Re: Poor PF spark gap motors

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Mark Broker by way of Terry Fritz <teslalist-at-qwest-dot-net>" <mbroker-at-thegeekgroup-dot-org>

Interesting gadget!  Although I do need a new DMM, I need Hfe readings far 
more than I need true power and power factor. :(  The Wavetek 235 looked 
like a winner until I saw the price tag.  :((

The 1/4HP motor (1/3, maybe?  don't remember, and it's 150 miles away) I 
used in my TC performed simlarly, using a typical analog clamp-on ammeter.
As I recall, I added 50 and 100uF of PFC, and saw no real difference.  I 
went with 50uF since the extra 50uF actually did help the PFC of the NSTs a 
bit.  However, I spent a significant amount of time finding the sweet spot 
again.  Not sure if that is because I bumped the motor while moving it 
around, the added motor PFC, etc.

Standard induction motors actually have a fairly good power factor when 
they are loaded down.  Unloaded, power factors plummet, as you have 
noticed, since the majority of the electrical load is the inductance of the 
stator windings.  As the rotor slows, more squirrel-cage bars cut through 
more magnetic flux from the stator windings, creating a higher current in 
the squirrel-cage bars.  The higher squirrel-cage current causes an 
increased magnetic field, which increases the torque on the rotor.  This 
process is similar to the leakage reactance in a standard transformer.  If 
the rotor locks, the principle electrical load is this leakage reactance.
Basically, the electrical behavior is very similar to a transformer.

I got a little long-winded. . . .

Regards,

Mark Broker
Chief Engineer, The Geek Group




>So I guess the spark gap motor in my small coil draws more current than 
>the NST!!  I never realized it was that bad.  The motor only needs 1.49 
>real amps to run.  Might as well correct for the 3.42 amps reactive.
>
>3.42 = 118.6 / Xc   Xc = 34.68 ohms
>
>34.68 = 1 / (2 x pi x 60 x C)    C = 76uF.  Four 20uF should do it...
>
>So with 80uF I get
>116.6 Volts
>1.81 Amps
>174 watts
>212 VA
>0.82pF
>
>Saves three amps!  I checked with my strobe light and it did not seem to 
>change the timing of the motor at all.
>
>I tried my other bigger spark gap  (1/4 HP motor much much larger rotor) 
>and got:
>
>116.2 V
>5.37 A
>217 W
>624 VA
>0.34 PF
>
>And again it is wasting about 3.5 amps...  So adding 80uF to 1/4 HP spark 
>gap motors can reduce the system current draw by about 3 amps.  Might be 
>enough to stop breakers from blowing in some cases.
>
>The Kill-A-Watt meter is pretty cool!!

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