date: Wed Oct 25 00:47:49 2000

 Re: 3 phase power for drive motors.

  There are commercial boxes of varying complexity for this purpose,
albeit expensive. The 'el cheapo' way is to obtain the use of a clip-on
AC ammeter, and a selection of motor-run caps. Hopefully also you can get
hold of a Variac of suitable capacity. (1 HP roughly equals 770 watts
equals about 3.5 amps at 220V [okay I said: roughly!])

  Disconnect the motor leads from the control circuits and remove the
media if possible, certainly lock the heads out of position. Think about
running the blowers first if you can.. perhaps they are single-phase.
Anyway, protect the media. Figure out what the direction of rotation is
supposed to be!!!

  CAUTION: Don't kill yourself for the Hobby, no matter how dramatic and
noble that might seem. 220VAC line will snuff you quick, and others too
who might try and come to your aid. If you are not expert in electrical
safety, you might want to seek someone who can help you with this.


  Attach two of the motor leads to the AC power source through a Variac
and/or a five-amp breaker or fuse, or, if nothing else, a 600-watt room
heater. Attach the third motor lead from one side of the AC line, through
the capacitor. Apply some reduced voltage, and read the current in the
three leads. Add and/or remove capacitors until the current with the motor
running is equal in all three leads. Voila! There you have it. Be sure
the direction of rotation is correct, if not, swap any two motor leads.
 
  Starting: MOst lightly loaded constant speed applications should start
up normally. If not, place a second capacitor of about twice the value of
the first, in parallel with the existing one, and place a switch in series
with it. Use a big pushbutton that will carry the load, if possible. Then,
when starting up, operate the switch to add enough capacitance to get the
motor up to speed and then Disconnect It.. else you will toast your motor,
and that would not be a good thing.

  I hesitate to include an ASCII line drawing, but I will be happy to
correspond off-list with anyone who wishes more info, clarification,
drawings, etc. It's easier than it sounds... just moderately dangerous
until you get the proper cap value settled on. For 220V 1/2 horse motors,
run caps might be in the 100-300uF range, 440/660VAC motor run caps, but
you need to select the one that matches the motor's characteristics. Find
surplus/used caps... if you have to buy a bunch new, it might
be... shocking.


   {sorry.....}


  I have run several 3ph motors succesfully using this method, one (in the
case of a milling machine at a former employer) for many years with no ill
effects at all. The clip-on ammeter is the key... get the currents equal
in the windings and you (and the motor) are happy.

  Cheers

  John

 Who lives in India where it's all 3ph 220v 50Htz... most of the time. :)
Received on Wed Oct 25 2000 - 00:47:49 BST