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R/C & Small Engine up to 30cc - DC Electric Dynamometer

Well I have collected some parts so far. The dynamometer I want to build will use a small diameter, external regulator alternator and some elements from a stove. Why a DC automobile? well it can handle high rpm, has a DC current output, excitation is controlled externally, reacts instantaneously to control inputs, handles high heat, and can output more current than I can make. The alternator I have right now is a newer style Chrysler and I believe it is from a 98 or 99 mini-van that is capable of being turned at 10,000 rpm. This is important because I want to use a 1:1 drive, this keeps the math simple. Most alternators are over-run at about 1.5:1 so that it can charge at idle and since most cars have a red line of 6,000 rpm, so that means the alternator has to be able to handle 9,000 rpm. So why don't I just use a prop and measure the rpm? Because props can vary from brand to brand and I don't want to have a large collection of props. At the most I would like to have maybe 3 props per engine, ideally 2 props should be enough. I guess the big reason for a dyno is that I want to get into making small engines so a dyno would be real handy.

Here is the process I plan to follow:
  1. Make plans, engineering documents.
  2. Collect parts.
  3. Assemble manual control rig first.
  4. calibrate dynamometer with an electric motor so I know how much power is required just to spin everything.
  5. Manually test rig, control alternator with a voltage regulator and meters to read alternator output.
  6. Make ADC, DAC board to interface between dyno and computer.
  7. Make software to control alternator (DAC's) and to read ADC's.
  8. Test rig using digital control.
     

Since most alternators can handle 24 volts output and 100 amps that should give me about 3 hp to play with. The important things are a small diameter so inertial losses are small. There is no sense in using a 200 amp alternator for 3 hp when most of my engines can only put out 1-2 hp at 15,000 rpm. I won't be revving any of my engines that high and if I have to rev that high then I will be gearing down the alternator and have to make new calculations. I may have to slow down the alternator so I'm prepared to modify the software to handle ratio's. Ratio's in themselves can cause problems, I've seen people who get their car chassis dyno'ed just to find out at the track that their car suddenly doesn't have as much power as the chassis dyno said they have. So out come the laptops, tuning kits, and anger. I think chassis dyno are great for tuning but, they are not a bible on how much power an engine is making, that's what engine dyno's are for and even they don't equate at the track. There are a lot of variables at the track that aren't seen on any dyno. Chassis dyno's are the worst though, you need all the gear ratio info, including final drive, unloaded and loaded tire height, and an operator that knows the dyno, how to input the correct data and how to read the info from the dyno.

Anyway if your going to follow along get a small diameter alternator like this:

 

Not a huge monster like these unless your going to dyno your 5hp or larger engine:

200 amp plus, great for 4hp and up or welding Older 60 amp Chrysler, has a very large diameter.

 

Well, check back later.

 

 

 

 

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Last modified: 03/08/15