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which is more efficient ac motor or dc motor??

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Deexith Hasan

Advanced Member level 4
Oct 24, 2014
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i'm intrested in building electric bike.....which motor has more speed and torque at low price


I'd say an asynchronous AC motor is cheap, has high torque and needs no maintenance.
So i'd prefer this motor.

But the power control electronic is more difficult than fir a DC motor.

Out of the two , torque and speed , for an e-bike , control of speed is first priority.

Since Ac motor control speed involves more cost in final product , DC is the suitable choice , as Klaus has noted.

That is why BLDC is the preferred choice for e-bikes , which FvM has already suggested.

But Your Topic title asks for which is efficient ? which altogether a different topic.


BLDC sounds like DC, but in fact it is similar to an AC synchronous motor. BLDC needs a lot power control circuit.


BLDC sounds like DC, but in fact it is similar to an AC synchronous motor. BLDC needs a lot power control circuit.
Yes, BLDC is a synchronous AC machine, usually operated with encoder feedback. I agree that the term BLDC is misleading, but it's somehow established. For small BLDC, e.g. fan motors with built-in electronics, the term describes the user view on the motor which has just two DC supply wires. But larger BLDC can't be distinguished from regular PM excited synchronous machines with or without sensor feedback.

Despite of slightly increased control effort, BLDC is clearly the way to go for E-bikes due to higher torque at low speed, small size and freedom from maintenance. Asynchronous machines with vector control are a principle option, but have larger size and increased weight.

The most efficient are both the BLDC and the brushed permanent-magnet DC.

As FvM has noted though, the electronic circuit on a BLDC controller is more difficult, since frequency/voltage control on 3 phases, which must be synchronized to a quadrature encoder.
But there are several ready-built controller boards that that remove the hairy design stuff.

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