[xrotaryguy]

I was browsing through the album on AustinEV several months ago and I saw a home-made electric bicycle which used a skill saw motor. I assumed that the skill saw motor was AC. I have been looking for it recently, and now I can't seem to find that particular bicycle. I am not sure how this person managed to use an AC motor on his bicycle. Is there a simple DIY way to do this? Or does it need to be done with an AC bicycle controller?

[Greenflight]

Most conversions of power tool motors use cordless tools, so the motor is generally a permanent magnet DC motor operating at around 15-25V, which is pretty good for that kind of application.

Also- a lot of devices (treadmills and stuff) just use small series-wound motors that can operate on either DC or AC instead of using dedicated AC induction motors.

[xrotaryguy]

I suspected that the saw motor could have been from a cordless device. thanks for the info.

So, if I were to use a treadmill motor that's rated at 120vac and 18 amps, would it be safe to assume that I could also run it at 24vdc and 75 amps?

120 vac x 18 amps = 24 vdc x 90 amps = 2,160 watts

[Mr. Sharkey]

No, wrong on several counts.

The internal resistance of the armature and field windings (assuming series wound here) will determine how much current a motor will draw at a lower voltage. (calculations based on simple resistance, a motor, being a dynamic device with internal inductance would return different but similar actual, measured values.)

120 v / 18 a = 6.66 ohms.
24 v / 6.66 ohms = 3.6 amps.
24 v * 3.6 a = 86.4 watts.

If you were able to load the motor (running at low voltage, and therefore a lower RPM) with sufficient drag to cause it to draw 75 amps, it would probably burn out, as the armature and filed windings are constructed with small, high resistance wire. The excessive current would cause a lot of heat to be generated, resulting in destruction of the motor.

If you intend on running a motor at a voltage far outside of it's design parameters, expect low performance and failures to be the rule.

[xrotaryguy]

Aah, so the motor would need to be rewound. That being the case, I would imagine that numerous other changes would need to take place. that's not worth it. I guess the best thing to do would be to find a way to feed it the correct amount of voltage and amperage and then gear the drive train accordingly.

[Greenflight]

Just to clarify, I wasn't suggesting that you use a treadmill motor... I was just using it as an example of the kind of motor often used in AC appliances.

It's probably not ideal for any kind of EV drive train... I don't know enough about them to say for sure.