Re: [TCML] Terry Filter Caps

[FIFTYGUY@xxxxxxx]

In a message dated 8/26/08 9:57:24 P.M. Eastern Daylight Time,  
yurtle_t@xxxxxxxxx writes:

>BTW,  I just blew up my second VFD :-(((.

    I killed two of my VFD's as well. They were old  Motortronics units, 
nothing I was too excited about, but they were perfectly  fine pulls until I used 
them to run the ASRG. 
    I thought the first one was on it's way out, since  it kept tripping its 
upstream MCP on power-up. It worked fine until I started  tuning the coil and 
getting more power out of it. Gave a nice flash and stopped  working. Did a 
power cycle and got a *BANG*. The other one (identical) didn't  trip the MCP, so 
I thought it would do better. Nope, lasted all of 20 seconds.  That's when I 
stuck the SISG under there!
    Autopsy revealed not only was the power module  blown, but the driver 
IC's were vaporized off the board! At that point I swore I  wouldn't pay more 
than $50 for a drive that I would sacrifice on the altar of  Tesla, and I set off 
to improve the transient protection of the control  system.
    I built the Terry Filter. I also put common-mode  MOV's at the jack for 
the cable going out to the RSG motor. I had also run the  RSG motor cable in 
the same flexible conduit as the two hots from the pig.  Despite the RSG cable 
shielding, that probably wasn't the best way to do  it!
    I bought a *really* cheap VFD. Apparently Dart  Controls, who makes a lot 
of fractional HP DC drives, did a stint of little  inverters! Discontinued 
now, and they just don't get more cheaply built than  this thing! I've got a 
*tiny* Mitsubishi as a back-up, but I'll be darned if  that Dart hasn't died yet! 
    It really does take 1/2 HP to accelerate a 12"  RSG with a 3/4" G10 
8-electrode disk to 3600 RPM in 20 seconds. It takes 2/3 of  that to keep it 
spinning at that speed! 
    At least one good thing about running a  series/parallel Y motor off 240 
is it gives some headroom for transients.  Running them off 480V with an 
inverter gives little room for noise in normal  applications - it's usually the 
first few windings that die when the switching  spikes burn out the insulation.
    I'm still undecided as to how to ground the motor  frame and cable 
shield, so I've just left them floating. :)
 
-Phil LaBudde 

Center for the Advanced Study of Ballistic  Improbabilities



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