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Re: 1/4 HP sync motor power



Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Mark,

Using the data from:

http://hot-streamer-dot-com/temp/4thHPsync.gif

We can find the "real" current.

Ir = Pr / Vr = 158.3 / 113.6 = 1.393 amps real

The reactive current is:

SQRT( 4.355^2 - 1.393^2) = 4.126 amps reactive (inductive).

The impedance to draw the opposite current from the AC line is:

113.6 / R = 4.126  so R = 27.53 ohms reactive (capacitive)

27.53 = 1 / (2 x pi x 60 x c)  so C = 96uF which is the theoretical size of
the PFC cap needed.  Then the motor should only draw 1.4 amps with an
almost unity power factor.  Theoretically ;-))


I worry a little about mixing too many big caps and big inductors on the AC
line in fear something may go 60Hz resonant.  The low input impedance of
the AC line should prevent that.  However, simply adding fast blow fuses to
the PFC caps eliminates that concern.  If the voltage or current goes too
high in the system, the fuse would simply open the cap stopping any
resonance.  The current a PFC cap draws is:

Vac x 2 x pi x f x C = Icap  

A 100uF cap draws:

120 x 2 x 3.14159 x 60 x 0.0001 = 4.524 amps.  So a five amp fuse would do
nicely.

One may still be able to easily use John's controller if the controller
were placed "after" the PFC cap.  That should not interfere with the
controller but still drop the AC input current to the system way down.
Theoretically :-))

One should be able to cut maybe 3 amps off the motor's draw which would
lighten the load on the AC enough to be useful especially if that extra
current could be put into the coil.  This is important if one is near the
point of popping the circuit breakers already.  One may be able to run a
15kV120mA LTR coil (six foot arcs :-)) and sync gap motor at full power off
a 15 amp 120VAC circuit.  Not too many VAs would be wasted in that case.

Cheers,

	Terry


At 12:46 PM 7/30/2001 -0500, you wrote:
>Terry, all
>
>I used a 1/3 HP split-phase motor foir my SRSG.  It drew about 5A (while the 
>TC remained unpowered).  For grins and giggles, I added a 50uF cap across 
>the motor, and the current dropped to 2.3A!  After some 
>experimenting, I decided that the cap reduced the overall wall-current draw 
>by more than adding it to the NST's PFC (total of 250uF for 15/90).  I left 
>it across the motor.  I'd use PFC across the Geek Group's big SRSG I 
>designed, but like you said, it will interfere with John's phase controller 
>that we'll be using.
>
>I can't see any reason why a motor should not have any added PFC.  The caps 
>merely phase shift the lagging current so that it more closely matches the 
>voltage sine wave, thereby lowering the apparent power drawn 
>from the wall.  The power company installs PFC caps on the overhead (or 
>underground) "HV" lines to attempt to negate the inductive loads and power 
>lines.  In a large industrial setting, the PFC caps are often on sight 
>next to the BIG HV transformers on their side of the power co's meter.....  
>In a REALLY large substation, a HUGE electric motor is often used as a 
>capacitor (the things I learn from a $.25 book from my alma mater....).....
>
>Mark Broker
>The Geek Group's Chief Engineer
>
>
>
>>Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
>>
>>Hi All,
>>
>>FYI
>>
>>I captured the following scope picture showing the voltage, current, and
>>power into my 1/4 HP A.O Smith motor that is converted to synchronous.
>>
>>http://hot-streamer-dot-com/TeslaCoils/MyCoils/SmallCoil/small_power.jpg
>>
>>http://hot-streamer-dot-com/temp/4thHPsync.gif
>>
>>It draws 158 watts of real power.  494.7 VA.  I think the jaggedness in the
>>current waveform may be due to the edges of the flats interacting with the
>>poles as they go by.  But I am not sure of that.  I wonder if power factor
>>correction would have any use for these motors or if that would cause
>>problems?  For some reason I don't think PFC caps and motors are supposed
>>to be used together...
>>
>>PFC caps would also interfere with John's controller...
>>
>>Cheers,
>>
>>	Terry
>>
>>
>>
>
>
>