Re: ARSG drive motor - was: [TCML] July 4th and why not to be in a hurry

[FIFTYGUY@xxxxxxx]

In a message dated 7/10/08 7:32:45 A.M. Eastern Daylight Time,  
drieben@xxxxxxxxxxx writes:

>As I said before, I run 0 - 120 volts to it via a 7.5 amp, 120 volt  variac, 
then run 
>this variable voltage through a FWB rectifier.





I've done this before for secondary winding jigs.  The nice thing is, without 
any feedback it's easy to stall the motor (no IR  compensation or armature 
current feedback). So it gives a nice, sloppy,  friendly "feel" when 
hand-winding. 
    I suppose it's fine for ASRG service, but it  might act a little weird 
depending on air resistance or bearing friction. I  guess if it runs a little 
slow at the same pot setting, you just "turn it  up"!
 
>I also place a 400 VDC, 3900 uFd 'lytic cap in 

>parallel with the rectified input to the motor. Not  

>really sure that this cap is necessary but I thought  

>that it may smooth out the ripple a  bit.
    "Necessary" would depend on the motor. DC motors  are rated by the Form 
Factor of the DC power that you drive them with.  Single-phase full-wave 
rectified juice actually has a better form factor than  single-phase phase-angle SCR 
controls. But obviously not as good as a 3-phase  full-wave source or a 
battery, of course. I dunno if your motor was meant for  single-phase SCR 
phase-angle control, since a 110VAC line gives you a max 90V  armature voltage (180 
from 220VAC). Maybe it was for a PWM drive?
    Form Factor is the ratio of the RMS voltage to the  *average* voltage. 
For a sine wave (your FWB) that's 1.11 . For a square wave  (PWM) it's 1.0 . For 
an SCR phase-angle control it depends on the phase angle at  the moment... I 
think the NEMA designation for "single-phase SCR phase-angle  control DC power 
source" is "K".
 
    I just found this site which should help:
 
_http://tristate.apogee.net/mnd/mfmdpow.asp_ 
(http://tristate.apogee.net/mnd/mfmdpow.asp) 
 
    Form Factor is also in the Basic Transformer  Equation, since the 
waveform will determine the current rate of change and  therefore the core flux. 
Waveforms also get interesting when mitigating  harmonics for power quality issues 
(like when somebody hooks up a big non-linear  lighting ballast/NST and a SCR 
control on the same circuit as sensitive  electronics!). 
 
    Is the 3900 uF doing any good? A PWM drive would  probably have fits with 
it, but it will certainly do *some* filtering of your  FWB waveform. If your 
motor is roughly equivalent to a 10 Ohm load, that cap  should cut the ripple 
down to about 20 Volts. I think that's a pretty  significant improvement, and 
should make your motor run noticeably cooler, if  not smoother. Easy enough to 
check - run the ASRG for 20 minutes with and  without the cap (obviously 
don't need to run the TC itself). See what the temp  is after the end of each run, 
with three hours or so in between for full  cooling.
    Operating temp is *the* killer of motors,  *especially* little DC motors. 
The heat burns up the brushes and commutator, and  causes the permanent 
magnets in the field to lose their strength. It's a  downward spiral from there! 
2.5 HP is a pretty serious little motor - it  probably is a "universal" motor or 
otherwise has a wound field. But even then,  field windings are usually many 
turns of fine wire, and IMHO are a weak link in  *big* DC shunt- and 
compound-field motors. We like to keep motors *warm*, but  not *hot*. 
    I think your cap is pretty much at the point of  diminishing returns for 
what you are trying to do. That must be a pretty  decent-sized cap if it's 
rated for motor-run duty! If it's a computer  power-supply cap it might be 
running past its ripple limits, in which case it  would be good to add another one 
or two of the same value, since they should be  cheap... and you probably don't 
need the unexpected excitement of a big  electrolytic vaporizing while you're 
trying to run a pig TC!
 
-Phil LaBudde
Center for the Advanced Study of Ballistic  Improbabilities



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