Re: [CAD_CAM_EDM_DRO] Roll-your-own Phase Converter

To: <CAD_CAM_EDM_DRO@yahoogroups.com>
From: "Raymond Heckert" <jnr@...>
Date sent: Sun, 18 Jan 2004 19:26:03 -0600
Subject: Re: [CAD_CAM_EDM_DRO] Roll-your-own
Phase Converter
Send reply to: CAD_CAM_EDM_DRO@yahoogroups.com


Ray

recently I asked about synchronous operation of the idler and
everybody graciously told me that it wouldn't work. The logic
seemed very sound and I agree that it probably won't work.
BUT Since I have most of the work done on a small motor that
was to belt drive the 15 HP monster through an adjustable sheave,
I am going to leave the small motor on as a pre-rotator to bring
the idler motor up to speed before I energize the circuit. That ought
to take care of the hi current necessary to bring that big rotor
up to speed. I also considered the Rope starter method ala lawn
mower. I have a large (4 ft. throat depth) horizontal band saw
that only gets occassional use for cutting foam panels. It has a
rope starter and it is only a half horse motor. It is just about all
I can do to get it up to speed without blowing a circuit breaker.
I didn't want to even think about starting that old Ball Bearing 15
horse power 1750 rpm motor.

Bob McKnight

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One thing... Check to make sure your idler motor has BALL
BEARINGS.
Many of the older ones have journal (sleeve) bearings. Also, the
'big', slower motors have 'hu-mung-ous' starting torque (current)
pull, as the (big) rotor inertia is overcome. A large, old 15 hp unit
may typically draw upwards of 120 Amps for a few seconds on startup!
(Ask me how I know).


RayHex



----------

> From: Paul Amaranth <paul@...>
>

<snip>
There seems to be a consensus that lower rpm motors make for better

>

phase converters as well; 1750 are better than 3500. I picked up > an
870 rpm 20 hp monster that probably weighs around 350lb for $10 > a
few weeks ago. That one will make a dandy converter to run my >
CNC
stuff (as well as the other machinery). > > Unless you have a very
rare situation where you burn out the windings, > almost nothing goes
wrong with 3 phase motors other than the bearings, > which are cheap
and easy to replace. Buying an old 3 phase motor is > generally a
good bet, although if you wanted to be sure you can always > check
the
winding resistances. > > Paul


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