Model Engine Maker

Supporting => Engine Ancillaries => Topic started by: Roger B on July 25, 2015, 05:55:33 PM

Title: Load Bank
Post by: Roger B on July 25, 2015, 05:55:33 PM
I'm not quite sure if it fits here or not but here we go.

As I have started load testing my horizontal engine I decided that I needed some form of adjustable load for the generator. A search in the useful bits of metal box found a piece of ready bent 5mm aluminium plate. I had a couple of power resistors already and bought some more along with some cheap switches from Conrad. The large resistors are 3.5 Ohm and the small ones 6.8 ohm.
Title: Re: Load Bank
Post by: PStechPaul on July 25, 2015, 09:17:48 PM
That's a good looking load bank, and it certainly should do the job. I assume you have a PM DC generator (motor) attached to the engine to produce power and then will switch in the loads as needed to measure voltage and current (or power), as well as monitor RPM. You can also make a mechanical dynamometer by mounting the generator on a rotational swivel and add a swing arm with a fish weight to measure actual torque. A 12" arm that reads 1 pound of force will be 1 lb-ft of torque.
You can also make an adjustable load bank using a PWM motor controller or a variable DC-DC converter into a fixed resistive load. In that way you can adjust the load from nearly zero to the maximum allowed by the resistance at the speed of the motor. But because of the PWM waveform and sharp edges with high frequency components, it may require extra filtering and special metering for accuracy.
Back to the load bank you have, it is possible to make it a little more flexible by using DPDT switches to put pairs of resistors in series or parallel. So two 3.5 ohm resistors can make 1.75 ohms or 7.0 ohms. It's also possible to make a switching matrix with SPDT switches to make a wide variety of series/parallel combinations. I did this for the recloser test set I designed and patented in 1980, where I used a matrix of six 28.8 ohm 2000 watt heater elements to get 15 different resistor values as follows:
Title: Re: Load Bank
Post by: Roger B on July 26, 2015, 06:18:05 PM
Thank you Paul. I built this to trial my horizontal engine (the vertical doesn't run well enough yet  ::) ),2821.210.html

I wanted something better than joining resistors together with crocodile clips, but tried to stop myself being carried away with too many refinements  :)
Title: Re: Load Bank
Post by: PStechPaul on July 26, 2015, 08:05:42 PM
Yeah, I tend to get carried away and then wind up not building anything because it has become too complex or I think of another "even better" way to do it. What you have is elegant in its simplicity and function. I would probably add a PIC circuit with various inputs and a display to read voltage, current, power, and RPM, and perhaps temperature and air pressure. It would function as a datalogger and send the information over a Bluetooth serial connection to the computer for analysis.
Using resistors is better than incandescent lamps because their resistance stays constant, whereas lamps can have about ten times lower resistance when cold than they do when glowing brightly. That can also make them present a tough load if connected during engine start-up. With a good flywheel they'll probably reach incandescence quickly enough to avoid slowing down the engine very much.
Title: Re: Load Bank
Post by: b.lindsey on July 27, 2015, 01:16:09 AM
I like the simplicity of your design. I hope you will also post a video of it in use when you have a chance.   :ThumbsUp:

Title: Re: Load Bank
Post by: Roger B on September 26, 2015, 06:32:55 PM
Here is a brief video of it being used to test my horizontal engine. I reached a maximum output of around 80W (10v at 8A). Unfortunately, as you can hear, the bracket for the generator was not rigid enough and started vibrating as the load increased.