RPC

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Dave T
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Joined: Sun Jul 14, 2013 5:42 pm

RPC

Post by Dave T »

I wheeled and dealed myself into the rotary part of a rotary phase converter. This is a shaftless motor with no tag or any info on it at all. How can I determine what size this is? The motor came off of a RPC that was locally built but the company that built it is gone as are all the people. If I can figure out what size it is then I can build another converter and move on to the next project. Thanks for any info. The dimensions of the motor are 12 1/2" long and 9 1/2" dia.
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wsippola
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Re: RPC

Post by wsippola »

Not an expert! From the size, I would guess at 3-5 hp. As it's shaftless, you'll need some caps to get it started. Then use a clamp on ammeter to see what the idle current is. Someone here will have a good idea what percentage of current a motor will draw at idle as compared to full load - I'd guess it's around 2/3. Of course it will be a 3ph motor running off single phase at this point, so that would probably change what one would typically see for idle current. Post the number and have other readers compare with their RPC's?
CaptonZap
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Re: RPC

Post by CaptonZap »

As a rough guess, I would say that it is no more than a horse or two, and that is just from estimating the HP of the motor from it's size. The test is to get it set up, and run it full load and check the temperature of the windings after a few minutes.
One thing that is advisable is not to run a RC at full load, but rather turn on another 3 ph motor in the same circuit, and let it run under no load. Between the two, you can get more power to the loaded motor, as each is acting as a RC.
The good part is that you get a better 3ph from a rotary than you do from a static converter.
BING 3 phase converters, and you can educate yourself about RC's.
http://www.bing.com/search?q=homemade+3 ... QBLH&ghc=1

CZ
spro
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Re: RPC

Post by spro »

While on bing "images" you may see your motor. By investigating that source, you may find its specifications.
Rich_Carlstedt
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Re: RPC

Post by Rich_Carlstedt »

Well at the risk of being called a nut, let me make a suggestion that has pretty well worked except for Chinese motors.
I have built well over 60 3 phase convertors of all types.
Sometimes you don't have the specs, so I know what the Op is looking for .
One Hp is equal to 700 watts ..but since motors are never 100 % efficient, lets use 900 watts
So if you start the motor ( single or 3 phase--no matter) and measure the idle current and voltage you can get an idea.

Most motors at idle run about 40 to 50 % of full current which is full HP .
Older motors, with lots of iron/copper are better IMHO.
So hook up 240 to the motor to two leads and spin it up with a kick or what ever and measure the current.
If it's on 240 and draws 2 amps, thats 480 watts or half of the 900 I mentioned, so the motor being at 50 percent at idle ( no load ) is most likely a ONE HP motor

Rich
Dave T
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Re: RPC

Post by Dave T »

I understand what you guys are saying and I am doing some research here at home. One of my problems is that the Idler motor is a blind [shaftless] motor made to be used as a RPC. This motor came off of a functioning RPC, but all the other parts are long gone. I didn't know if I could plug it into 1ph to start it without damaging it. I do not have any type of a starting circuit for it yet, so how do I get it to spin or should it start on it's own ? I am gathering up some stuff , I have access to a bunch of odd lot capacitors and switches and am sorting to see what I can maybe use. Also how do you check a capacitor to see if it is good without getting bit to hard ? I learned that lesson along time ago. Thanks for the input , I'm learning slowly.
Rich_Carlstedt
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Re: RPC

Post by Rich_Carlstedt »

Yes Dave , With a shaftless motor, its hard to spin the armature.
You will need to add a capacitor "temporarily" across the third wire to start the motor spinning
The general rule is 100 Microfared ( 250 volt rated) per HP. but based on the size that you mentioned, I bet you could do it with 100 to 200 Mfd . It should do it in just 2 or 3 seconds before disconnecting . There may be a large spark when breaking the connection, so be careful, you are playing with 240 volts . If the cap is bad , nothing will happen except the growling of the windings .
To check a cap, you need a volt/ohm meter.
Short out the terminals first with a plastic/wooden handled screwdriver
Then check the resistance with the ohm meter.
It will show a short, or near short and then resistance will climb to several million ohms.
The reverse the leads and you will see a dead short as the cap unloads the low voltage the meter charged it with.
DO not let up as the meter will then start climbing as the cap charges in reverse also to several million ohms.

Rich
spro
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Re: RPC

Post by spro »

Great advise. There is something else about measuring capacitors. Watching the sweep of the pointer/hand/needle is great when there is a network calibrated to sensitivity of the meter. If it isn't there, the discharge of the capacitor can peg the needle or blow out the jeweled movement. Calibrated correctly, a sweep is seen and then go to next range and read the charge. Old hands at this have seen charge, then a point where it falls off and becomes a resister. It is difficult to describe but a capacitor is not supposed to break down and become a resistor/path of some measurable current when it is dormant. The whole time, a digital meter is running numbers like a blur The point is, that protected DVMs can direct you in the area to which more sensitive analog meters allow you to actually see the analog action without blowing them out.
spro
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Re: RPC

Post by spro »

Within seconds I see this is so old news. So many new devices now. Just don't take Dad's or Grand Dad's pride meter out of the box in the closet for checks of capacitors like these.
spro
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Re: RPC

Post by spro »

When testing capacitors with a VOM, the ohms/ resistance section is being used. This is a delicate area and the power source of the meter is a supply towards charging the capacitor. From this charge, you can see the charge slowly sweeping the needle as the capacitor charges and switch the meter to a higher range.
If you reverse the leads, there is voltage not resistance. The most sensitive section of the meter is hit with a charged capacitor. Back in the days when techs had VOMs some techs repaired the VOMs. There were bent needles where they had slammed so hard against the limit posts, the posts were bent. (delicate posts). Some meters showed signs of loose movements where the needle had been scraping against the inner bezel or register face. The rare, yet noteworthy, were those completely blown out of the ruby bearing movements.
I should add that there were many voltages around. Millivolts, AC and DC in the hundreds. Just tools.
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