Q&A Forum
Hi everyone,
I am trying to build a small engine dyno. I want to use a 140 amp car alternator as an eddy brake. This dyno will be used for testing performance modifications made on two cycle chainsaw engines ranging in power from 2hp to 10hp. I will use YourDyno controller but I need help building a suitable transistor board that will load the alternator using PMW from the YourDyno controller.
I don't quite understand enough about this to pull it off by myself and am hoping someone in the forums can point me in the right direction. I was an automotive technician at a dealership for 18 years so I understand some electronics and can read schematics but I have never designed and built any electronics before.
I found this MOSFET on amazon and am wondering if it would be suitable for supplying the power to the alternator rotor winding.
Also, here is an image of the alternator, torque arm, load cell setup I plan to use. https://ibb.co/56nRpdZ
Thanks
Can I just use a dc power supply to vary the voltage to the rotor coil of the alternator like some people would use a manual valve to vary water supplied to a water brake? I would still be using YourDyno controller for data acquisition. Saws would be tested starting out at wide open throttle and no load and then gradually bring in the load by adding more voltage to the rotor windings.
You can use a transistor that shorts the output of the alternator, controlled by a PWM signal from YourDyno.
The H-bridge you linked to may work, but there was very little information on it. Attached is a drawing of a typical h-bridge. Current normally flows like the red line in the drawing. You actually only need 1 of the transistors in the h-bridge, so you can connect two in parallel if you use the h-bridge.
@admin Thank you for the response, Jostein, I found a wiring diagram for the alternator which I plan to use. It is 140 amp unit from a 2005 GMC yukon denali. The alternator has a solid state regulator attached to the back of the alternator. Please see attached schematic.
The vehicles powertrain control module uses duty cycle on pin C of the solid state regulator to control the alternator rotors field coil. Can I just hook up yourdyno PWM control to pin C of the alternators solid state regulator to control the field coil and thus control dyno braking?
I would also have a car battery hooked up to the alternator with positive hooked up to BAT terminal on alternator and the battery negative hooked up to the alternators metal case. Thanks for your help. If this arrangement would not work I think I understand how to use a high speed switching transistor as you describe.
@admin Thank you for the response, Jostein, I found a wiring diagram for the alternator which I plan to use. It is 140 amp unit from a 2005 GMC yukon denali. The alternator has a solid state regulator attached to the back of the alternator. Please see attached schematic.
The vehicles powertrain control module uses duty cycle on pin C of the solid state regulator to control the alternator rotors field coil. Can I just hook up yourdyno PWM control to pin C of the alternators solid state regulator to control the field coil and thus control dyno braking?
I would also have a car battery hooked up to the alternator with positive hooked up to BAT terminal on alternator and the battery negative hooked up to the alternators metal case. Thanks for your help. If this arrangement would not work I think I understand how to use a high speed switching transistor as you describe.
Never mind this last idea, I don't think it would work out. I think I have a better idea I will run it past you when I have it all sorted out. New idea utilizes a pair of mosfet's in parallel to control the alternator field coil and the mosfets can be switched using PWM from YourDyno controller.
@admin Hi Jostein, how about using this mosfet module? https://a.co/d/0XXWXqO
I think I can wire it in with the yourdyno controller as I have drawn in the picture down below.
Do you feel this would be an ok setup? Thanks for your help!
@admin Hi Jostein, how about using this mosfet module? https://a.co/d/0XXWXqO
I think I can wire it in with the yourdyno controller as I have drawn in the picture down below.
Do you feel this would be an ok setup? Thanks for your help!
The transistor board should work. However, you cannot hook the alternator up to a 24V power supply and "charge" that. You could connect to a discharged battery. But I think it will work if you just short the output instead.
Connect the negative of the alternator to earth ground and to 0V of YourDyno. The rectifier of the alternator must be used too, so you hook up to a DC voltage.
Sounds good! I appreciate the support!
My plan with the power supply was to use it only to power the field coil through the transistor board.
The alternator "charge" output from the stator windings would be dumped into a few 600 watt 12 volt heating elements that will be wired in series and submerged in a bucket of water. I was considering using these units here... https://a.co/d/5KRO3l7 This "dumping" circuit would consist of the stator winding output from the alternators large positive charging terminal running through the heaters and then grounding back at the alternator case.
The field coil would be its own separate circuit using the power supply which I agree should probably be 12 volts power supply instead of 24 volts. My initial tests of the dyno might be performed with the field coil circuit powered by a charged up 12 volt car battery.
Thanks for the help, Jostein!
@admin Jostein, will 24 AWG conductor be sufficient for the PWM circuit from the yourdyno controller to the transistor board? Thanks
As a general rule of thumb a 12v alternator (actually 14.4v normally) consumes approx 1hp to output 30 amps, so you need to size you alternator accordingly
This is a testbed measured figure, not one pulled off the internet
Some positive early test results using the car alternator as a small engine brake. Almost enough brake effect to stall the engine. There's more room to improve the braking effect too.
Clockwise these meters are source voltage(red meter) 12v car battery, saw crank rpm, alt out voltage, alt out amps, and the voltage Im injecting into rotor winding. 60cc McCulloch 610 Pro Mac chainsaw.
https://drive.google.com/file/d/1xNgjmVH-bhzZzqw9wM7JwrqyHM5eI8G_/view?usp=drivesdk
Update on my home built small engine dyno for those that may be interested. Everything is working great. I will post a video below of a run I made on a saw of mine. This saw was 3.25hp stock and now its pushing over 6hp with some intake and exhaust port modifications and I home built an expansion chamber for the engine, little more ignition advance, bigger carburetor from a bigger saw. It's been a lot of fun.
Here's screen capture video of a 6hp run. The expansion chamber sounds amazing! Sounds really mean around 10,000 rpm as if it's trying to open a wormhole to another dimension or something 🙂