Apply constant load to an electric motor - low tech way

Question

I want to know how much I can over-drive a small electric motor. Its speed at the rated voltage is too slow, so I want to find a higher voltage that won't blow it. It will only be used intermittently, but I figure if I run it for 100x the max expected duration with the expected load then it will probably survive repeated intermittent use.

So my question is, safety concerns aside, how can I apply a relatively constant load? I can't think of a reliable way of using friction, such as attaching a disk and hold something on it, vaguely resembling a vehicle brake disk. I could attach a weight via a pulley but it would quickly run out of length. I could use a second motor and load that electrically to increase the torque, which is IMO the best idea, but I'm wondering if there are any other mechanical solutions? I think I'm missing something really obvious.

Answer 1

Probably the easiest way to add a reasonably constant mechanical load to a motor is a fan. Because aerodynamic forces are exponential in respect to speed this should be consistent enough for evaluation purposes and has the advantage that you don't have the problems of wear and adjustment associated with solid friction brakes. Here the obvious solution for a small motor is to scavenge the fan from a cheap PC cooler. If the fan has an enclosure you can even vary the load by the simple expedient of using duct tape to restrict the flow.

For a small motor a fan in air should be fine, for higher power applications a paddle wheel immersed in a fluid is more compact,albeit more complex to set up.

If you want to be able to measure of vary the load then a friction band brake is a good solution as you can measure the tension in the band to give a crude measure of torque and even a simple setup should at least give you a consistent range even if you can't derive precise figures for power.

You can also get cheap handled non-contact tachometers to measure rpm

If you want more detailed measurements of load then you could always connect the motor to a generator and measure the power generated. This can give fairly reliable direct measures of power output and a simple optical or magnetic tachometer will give you rpm and thus torque. With the right instruments this setup also allows for sampling of variations over time. With some basic programming and electronics knowledge building a data logger to a PC is realistic (especially with a legacy parallel port).