1 leg of phase converter too hot. Series 1 cnc problem

[cnc1000]

I have a rotary phase converter rated at 5 hp running a Bridgeport Series 1 cnc. The spindle motor is 2 hp. The manufactured leg of the RPC is 275V and the other two are 242V. I'm afraid I will blow out transistors in the drives unless I can drop the input voltage or otherwise drop the drive motor voltage. These are open circuit voltages and when hooked up, will be somewhat lower.

Is there some easy solution to this? Thanks. cnc1000

[Al_The_Man]

You may want to try tuning it, see the old RPC build by Fitch Williams, look at the FRW files.
metalworking.com - /dropbox/_1998_retired_files/
Are the drives on all three phases? or can you ensure they are on the 240v legs if 1 phase input?
Al.

[underthetire]

This is interesting on rotary phase converters. Sounds like you may need some capacitors..
SINGLE to 3 PHASE CONVERTER

[cnc1000]

I spoke to Arco-Electric, who made the converter and they said the open circuit voltages sound right for that converter. The previous mill owner said he made sure the electronics were driven from L1 and L2 and don't let L3 (the manufactured leg that is too hot) go to the electronics. I'll check that.

Also, I noted something odd. On the high current transistors there are some diodes across the base-emitter junction. I understand what they are trying to do, but I don't understand why they are not across the COLLECTOR-EMITTER junction. That is where the flyback diodes should be in order to shunt the reverse voltage from collapsing motor fields.

NPN bipolar transistors need flyback diode protection across C-E. Maybe they are installed somewhere else, preferably close to the motors.

Ideas? Thanks for the help. cnc1000

[cnc1000]

Further info: Replaced the blown transistor on Y and it seemed fine until (of course) I tried to demonstrate it to someone. It immediately blew a transistor on X, this time. Arrggghhhh!

Reading the schematics, it appears that all 3 phases are used to run the xyz steppers, one per phase. Hence, I can't move the motor supplies to simply the L1 and L2 from the street. Is there a way to reduce the voltage driving each motor, maybe? Transformer taps or something? At this point, I'm unable to do anything, since I fry transistors almost immediately.

Ideas? Ideally, I'd like to drop the manufactured L3 leg down from 275V to 240, like the other two.

[Al_The_Man]

Originally Posted by cnc1000 Ideas? Ideally, I'd like to drop the manufactured L3 leg down from 275V to 240, like the other two.

I originally built mine from the F.Williams details in the link I posted, if you look at the FRW-7 it shows how to tune it, You need an AC voltmeter and Ammeter, and a selection of ~20ufd oil filled AC motor rated caps..
Al.

[cnc1000]

Ah! Ok. Looks promising. Unfortunately, the posted picture text is so small, I can't read some of it. At least now I know it's possible to lower that third leg by messing with the capacitors. Tried zooming in on it and it's pixelated, so it's as resolved as I can make it. I'll have to go back and review how 3 phase motors work. Is there a better schematic somewhere that more clearly shows the small text? I have AC current and voltage measurement capability here.

I assume it would be best to balance it with the mill running. The text I can read says that this might make it run rougher, unloaded. I really have two loading conditions. One, is where the electronics are on, including the stepper motors. Any time they are on, I run the risk of popping transistors if any of the phases are too hot. So, I need to strike a compromise between loaded (with the 2hp spindle motor running) and when the spindle is not running.

This is the only 3 phase load I will be running with this RPC, so I can tune it just for this machine. Thanks very much for the help. Looking forward to some help reading the small type on that schematic. cnc1000

[Al_The_Man]

See if this is better, also I have an old pdf from a long gone site.
For some reason the PDF or ZIP files won't load, if you want to send me a PM with the email address I can send it.
Al.