[Gnome]

I have been trying to get my RPC running using the plans from http://home.att.net/~waterfront-wood...econverter.htm
I followed the recommended capacitor values for my 5 hp motor however, when the power is applied to the motor it makes a bad chattering noise and the motor only runs at about half or less of its rated RPM (1750). I have tried changing the capacitor values and also turning the motor shaft initially with a cord all without any luck.

The motor is a 4 pole induction motor - 3 phase (obviously) that is wired for 220 but can be wired 440. I do not know the history of the motor but when i performed a continuity check btwn each of the winding wires and the outside motor case and it seems that a couple of windings (3 to be exact - one on both the L1 and L2 lines and one on the 3 wires that are tied together to specify 220 volt input), do not have continuity with the motor case.

Does this mean that the motor is fried? Am i doing something wrong?
If the motor is fried, does anyone have a 7.5 hp or 10 hp 3-phase idler that they would want to get rid of cheap? (preferably close to vancouver BC, Canada)

Thanks a lot for any help

[m_c]

None off the winding wires should have any continuity with the case. If they did, it would mean the motor case would be live when power was applied to the motor.

Have you tried connecting just the motor to the power supply without any capacitors, and starting it off by hand? If the motor is OK, then it should run at almost full but be a bit rough/vibrate.

For starting the motor by hand in this way, I used to get a bit thin rope long enough to wrap it around the motor shaft several times (just wrap it around on itsled so that it self tightens onto the shaft, and will come staright off once unwound). Then I'd pull the rope off as hard as I could to spin the motor, then flick the power on.
When pulling the rope off, make sure you hold it, so that should it not release/get pulled back into the motor, that it'll be pulled out your hand (ie. don't wrap it around your hand/fingers), instead off you getting pulled into the motor.

[Gnome]

I have tried the string trick with no capacitors at all hooked up. I even went so far as hooking up a second motor to my idler motor to get it up to full speed before I throw the power on. All I got was smoke and some sparks – eventhough I only left the power on for a couple seconds.

So I guess that means my motor is done for. Does anyone have an extra motor? – I can’t seem to find them online or really anywhere.

If I went with a commercial rotary phase converter, is there any company in particular that you guys would recommend? Doing a quick ebay search shows tons of results of cheap phase converters – seems like a shame cause it doesn’t look that tough to build myself.

The phase converter does need to be balanced because it will be used to drive a cnc mill.

[murphy625]

I have tried the string trick with no capacitors at all hooked up. I even went so far as hooking up a second motor to my idler motor to get it up to full speed before I throw the power on. All I got was smoke and some sparks – eventhough I only left the power on for a couple seconds.

The phase converter does need to be balanced because it will be used to drive a cnc mill.

Contrary to popular belief, motors do not run on electricity..

They run on smoke.. If it leaks out, the motor is no good..

Fastest (but not the best) way to check a 3 phase motor is to do a conductivity check between each of the 9 wires coming out of it to ground.
Put your meter on x1 ohms and nothing should happen..
Then,
Then check L1 to L2 (this should look like a short)
Then check L2 to L3 (another short)
And finally L1 to L3 (another short)

If all these check out, your motor is probably good.. The above is just a cheap test that will prove positive in 90% of all things that usually go wrong with 3 phase induction motors.

Make sure your motor is wired for 220 volts and not the 480 volts.

Balancing the 3 phases is NOT always required for CNC applications..
My system (although not a CNC machine) uses servo motors and CNC controls. My RPC is not balanced and everything works fine. The reason for this (I discovered while trouble shooting) is that all 3 phases get rectified into one within the servo driver anyhow. Upon a little research, I found that this is popular in many drives.

Make sure you have a good motor before calling it quits..

Murphy

[m_c]

Certainly sounds like your motor is done for.

Rotary phase convertors aren't hard to build. I've built two of them (well same one rebuilt). First was the simple 240V similar to what you're trying to build, which was fine up until I got my milling machine, then realised I needed 415V three phase to operate the contactors. So the original one was stripped down, a step-up transformer found and rebuilt to provide 415V three phase. At this point, I also added push button start/stop (old one was just a case off flick the main power switch on and it started up). I would post up a schematic, but I don't know if I've still got a copy.

Personally I don't think commercial units aren't good value for money. When you see what goes into them, you can buy the indivdual parts far cheaper.

As for balancing them, this is just a case off selecting suitable capacitors until you get the voltages balanced.
I balanced the voltages for when the milling machine spindle is running, and everthing seems to work fine, although the voltages are somewhat off balance when the convertors doesn't have any load, but I'm not running anything that's voltage sensitive.

If you want to be more accurate, balance the convertor while it's running without any load, then add suitable capacitors to each motor so that the convertor remains balanced when each motor is running. I've heard off several people who've done this to ensure they get the motors running as smoothly as possible.

[mcmmach]

Also make sure if you need to tap off 120 volts for relays or such that you don't use the "high" leg, ie all rotary convertors I have build will have 220-240v from leg to leg but from leg to ground you will have @120v on 2 legs and 240v on the third leg. Will fry a 120 relay pretty fast!