"Ignoramus5833" wrote in message
...
On Tue, 26 Jul 2005 17:23:53 +0000 (UTC), Christopher Tidy
wrote:
Ignoramus5833 wrote:
I am going to build an RPC. I will go to an industrial junk yard to
pick a used 3 phase motor, tomorrow. I do not want to buy a wrong
motor.

Some questions, possibly very ignorant:

1. Is TEFC better than drip proof, for me?

2. Is there a difference in wye vs delta wound, as far as I am
concerned?

3. Do I need ball bearing or I can get away with sleeve bearing
(remember, it is an idler, no load or thrust on shaft other than at
startup).

4. What HP to choose? With these motors essentially free, the cost
difference between 10 and 7.5 HP is minimal. 10 HP, though, draws a
lot more current at startup. My garage has a subpanel that is on a 60
amp breaker.

5. I salvaged a 1/4 and 1/3 HP motors from two broken vacuum pumps
that I had. I would buy a 5/8" ID pulley on one of them. Would they be
able to spin up a 7.5 or 10 HP motor? Jim Rozen reportedly used a 1/4
HP pony to spin up a 7.5 HP motor, that makes me hopeful.

6. How fast would the idler need to spin before I can remove the pony
and apply 220V to the idler input?

7. Is there something that I forgot to ask?

If you want a 220 V three phase output, just make sure the idler motor
is rated to run at 220 V on the data plate.

good point.

This will most likely mean a delta (mesh) connected motor. Should
you want a 440 V output (you'll need a transformer for this) you'll
need a motor rated for 440 V operation. Depending on the size this
could mean either a star or delta connection.

No, I am only looking at 220V.

Personally I like sleeve bearing motors. They're quiet and there's
something nicely "old school" about them. Some of the poorer ball
bearing motors don't even have grease nipples. I would check that there
is a way of lubricating the bearings from the outside.

Okay, I will count your post as a vote for sleeve bearing motors.

If that big transformer happens to be 220 V - 440 V, keep it! It might
be useful for a phase convertor designed for running larger motors.

No, it is a POS from a big UPS.

I am very tempted to build a phase convertor now. It would mean I
didn't have to build individual capacitor start and run systems for
each machine. I built one for my power hacksaw, but then it stopped
working last winter. Probably too complicated and the damp got to
some of the components. I will have to dismantle it and locate the
problem when I get chance.

Yes, I rather like the simplicity of the concept of an RPC.

I can get 2.5 hp and 22.5 hp motors free for a phase convertor. I reckon
22.5 hp is too big, both to carry and in terms of current demand!

22.5 HP is kinda hard to "carry".

i
One other thing to look out for is the speed; I built mine with a 10HP TEFC
motor, but it's 2 pole so it runs at 2800RPM (on 50Hz, would be 3400RPM on
60Hz) and it's damn noisy; you will be better off with a 4 pole 1700RPM
motor. Another point to watch for is the orientation, I happened to get one
that had bearings rated only for use with the output shaft pointing
downwards so I've had to build a frame that mounts it that way up; the
positive side is it takes up less floor space and there's no way I'm likely
to get caught up on the spinning shaft.
Martin

--
martindot herewhybrowat herentlworlddot herecom