[Sneap] GVM needed for General IonX Tandetron

Thu Jan 17 13:54:11 EST 2008

Klaus,

Indeed, it did help. I was just about to recover the SF6 from the tank without first checking the obvious things you mentioned. Except for mechanically checking the bearing, all of the things you mentioned can be electrically tested on the air side of the motor without opening the tank.

The phase-shifting capacitor is the most logical component to fail. On large split-phase motors delivering significant power to a rotating load, this capacitor will have a lot of current and high voltage stress. This is known to cause overheating and subsequent dielectric failure if the capacitor isn't properly sized and rated for this type of motor service. Fortunately, for the GVM application, there is only a small "windage" load so the capacitor is only lightly stressed. Alas, it measures 4.27 microfarads using my BK Precision Model 5360 DVM and has no measurable DC leakage resistance using the same meter. So, I conclude the capacitor is probably still in good condition.

The motor windings are another story. This motor has four wires coming through the bulkhead fitting. One of these is a "shield" that is connected to one of the four mounting screws securing a 4-pin Amphenol circular MIL-style bulkhead connector. This connector brings in two of the three phases plus neutral to power the motor and the electronics in the GVM. Two of the other three motor wires connect across phase A and phase B of the three-phase 208/120 VAC wye feed from the Tandetron. The third motor wire connects to one side of the phase-shifting capacitor, and the other side of the capacitor connects to phase A. One would expect to measure motor winding resistance between any two of these three wires, but each wire indicates an open circuit to the other two. One wire, connected to phase A, measures about eighty ohms resistance to ground. So, not only are the windings open, one of them has shorted to the case. The outer shell of the GVM gets quite warm when the AC power connector is plugged in. I don't recall it ever heating up during normal operation, but my memory could be faulty.

Well, on with the task of removing as much SF6 as I can using the liquid nitrogen sublimation process... 

Thanks to everyone who has offered help so far. I sure hope I find a Rotron motor inside that has a modern replacement!

Howard

-----Original Message-----
From: sneap-bounces at tunl.duke.edu [mailto:sneap-bounces at tunl.duke.edu]On Behalf Of Klaus Bahner
Sent: Thursday, January 17, 2008 11:00 AM
To: 'Symposium of Northeastern Accelerator Personnel'
Subject: Re: [Sneap] GVM needed for General IonX Tandetron

Howard,

First of all you should find out why the motor has stopped working. 
Is it blocked mechanically, due to a defective bearing? -> replace bearing
Is the phase shifting capacitor defective -> This is the most typical failure mode. Replace the capacitor and the motor will work again.
Are the windings OK? ->If one of the motor coils has opened or short circuited, check whether there is no overvoltage/overload on the motor (without overload motor coils typically last forever) and get the motor to the next motor repair shop, where they will re-wind it for a comparatively small amount of money. I did that with our GVM motor a couple of years ago, because re-winding it was much cheaper than any new motor and saved me from making any mechanical changes. And it took less time to re-wind the motor than getting a new one - just one day.

By the way, precision bearings may improve your mechanical noise and jitter performance of the motor, but will certainly have no effect on the lifetime of the motor. 

Hope that helps,
Klaus

  _____  

From: sneap-bounces at tunl.duke.edu [mailto:sneap-bounces at tunl.duke.edu] On Behalf Of Howard Evans
Sent: 16. januar 2008 22:26
To: Symposium of Northeastern Accelerator Personnel
Subject: [Sneap] GVM needed for General IonX Tandetron

SNEAPers: 

The motor in the Generating Volt Meter (GVM), used to monitor and provide negative feed-back control of the terminal voltage in our 1.7 MV Tandetron, has stopped working after only twenty-seven years. Does anyone reading this message have a complete GVM assembly (including the weldment that attaches to the side of the accelerator tank) they don't need? It doesn't have to be in working order, although that would be nice. If we have to replace these things every twenty-five or thirty years, I would like to have a spare on hand.

Actually, what I really need right now is the motor. All I know about it at the moment (before I open the tank later this week) is it is a split-phase induction motor, with a 4 microfarad oil-paper phase-shifting capacitor, operating on single phase 208 VAC. It is General Ionex Corporation part number A-08723, which is probably meaningless in 2008, but may have made sense to someone in 1981 when it was penciled into a drawing.

As someone posted earlier in this forum, regarding ordinary versus premium bearings, we would like to invest in a motor with "premium" bearings in the hope that the motor will operate for a longer period of time.

I tried using the terminal voltage monitoring tap on the voltage-grading resistive divider to control the terminal potential... it seems to have been designed for this purpose originally... but the results are not satisfactory. The ion energy wanders at will and seemingly randomly. We want to do an implant that will take several days to complete, but we cannot spare someone to continuously monitor and re-adjust the beam energy and/or position. So I need to get this GVM repaired or replaced as soon as possible, if not sooner.

Howard B. Evans, Jr. 
Engineer, Materials Laboratory 
UES, Inc. 
4401 Dayton-Xenia Road 
Dayton OH  45432-1894 

937-426-6900 ext. 116 (office and voice mail) or ext. 121 (lab) 
937-426-5718 fax 

hevans at ues.com e-mail 
http://www.ues.com web site 

"Things should be explained as simple as 
possible, but not simpler." -- A. Einstein 

******************************************************* 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://daytona.tunl.duke.edu/pipermail/sneap/attachments/20080117/1cf92edf/attachment-0001.html 