(ET) Elec-trak DC-AC converter thoughts

[Christopher Zach <czach computer org>]

This evening I did a test of the Bill Gunn Elec-trak AC rotary 
converter. Bill did a really good job on this one; before selling it he 
not only replaced the brushes and bearings, he also replaced the outlet 
because apparently it had a tiny crack.

I'll say this for Bill: He sells good things.

Anyway, I hooked it up to the tractor, and used a Watt monitor to check 
it out. With no load it was running 134 volts, 66hz on the AC side. Not 
bad actually; bit high on the Hz but that's probably because my E20 has 
a slightly higher resting voltage (40 volts) due to the NiCDs.

Under a 600 watt load it was down to about 118 volts, 60.1 hz with a 
power factor on the load of .96 (AC motor). Not too shabby, holds the 
frequency stable and seems to work well. Time for the leaf blower.

Plugged that in, and noticed something: At low speed the blower pulled 
800 "watts" but only about 4.x amps. Checked the power factor; about 
.54. Apparently the blower's low speed is probably just a diode in 
series with the motor. Makes sense, yet the voltage on the AC side was a 
nice solid 120 volts. Power factor is not the key on this device, unlike 
with solid state inverters. Amps are.

Went to high speed. .94 power factor, 9 amps, 900 watts. Voltage had 
dropped to about 98 volts, frequency was at 59. I guess they use a shunt 
motor to keep the rotor going at 3600 as much as possible. Still the 
blower was useable, albeit a bit slower.

I think this is how it handles things like chainsaws. When the saw hits 
hard wood it simply will slow down a bit as the voltage drops. Not the 
end of the world, just cut a bit slower.

Will discuss the problem in the next email.

Chris