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Rotary converter draw issues, and popping motor start control breakers.

Mr.Mindless

Plastic
Joined
Oct 1, 2020
I'm new here, and looking for some assistance with a new setup. 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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I would be questioning the static converter, not that I have any experience with them. I seem to recall that the static converters only supply power to 3rd leg on startup, but could be wrong. Sizing might be your problem too, without knowing what the idler hp is, all we can do is guess. General rule of thumb is you need 2x the hp for an idler, so if your mill is 2hp, you would want 4hp (realistically 5hp because a 4hp motor is unicornish).
 
once the idler starts (which it does without any issue), the static converter can be completely removed from the picture. I can turn off the SPST switch on the B leg, and nothing changes at all.
 
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.
 
European motor starters usually trip on current imbalance; i.e. if one leg is carrying significantly less current than the others. This is to prevent single phasing, which is essentially how a rotary converter works. I don't think that style of breaker does, though.
 
If the breakers trip on less than their rated trip current, then they must be tripping on something else, OR they are defective.

Any thermal element breaker is sensitive to external heat. If the contacts are dirty and heating up, that can cause early trip, but does not have to. It depends on whether the heat affects the element. With fusible alloy types (with heaters) that is less likely, and that one is stated to be a melting alloy type.

The rating is 18A, but that may be a max, the actual melting alloy element (or heater) used is what sets the current for the actual unit.

Another site says 3 "thermals" are required. So that indicates the elements will determine the trip point.

You need to determine what element or heater is installed.

Replacing thermals... video:

Installing Thermal Units & Resetting NEMA Class 2510 Manual Starters | Schneider Electric Support - YouTube

tech sheet (not helpful)
https://www.zoro.com/static/cms/enhanced_pdf/ZPcjMiJdsnHXZjMcg4WMmqQgeszPVfmrOiUfSdqggiQiUe.PDF

Pictures of yours?
 
Thanks for the links!

After after monitoring draw and lack of any notable wiring temps I decided to bypass the breakers, and after about 10x the duty cycle of what the breakers were limiting me to, I had no change in current draws and all wire temps continued to be nominal.

The guy who sold this machine to me stopped by today and I learned it was on 440 power in its previous installation. We wondered if breakers were specifically sized to that and are popping since I now draw twice that on 220.

I’m still very curious why I don’t see draw on the third leg from my idler motor, but the immediate problem is resolved in a slightly bush-mechanic fashion.

I do recall seeing markings on the “thermals” as I connected my load circuits above them. I’ll try to cross reference those somewhere. Hopefully I’ll find they are 4a or less. I didn’t take any photos.
 
If you supply the machine with a different voltage, you did check the motors were wired for 240, right?

They could well just be doing their job.
 
Thermals rated for the draw at 440V will indeed likely trip after some delay, when operating at 230V. Just the idle current will be close to their rating.

.....

I’m still very curious why I don’t see draw on the third leg from my idler motor, but the immediate problem is resolved in a slightly bush-mechanic fashion.

I do recall seeing markings on the “thermals” as I connected my load circuits above them. I’ll try to cross reference those somewhere. Hopefully I’ll find they are 4a or less. I didn’t take any photos.

The draw at idle on the "generated leg" will be low at idle, because the voltage generated by the converter and the voltage generated by the motor are about equal, without boosting by capacitive compensation ("balancing") or other means. With "balancing", the phase is often changed enough to affect current draw.

When the motor is loaded, it slows, and the back EMF falls, so the converter output exceeds the motor back EMF, and current is supplied.

You might check to make sure that current increases with load.
 








 
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