Phase convertor problems

[John RandallParticipant @johnrandall96767]

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Edited By John Randall on 19/11/2017 23:45:10

[John RandallParticipant @johnrandall96767]

[John RandallParticipant @johnrandall96767]

Posted by John Randall on 19/11/2017 23:41:26:

Earlier this year I purchased a 3 phase convertor to run a 13 inch Harrison lathe, the lathe has 2 speed motor which give 5,5 and 3 HP respectively. which the invertor is rated for. The problem starts when stating the lathe is a contactor in the invertor bounces violently in and out 4 or 5 times on both motor settings before the invertor settles down to run the lathe motor. I had seen the lathe run before I bought it, indeed the invertor runs the lathes own coolant pump.And although smaller runs both my myford and my milling machine ok. The instructions say to wire the started across L1 & L3 phases I have checked these and they are wired correctly. but the problem still persists. Does anyone have any idea how to cure this please ?

 

Edited By John Randall on 19/11/2017 23:48:04

[not done it yetParticipant @notdoneityet]

If it was a new item, what response have you had from the supplier?

[John RandallParticipant @johnrandall96767]

I returned it and they say no fault found from there service department . They are saying its not faulty hence the question,

[Tractor manParticipant @tractorman]

I had the same thing running a cut off saw off my rotary converter. It was rated ok but made a horrible noise on start up. The manufacturer advised me to note the wiring positions in the plug and rotate them all one place on and try the machine, if that didn’t work rotate them again. Hey presto two swaps and no more problems. Something about the way the converter does its job. Worth a try as it worked in my case..

[Clive FosterParticipant @clivefoster55965]

Presumably its a rotary phase converter with a step-up transformer to give 440 volts and the contractor coils are correctly connected across the transformer outlets.

If so its under-rated for the job. Those two speed motors are notorious current hogs on start-up and if the transformer cannot supply the full current needs the voltage drops enough to let the contractors pull out. As soon as the contractors pull out full voltage is restored so they pull back in. Effect on motor is sort of like getting a dead car rolling by rocking it back and forth. After three or for chatters (rocks) the thing gets moving on the the current the transformer can supply.

Electrically savvy folk with appropriate knowledge, experience, understanding and instrumentation can sort such problems. Usually doesn't take a lot to go from almost works properly to does work properly, albeit with perhaps less safety margin than you'd ideally want. If you know what you are doing.

For ordinary guy in the shop best solution is brute force with a bigger converter. On home built converters enlarging the rotary element / pilot motor can fix the problem. Were I to do a home brew (which I never shall again) I'd probably use a 7.5 HP pilot and a theoretically oversize transformer.

Commercial manufacturers have to hit a sensible price / performance ratio so sometimes a device that works perfectly well for all normal use will fail for the edge cases.

If its a static converter basically fergedditt. Generally running such motors is beyond their pay grade.

Clive.

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[John RandallParticipant @johnrandall96767]

Gentlemen thank you for your reply's it is a static convertor so I guess I will have to come up with another plan to run the lathe before the invertor expires.

[Nathan SharpeParticipant @nathansharpe19746]

I have a Harrison L5 with the dual speed motor, it runs perfectly on my Transwave Static Converter as it did eventally on the original Silverdrive unit I bought. Have you set the selector switch correctly to give as close to motor voltage/slightly over? Do you keep the motor warm? I have a small tubular heater wall mounted next to the motor and it seems to help. This heater was fitted because initially the Silverdrive had the symptoms you describe. Nathan.

 

Edited By Nathan Sharpe on 20/11/2017 20:00:02

[John RandallParticipant @johnrandall96767]

Nathan at the moment I am prepared to give anything ago so thanks for that John

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[BramhpParticipant @bramhp]

Had exactly same problem, trans wave static, been working fine changed lathe and encountered same problem as yourself. Thought lathe… nope.. thought capacitors in converter and took back to transwave they checked it over and found nothing wrong…. bought slave motor and connected that up, this helped in that I could get it working but still wasn’t brilliant. Then moved wiring around as per ‘tractor man’ bingo cured all issues sweet as a nut, doesn’t really need slave motor but bought it wired it fitted it under bench so it’s staying. Anyway I would try this before anything else as it won’t cost anything apart from 10mins of your time.

