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3 phase power issue

Mr. Mindless

Well-known member
Joined
May 27, 2020
Member Number
1584
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175
Loc
Rochester NY
Cross posted on practical machinist...

I'll share some unnecessary detail for background in case it turns out to be important...

TL;DR: my "made up" leg has voltage, but my motors are drawing very little on that leg, and I'm popping 3ph disconnect breakers on my mill. All seems perfectly fine other than the breakers popping after a few minutes of run time, and investigation of things turned up only that one thing: minuscule amperage draw on that "made up" leg.



I got a rotary converter setup along with a lathe. It was in use at a shop together, and worked fine for me after transport. It's an idler motor with a phase-a-matic PAM-600 static converter, and a barrel switch for reversing the lathe motor. Neither idler nor lathe motors have data plates, I'd hazard a guess at 3-5hp based on motor size, and that's also the power range on the PAM-600.

I added a Tree milling machine to my setup, wiring all motors and the phase converter in paralell. Both motors on the mill needed new leads, and I had a motor shop do that for me. The main 1.5hp motor test good after changing the leads. The crossfeed motor wasn't test run, since it was missing a support bearing while removed.

My feed from 240-1ph is a knife switch, which had 30a fuses in it. I reversed my lathe too quickly and blew ONE of those fuses, which then blew the single capacitor and cooked a 10w500ohm resistor in the phase-a-matic. I replaced those with spec-matching components, and motors now start great, and the lathe runs fine. That switch is no longer fused. It is fed from 50a breakers in my panel via 8ga wire.


Each of the two mill motors run through their own Square-D 3 phase disconnect/breaker. Square D - 2510MBG2 - Starter | SuperBreakers.net

After 3ish minutes of run time, those breakers trip for either mill motor. They trip more quickly after that if immediately reset.

I checked continuity of all wiring from phase converter to barrel switch, and found no issues. I checked running voltage across all legs and found they're a bit off; from some reading I did in the stickied thread on rotary phase converters, it looks like that's expected without additional capacitors in the setup. I have 200/209/233 across ab/bc/ac. I have 15-18a draw on legs A and C (those supplied directly from my 240), but I see a startup spike on leg B and then only .5a running.

I checked draw on the lathe legs, and it's 12.8a on A & C, and 2.4a on B.

I switched legs on my idler motor, and it made no difference.

I verified motor wiring, and all are 1-7 2-8 3-9 4-5-6

I reread the phase-a-matic documentation and saw it indicated to put a SPST switch in the B wire if using an idler motor. I did that, and symptoms did not change.
 
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If you are running a static phase converter you won’t see current on B phase while running. You only get 3 phase power on start up then the capacitors are disconnected from the motor windings and you run on single phase. Basically why you get 2/3 horsepower you are single phasing the motor.
 
and I should see load (amps) from the idler motor to the running machine after startup. but it's minimal, nowhere near what the idler is drawing on its other two legs nor what the running machine is drawing on its other 2 legs.
 
I would get rid of the 10 gauge wire after you figure this out.
 
it's awg 8, not 10. you made me double check. editing OP.

took some further measurements, and verified that the thermally controlled breakers are getting hot to the touch. I almost wonder if the contactors are dirty. it's supposed to be at 18 amp break, and I'm pulling 4.5 through it.


N-A 118v
N-B 167v
N-C 115v

A-B 199v
A-C 233v
B-C 210v

head motor 1.5hp 4.5a
A 4.6a
B .49a
C 4.5a

feed motor .5hp 2a
A 2.1a
B .47a
C 2.3a


I disconnected the idler motor and ran on just the static converter with head and feed motors. didn't measure amp draws, but breakers still got warm and popped.

I'm thinking I have faulty breakers.
 
What kind of breakers? If you have the what I call the euro trash they have leg loss tripping. If one leg gets to low compared to others it will trip. You may have to replace them with a old school square d breaker.

my Local automation Supplier and get in battles over that shit. They love those tiny Schieder motor starters and overloads. I hate them with a passion. The old school nema contacts and breakers were awesome. They just flat out work in harsh environments.
 
They are Square D - 2510MBG2, linked in first post.

Find a lot of places to buy them for a bit under $200ea, but didn't have any luck in looking for leg-loss or differential power issues. makes me want to sell them for half that and put dumb start/stop switches on it and let the panel feed breakers take care of any issues. But when I don't know if they're working right...
 
I would start looking on how the leg label B is only drawing 1/2 a amp.
 
Are you sure that you have the motor wired right? You have it labeled as a Y. Delta would be T1-1-7-6, T2-4-8-2, T3-3-5-9 or at least that what google says
 
Your voltage readings phase to phase dont look good at all. Were are you measuring phase to N at? Nuetral is only used in 4 wire.
 
Are your voltage readings while it's running? Obviously your current readings are.
 
I dont know much about phase converters but it appears a and c phase measured to b is putting out 208 voltage. Someone mentioned you have it wired y. That sounds like your issue.
 
Both mill motors are wye, they have data plates intact.

previous setup with the phase converter and lathe wiring were not touched, and were wired wye already. data plates gone.

the voltages don't change with load, have measured both ways. I'm grabbing neutral from a nearby 120v outlet that originates at the same panel.
A large rotary converter thread at practicalmachinist tells me my voltages are relatively as expected without adding other capacitors between legs to correct. It was somewhere in this 10 page thread https://www.practicalmachinist.com/.../rotary-phase-converter-designs-plans-101231/

I posted over there as well, the most recent reply in my thread indicates that low draw on the third leg isn't necessarily crazy, since that leg is just kinda idling between the motors until one is under load. That certainly makes sense intuitively. I don't expect I'll ever put REAL load on that motor except when I run out of talent and engage tool breaking mode. It was also suggested that the thermals in my breakers may have been swapped to match load, and I do know that my machine was previously running on 440, so that load has now doubled. I'll see if I can get any meaning from the markings on them.

