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
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