Taken from a popular KR website:
"This "dynamo" is from a John Deere model 670 lawn tractor (any X70, actually), and weighs 4 pounds with the regulator. Output is 20 amps minimum when driven by the mower's 6" pulley running at 2500 rpm. Part numbers are AM877557 for the alternator ($160), and AM101406 for the regulator ($67). The wonderful thing about a dynamo is that it uses permanent magnets rather than field excited electromagnets, so it doesn't "need" electricity for the windings to produce electricity. "
The JD dynamo alternator is basically the standard for corvair conversions. Probably 80% of the flying corvairs use this alternator. The reasons are mostly because they are very lightweight(4lbs with regulator vs +/- 6lbs for a Denso 40-60 amp internal regulator field excited that is very commonplace on homebuilts) and really compact. The denso is by far the most common second choice, used mostly if either the owner feels they need more than the 20amp the JD puts out, or they just feel uncomfortable with the fact that the dynamo charges full draw all the time. FWIW, both alternators have performed very well on the corvairs, and both have proven to be quite reliable.
One thing I am curious about is the constant mechanical power draw from the JD dynamo. The dynamo, being a permanent magnet type, will not reduce the armature resistance with reduced charging needs like a field excited (Denso)alternator will. I have one of these JD Dynamo alternators, and spinning it by hand for just a few revs takes a suprising amount of effort. Granted, these units were designed to be flat bulletproof for small diesel engines tractors/mowers, so there was certainly little concern over a bit of potential extra engine power draw from a permanent magnet unit.
Here's a retyping of a snipet from Kent Paser's "Speed with Economy" book that got me thinking on the subject.
ALTERNATOR CUT OFF SWITCH
`A normal aircraft or automobile alternator, when operating at it's rated output, can draw as much as 3-4 engine horsepower to operate. This is not caused so much by the mass of the armature because the armature rotates in ball or needle bearing; and, once the mass of the armature is spun-up, it doesn't require much energy to keep it spinning. Rather, most of the energy (horsepower) required to spin the armature is due to the armature spinning within the magnetic field created by the field coil windings. In an alternator, the field windings must be energized by an outside source of DC current (the battery) for the alternator to start producing an electrical output. I have installed a seperate switch on the instrument panel so that I can cut off the current to the field windings; thereby, freeing up as much as 3-4 engine horsepower which can then be used to either increase the aircraft's speed or decrease the aircrafts fuel consumption. Of course, when I shut off the alternator, I also minimize other current draw from the battery so that the battery doesn't discharge completely during a race. Actually, since the engine's ignition is provided by the magnetos (which requires no outside electrical power source), no electrical draw from the battery should be required during a race unless the race format requires radio communication during the event."
Granted, there could be some debate as to whether it is prudent
to be able to intentionally shut down your charging system in flight. But I'm more intrerested in finding out how much power a small permanent magnet alternator such as the JD dynamo draws from the engine, especially as it is running at full resistance ALL THE TIME!
Since my application is in the 100-120hp range(+/- 60-80hp at cruise), if the JD absorbs 3-4 engine hp at cruise setting similar to what Kent Paser suggests a regular Denso type -may- be absorbing, CONSTANTLY, regardless of charging load, that may be something to consider.
My understanding is that a typical internal regulated field wound alternator (such as the popular Denso units) takes up less and less energy to turn as the charging requirement diminishes. So say a 60 amp Denso charging at full rated power draws 3-4 engine HP, that would presumably drop to say 1 or less than 1 HP under the lowest charging demand?
If it is reasonable to assume(and we all know what happens as a result) that a denso unit will draw 1hp or less during low charging times (probably most of the time) and the JD dymano could consume 3-4hp all the time, a functional 2-3hp usable engine HP output difference on account of the different alternators could result. As the mounted Denso functionally only weights almost exactly 2 lbs more than the mounted JD dynamo, that would seem to be a fair trade for the potentially significant drop in resistance.
A member of the corvair conversion community has very recently purchased a $40,000 engine dyno for his business for R&D and customer work with corvairs, so perhaps a proper back to back set of tests can be done to put some hard numbers on the issue in the near future. In the meantime, I'd be happy to hear any thoughts/opinions on the subject.