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Re: RSG Treadmill Motor

Original poster: "Dr. Resonance" <resonance@xxxxxxxxxx>

We use this same DC treadmill motor arrangement in most of our medium and
larger coils.

We prefer to use pulleys to gear the motor speed down to a 2:1 ratio so
around 2,000 RPM is top speed.  We use this with a rotor containing 12
electrodes to keep max breaks in the 400-500 bks range.  In many instances
it seems that breaks around 150-250 bks seem to give the longest sparks.
This pulley reduction scheme allows the DC treadmill motor to run at a
higher speed which gives it more horsepower than trying to run a 4,000 rpm
motor at 1,500 rpm where it might start overheating because it's "lugging".

We use a rotor with an extended shaft on each end of the hub.  This is
mounted in a standard McMaster-Carr pillow block arrangement.  Use a pillow
block on each side of the hub with one end of the shaft sticking out 3-4
inches to allow it to go through the pillow block and still have room to
attach a suitable pulley.

This setup allows easy experimentation with break rates by changing pulley
sizes to give your coil the best spark length.

Another trick is to use wood not metal to support the pillow blocks.  It's
truly amazing how much quieter the wood is than a metal mount.  Sometimes
you can hardly tell the rotor is running after it's turned on.  We used a 10
ft long piece of 6 x 6 inch wood known as "shoring timber" which is used to
hold soil banks in place on hill sides.  They are available in most lumber
yards.  One pole provides enough timber for dozens of pillow block supports.

Metal blocks always seem to have a small resonance which makes them much
noiser than the wood.  Wood absorbs and dampens these vibrations.

Dr. Resonance
>  >
>  >For those contemplating building an asynchronous rotary spark gap,
>  >the
>  >following may be useful:
>  >
>  >I bought one of the new $10 surplus treadmill motors from Surplus
>  >Center
>  >(www.surpluscenter.com), Item # 10-2167, and I am most pleased with
>  >it.  It
>  >is a permanent magnet DC motor rated at 2.25 HP at 260 volts and 5
>  >amps.
>  >The bearings are tight with inperceptable end play.  It comes with a
>  >6 inch
>  >diameter combination flywheel, cooling impeller and pulley that is
>  >perfectly
>  >balanced.  The temptation to use it as-is as an arbor for the RSG
>  >rotor was
>  >too much, so that is what I am using.  As there are 11 impeller
>  >blades, I
>  >had to drill 11 mounting holes between the blades through the rim to
>  >attach
>  >the rotor.  Any other number of bolts would result in imbalance.
>  >
>  >With the hub at 6 inches, the rotor must be 12 inches in diameter to
>  >allow
>  >enough space between the electrodes (mounted at 11 inch diameter)
>  >and the
>  >"arbor" rim.  This is not a problem - there is power to spare to
>  >turn this
>  >relatively large rotor, a 12 inch x 3/8 inch piece of GP03
>  >(electrical grade
>  >fiberglass) from McMaster Carr (8549 K47), cost about $13.
>  >
>  >A small 0 - 120 VAC variac powers a full wave bridge rectifier with
>  >a 1,000
>  >uF filter capacitor.  Here are some test results:
>  >
>  >Rotor RPM    AC Volts   AC Amps
>  >1000                       30          0.6
>  >2000                       56          1.1
>  >3000                       82          1.9
>  >4000                      106         2.8
>  >
>  >At 4,000 RPM, the motor power needed is only about 0.4 HP, and it
>  >runs cold
>  >and is very quiet.  I will be using 12 rotor electrodes, so at 4000
>  >RPM, the
>  >break rate will be 800 which is more than adequate for most
>  >purposes.
>  >
>  >Bottom line - this is a winning combination, and I commend it to
>  >anyone.
>  >
>  >--Steve Y.
>  >
>  >