In the mean time I've bypassed the thermals and I'm just using the switches. No increase in wire temp nor current draw with extended use, so I'm comfortable with that situation for the moment. It's still got a disconnect right there, and a 50a in the panel box.
 
You still have the 480 thermals in? Doesn’t the cover have a chart to size for the corrected amps?
 
Not familiar with how your phase converter operates, but if you disconnect your idler motor and have the same symptom, I'd be suspect of the idler motor, the wiring, or the phase converter itself.

I thought phase converters were either static or rotary, not both. With my phase converter, if I disconnected the idler, there wouldn't be a third leg at all, and none of my three phase motors would start.
 
I suppose the accurate way to say it would be a static phase converter with idler motor?

I don't think I ever looked at the inside of the front covers on the switches. All photos I have, the covers are right side up and can't see the inside of them. Figures!
 
Might be time to ditch the rotary and go with VFD's on everything. Gets rid of some bulky equipment and gives you variable speed.
 
Lots of three phase machines already have variable speed built in via gearboxes, or belts and pulleys, and three phase motors running at lower than rated hertz have much less power than at rating. Doesn't really matter when you're driving a constant load motor application, but stuff like mills, lathes, sanders, it's an issue.

Also, some VFDs don't like interrupt style switching, so the machine's stop/start switches, or fwd/rev switches, become useless. More expensive VFDs that don't have that issue become more expensive than a decent rotary setup very quickly.

Source: had a vfd on my bridgeport for a bit (barely worked) and a vfd on my rockwell lathe (worked ok, not great), then a static phase converter (worked, but slow), and now a rotary setup. 10hp rotary setup powers my bridgeport, leblond lathe, 10x72 belt sander, and 5hp air compressor today. Still have a VFD to power some three phase fans in the garage, and it works great for that.
 
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My Tree mill has constant variable belt drive. the lathe has 4 belt speeds that are annoying to change, plus a high/low shifter on the motor drive. I have plenty of speeds, and I'm a cheap fuck.

No interest in changing what's working (now that it's working) for the sake of changing it. I have $200 and a road trip into the lathe and phase converter, and the cost of motor lead replacement into the mill. I'd rather spend on tooling.
 
When you took the amp readings that was in a no load condition?
 
GLTHFJ60 said:
Lots of three phase machines already have variable speed built in via gearboxes, or belts and pulleys, and three phase motors running at lower than rated hertz have much less power than at rating. Doesn't really matter when you're driving a constant load motor application, but stuff like mills, lathes, sanders, it's an issue.

Also, some VFDs don't like interrupt style switching, so the machine's stop/start switches, or fwd/rev switches, become useless. More expensive VFDs that don't have that issue become more expensive than a decent rotary setup very quickly.

Source: had a vfd on my bridgeport for a bit (barely worked) and a vfd on my rockwell lathe (worked ok, not great), then a static phase converter (worked, but slow), and now a rotary setup. 10hp rotary setup powers my bridgeport, leblond lathe, 10x72 belt sander, and 5hp air compressor today. Still have a VFD to power some three phase fans in the garage, and it works great for that.
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The machine switches just need to be rewired to operate the logic side of the vfd, they all have it. Fwd/Rev, stop/start, etc. A vfd is a great solution to any 3ph single motor less than 5hp that you need to run off single phase.

Where you run into issues are multiple motors and contactors, switches, and solenoids that would need an entire panel built with many drives and a plc to control the machine.

I've got vfd's, rotary, and static converters in my shop. All were chosen to be the best fit for certain machines.
 
Dethmachinefab said:
The machine switches just need to be rewired to operate the logic side of the vfd, they all have it. Fwd/Rev, stop/start, etc. A vfd is a great solution to any 3ph single motor less than 5hp that you need to run off single phase.

Where you run into issues are multiple motors and contactors, switches, and solenoids that would need an entire panel built with many drives and a plc to control the machine.

I've got vfd's, rotary, and static converters in my shop. All were chosen to be the best fit for certain machines.
Click to expand...

Right, I'm aware of that. This however does not fix the reduced power problem at lower than rated hertz though. Furthermore, if you're using the VFD just for creating the third leg (and start/stop, fwd/reverse) then you're paying for features you're not using, aka the variable part of variable frequency drive.

The point I made was that single a rotary phase converter setup, when working properly, is usually more economical, and certainly less labor intensive, than setting up VFDs for each one of a multitude of machines. Given the choice of a VFD on each of my 4 three phase machines (with each machine rewired to work properly with it) vs a single rotary phase converter to feed them all, the rotary wins every time.

To each his own. There are definitely pros and cons to each approach.
 
Isnt his problem that he is using 240v delta grid power and converting one 3rd phase to output wye 208v? Or maybe it doesnt mater. I believe that's why his volt readings are not matching. Maybe that's why the breakers keep tripping from voltage imbalance on the motors. When there is a large imbalance some of the phases on the motor will work harder to compensate.

I'm sure the shop that the equipment came from probly had 208v grid power at the main panel.
 
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I would change to thermals for the 240volt amp draw and read the amps on the low leg under a load. The mill without out the load is acting like the idler.
 
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