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. 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.

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.

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.

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.

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! Any
idea how big a load I could run using a 2.5 hp idler motor? By the way,
it is a 940 rpm motor. Would a flywheel to store extra energy improve
performance?

Best wishes,

Chris

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?

I like an open frame or drip proof motor, but I've used TEFC with no
problems. If there's much of anything in your shop- dust, overspray,
etc. for a motor to pick up the TEFC will be more convenient 'cause you
won't have to blow it out.


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

I'll let someonne else answer that, but I think you'll find wye more common.


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).

No big deal here, as Chris Tidy said a sleeve bearing should run quieter.


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.

I can start a 10 hp on my 50 amp breaker, you should have no trouble there.


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?

The closer you get it to the nominal speed the less current surge
there'll be when you switch it in. 2/3 nominal will work well.


7. Is there something that I forgot to ask?

Take a good ohmeter with you to check the motor windings. The two
problems you may find is a short to ground and a shorted winding.
Neither makes for a good convertor;-) In my experience it's not common
to find a cooked motor that's not shorted to ground in a big way but it
happens, measuring resistance from the windings to the frame is probably
adequate. If you have a megger all the better, but it's not absolutely
necessary for checking a junkyard motor.

John


thanks

i

In article , Ignoramus5833 says...

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.

OK, the idler motor in that setup is *five* hp, and it is spun
up by a 1/8 hp repulsion-induction motor. It takes about one
second to do this.

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

It should be real close, or slightly higher than nameplate speed
to mimimize the inrush currents that flow when you energize it.

7. Is there something that I forgot to ask?

You should ask about balancing capacitors and phase angle correction
capacitors. Also you really should have at least one contactor
in the setup which will drop out (self-holding) if you lose power
while the converter is running.

As a data point, my setup will easily start and plug-reverse one hp
load motors, and is powered off a 15 amp 240 volt branch ckt.

Jim


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