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

[Quarkster <quarkster@xxxxxxx>]

Phil -
   
  I've also found this to be true with several secondary winding jigs I've built.
   
  I use right-angle DC gear-motors that are rated for an output speed about 50% greater that would ever be practical for winding. You can apply a reduced voltage to the field windings, and then power the armature through a variable resistor (foot-operated carbon-pile speed control from a sewing machine, or a foot-pedal speed control from a die-grinder). This gives good speed control, but with relatively low torque, which is great if you are working with small diameter (#30 AWG or smaller) wire. I've even found one or two gearmotors that had enough residual magnetism in the field cores that only armature power was required, which again gave good no-load speed, and reduced torque to a small but managable value. The typical PWM DC motor speed controller that pumps in more current at the slightest reduction in speec is exactly the opposite of what you want to drive a secondary winding jig.
   
  Regards,
  Herr Zapp
   
  

FIFTYGUY@xxxxxxx wrote:
  

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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