[Clive FosterParticipant @clivefoster55965]

I'm impressed that Nathan is able to get a static converter running well on both speeds. Ones I've run into usually won't play ball. Either can get one speed going decently and the second one either not working at all / barely limping along or both clearly unhappy and not giving full power.

Got a 4 HP max MotorRun static way back when I first installed my Bridgeport from a guy who was selling it because it wouldn't run the two speed motor on his new-to-him lathe. Colchester I think.

Adding a pilot motor to make a static converter into a crypto rotary has always worked well for me when arm twisted into sorting problems for other folk. The old MotorRun ran my Bridgeport OK but never seemed very happy and changing settings was sometimes desirable when using larger cutters. Had a 5 (or maybe 3) HP motor about the place which I connected as a permanently running pilot. Transformed performance. Much happier running and a clearly faster, better, start. Sold by it mail order complete with pilot motor to a bloke who rang me up later to say how pleased he was with how well it ran the two speed motor on his Colchester lathe. Which was nice but, frankly, I'd have not risked selling it to him if I'd known it was going on a two speed motor.

Connection dance still happens even with a factory rotary. My mate Mike the Pilot has a Bridgeport on a Transwave rotary which will only run properly on one specific arrangement of connections. I'm pretty sure we tried the lot. Both motor and control gear. More than once. On wrong ones it either wouldn't go at all or stopped after a random few minutes. Even sensitive to which side was live and which neutral relative to the converter input. Which is seriously odd. Been working fine for getting on for pushing 20 years since we fixed it! So I guess its sorted well enough.

Clive.

[John RandallParticipant @johnrandall96767]

Clive I a running at the convectors maximum out put at 5,5 hp.so is a pilot motor an option still an option ? I tried tricking the convertor by staring up the 3 phase coolant pump first on the lathe which runs ok but when you start the lathe main motor in either 3 or 5 hp setting its back to the alarming contactor bouncing until the lathe picks up speed,

[Clive FosterParticipant @clivefoster55965]

John

Short answer is I don't know. No experience of a system with one motor so close to the nominal maximum of the converter output. Really has to be try before you buy.

Fundamental limitation is how much current the step up transformer can supply. The transformer is considerably over-rated for load current because its supposed to supply the start up current surge without too much voltage drop so there should be ample headroom to supply the pilot motor. A 2.5 or 3 HP pilot will want around half a horsepowers worth of current. Its said that for best performance the pilot motor should be as large as, or larger than, the biggest motor but even a 2.5 or 3 hp one will give a very useful boost to the third phase of the lathe motor on start up.

The pilot motor ensures that there is properly synchronised 3 phase power on the generated leg even at start up so its much less current hungry than a static converter. More than enough saving to supply the pilot motor. Not only does a static converter effectively have to get going on only two phases it also has to fight the capacitor bank to some degree as it runs up as the third phase generated with the help of the capacitors is seriously out o sync relative to where it should be.

Clive.

[Nathan SharpeParticipant @nathansharpe19746]

John. I should have explained that the heater was there because I initially suspected a damp motor/workshop was to blame. I eventually took the Silverdrive apart , I found that a lot of switching was carried out by reed switches of unknown quality. These were replaced and all was fine until the also cheap pcb, cracked after 6 years. I replaced it with Transwave after too many repair sessions!

Clive. I've never had a moments hesitation from the Transwave from day one. The Harrison is still on it's original starter . If I have to do anything at all to start, I may have to adjust the boost/voltage control in cold weather. I wired the workshop and installed a circuit just for the lathe , no domestic light or power problems , my own installed VM shows voltages within the regs . Nathan.

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