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I am planning to build an e-bike and I realized that a car starter motor would be a good motor, as it has enough power (1kW, enough for my bike) and the voltage is practical enough. Will it handle the stress?

A mechanic told me that the coil in it would be destroyed after 30 seconds, as it isn't meant to be used a lot. I am only interested in the motor, not the solenoid. I will be using it at 75% of its power, at most, so not too much stress. Will it work? It's going to replace a 50cc motor of a piaggio ciao.

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It might not burn out in 30 seconds, but I suspect your mechanic is probably on the right track. All motors are going to generate heat as they run. They differ in how quickly they heat up (and what temperatures they can withstand). For a given load rating (say 1 kW), a smaller motor will heat up quicker and a larger motor will heat up slowly. Your starter motor is probably relatively small for a 1 kW motor. It's going to heat up quickly with operation. But that doesn't matter for your car, because it's only intended to work for a short period of time. It's on for 10s or so, starts to heats up a little, but before it gets close to its maximum temperature, it gets turned off and given a chance to cool down for a long time (hours maybe) before being operated again. That's very different from what you are going to be doing with your bike. You'll be running it for a lot longer periods. The motor is going to get very hot very quickly, and you won't be giving it any time to cool down. It might not burn up in 30s, but I don't think it will last for 10 years either. You'll want a motor that is designed to be operated at 1 kW continuously.

Duty Cycle: What you need to know page might help.

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  • $\begingroup$ What if i cool it somehow? Feed it lots of air $\endgroup$ Apr 30 '17 at 13:10
  • $\begingroup$ I'm sure that would help, but it's hard to say how much. Depending on the design of the motor it's usually easy to cool the stator with additional external cooling, but sometimes it's hard to get the cooling air to the rotor. $\endgroup$
    – Daniel K
    Apr 30 '17 at 14:20
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Starter motors are designed to be most efficient at starting engines : getting a large mass moving to a particular speed - but not for extended long period running. This will affect the design of the bearings etc The design of e-bike motors are optimised for that duty. So, it may work, and you may be happy with the performance, but other types of motor may give you more range or speed.

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Starter motors are optimized for high torque and to survive the rough environment of a car. They are not designed for sustained use, and are not particularly efficient. Both those latter points make them unsuitable for a bicycle.

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As touched on in other answers: a car starter is in a harsh environment, so it is pretty well sealed against dirt and moisture, and has no provision for cooling. In operation, a starter like all motors will generate heat due to electrical resistance in the windings. Its normal job as a starter requires it to run for a few seconds and then has minutes, hours, or days to get rid of the heat that builds up as turns the engine. In continuous operation, it will generate heat much faster than it can get rid of it, eventually causing something to fail (maybe melting or even ignition of insulation). Any motor suitable for continuous operation will have provision for cooling - the windings will be visible through ports in the housing and there will almost certainly be a fan to move air through it.

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