What Have I Gotten Myself Into?!? - Hooking Up a Phase-Quest RPC to a T-Wound Trafo

[crshears]

Hey all,

I agreed to help a neighbour install & commission an industrial lathe driven by a three-phase 550V motor. His wiring is excellent...but the equipment he's using to supply it is a mixture of new & used...

Rotary phase converter output has been wired to a three-phase single-throw disconnect switch, enabling the RPC to be started without any connected load, as specified in installation instructions; so far, so good...I think.

P-P RPC output voltages as measured were [if I recall correctly] ~240, 265 & 270 volts with disconnect open.

Now here's where it gets interesting, and I wish I had had time to take some nameplate pictures, as the step-up trafo is T wound on both the LV and HV sides; P-P output between two of the HV terminals [I measured at the ones labelled 575V] were ~590V, and the other two were >600V [unfortunately my digital meter won't read higher than that]. In lieu of those numbers I also went P-G and got - again, if I recall correctly, ~350, 380 and 400 something.

I realize more info may be needed, but the question I have is, should the real output of the RPC be connected across the cross-arm of the LV Tee, and the two derived connections go from each of the two ends of the cross-arm down to the base of the stalk? Or the real voltage applied from one of the two ends of the cross-arm down to the base of the stalk?

I also intend to see that the lathe control circuitry gets connected across the realest two phases that show up at the lathe.

I still plan to take some pics to post here, as I ran out of time yesterday.

Thanks for any recommendations.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

I'm assuming that your RPC output is 240 Volts.
The cross arm should have 240 Volt taps and the vertical arm should have a 240 Volt tap.
Voltages around 600 Volts should be OK. (575V motors are normally fed from 600V supplies)
The wye voltage of a 600V circuit is 347 Volts so your 350V, 380V and 400 Volts are probably OK.
Measure again when the motor is connected.
You should be closer to 600V and 347V.
You should be even closer when the motor is loaded.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Thanks, Bill; I was wondering whether the inherent dysphasia/imbalance of the RPC output would best be addressed by connecting the "underived" 240V output of the RPC to the cross-arm, and if I understand you correctly that is the best way to go.

I will let you know how it turns out.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[bacon4life]

Might want to double check the rating of meter to see if it is rated to withstand more than 600V.

 

[waross]

Once the motor is connected it should balance the voltages somewhat.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Thanks, bacon; for sure my meter didn't like reading anything greatly in excess of 600V, but the withstand was there, at least...and the neighbour's digital multi-meter proved to have a range just wide enough to show no-load readings of ~580, 600 and 640...and did the RPC ever quiet down as soon as we started the lathe motor!

We didn't finish taking all the readings though before one of the 10A fuses to the lathe popped, something that din't worry me overly much once the neighbour told me they were quite old; I've heard that with age the fuse wire alloy tends to go crystalline and greatly reduce the amount of current needed to blow the fuse...

Will still try to get some pics and report the as-built readings once we've collected them all.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[itsmoked]

CR; I don't know if I agree with the 'old fuse' theory. You may be in for a ride in the barrel. If the phases still aren't very even you could have lots of fuse-loading circulating current.

Do also realize that if you have so much as a 10W light bulb hooked to the RPC when you turn it on you will let gobs and GOBS of smoke out of the RPC. I'm always leery of a disconnect between the RPC and everything else as all it takes is a bad shut-down process by the user to toast the RPC. I'd prefer a timer and a contactor. That way when, say, the owner gets in the groove of "turn off the load disconnect then turn off the RPC disconnect" a visitor uses the tool and turns off only the RPC. Next time the owner shows up and throws the RPC switch he has to leave the shop by crawling along the floor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[crshears]

Yeah Keith, I did mention that to the neighbour and will push harder to have him include that; I might even have an old Agastat relay lying around that can be used to close in the transformer energizing contactor, say, five seconds after the RPC starts, just to be on the safe side; it seems to spin up to speed in less than one.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[crshears]

Finally got back to this today...

Curious thing: lathe motor is acting as if it's single phasing; it's only rotating slowly, yet the currents in the leads to the lathe are 27, 31 and 51 amps respectively. Asked owner if he was aware if anyone had messed with the connections between the motor contactor and the motor itself, and the answer was no...he himself reported having disconnected it from its installed location, and has never re-wired anything other than the supply to the entire lathe...and reportedly it ran fine previously.

May or may not be relevant: lathe has been in dry but unheated storage for some time...

No-load phase to phase voltages to lathe are ~590, 610, & 640, which Phase-Quest rep has said should level right out when motor is connected; haven't verified that...

This is a pic of the nameplate of the step-up trafo we're attempting to use:

The "real" 240V output from the RPC has been connected to the X2 and X3 leads on the low side, with the virtual/phantom/wild phase connected to X1. Although the X0 is shown on the nameplate, its lead isn't actually brought out to the headboard...not that we're using it.

HV output has been connected to the 575VAC terminals.

What's happening? Circulating current or something?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Is the RPC new or used?
Have you checked the capacitors for a bad cap?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

“In storage for some time...” if not powered up for a couple of years that generally leads to capacitors deteriorating. The aluminum oxide layer that they need internally to prevent shorting out will dissolve back into the electrolyte over time so when re-energized without it, the layers in the capacitors become a dead short and burn. There is a “reforming procedure” to re-establish that AlOx layer again to prevent failure that can be done BEFORE trying to use them, but once energized without doing that they fail quickly and are toast. That would be my suspicion.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Skogsgurra]

It would surprise me if electrolytics were used in a pure AC circuit. Or is there a VFd involved?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[crshears]

New RPC & starter box, so don't suspect bad caps...but you never know...

Used trafo & lathe; don't see any caps on it.

No VFD involved.

Despite owner's doubts, I'm beginning to wonder if lathe motor is delta connected and has an open in there somewhere; would explain why leads are recording current values reported above...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Roll the connections and see if the high current stays on the same RPC phase or if it follows the motor.
If the current unbalance follows the motor then suspect a motor issue.
If the high current stays with the same phase of the RPC it may be normal.
Also, if the voltages do not even out when the motor is connected suspect a motor issue.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Thanks, Bill et al; will see what else I can figure out, but it might take a few days...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[crshears]

Hey all, situation has been resolved, as follows:

Owner [a retired machinist with 42+ years of experience] did some experiments, first determining that the RPC would successfully start and run a 5 HP 3-ph 600VAC table saw [it did].

He then reconnected the lathe and tried starting it with the motor wedged up so the drive belts were slack; the motor spun up to speed without issue, indicating a mechanical problem with the lathe...

Partial tear-down of lathe revealed that both the clutch and gear shifting mechanisms were way out of adjustment, meaning the motor had been punching way above its weight by trying to accelerate a load well in excess of its locked rotor torque. Clutch was adjusted so that it was disengaged both forward and reverse when its control lever was in neutral; similarly the speed shifting levers were re-calibrated so the pointers matched the gear ratio selected.

All is well and owner is very appreciative of the advice and assistance.

"A merry Christmas to all, and to all a good night!"

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[itsmoked]

Thanks for the ET Yuletide comeback gift!

Keith Cress
kcress -

http://www.flaminsystems.com