Working out motor power draw

[Wizard9956]

Hi all , could someone tell me if i've calculated this right .

A 250 watt motor , 36 volt battery , to work out the ampage draw i used
I = W / V So W=250 V=36 I=6.94 amps current draw at full throttle , have i got that right .

Thanks Wizard

 

[flecc]

Your maths and physics are dead on Wizard, but in practice the answer is wrong.

The 250 watt rating is a legal nicety, an average supposed power rating to comply with the law's requirements.

In practice, the current drawn varies from the maximum at up to the motor's peak torque point, then declining until the point of maximum efficiency, which is a bit short of maximum speed.

The peak varies enormously, for example, the Powacycle NiMh Windsor at 272 watts, and the Powabyke at 700 watts, both nominally 200/250 watt motors.

Your bike's peak gross power is probably about 400 to 500 watts, a very common power area on our bikes, so the peak current draw from 0 mph to around 8 mph typically for the peak torque point will be a peak of about 11 to 14 amps, and at a mile or two per hour short of top assisted speed, about 4 to 5.5 amps.

Remember, I have only estimated your motor's figure, it could be higher, but we are often reduced to such guesstimates since Chinese manufacturers are notoriously difficult to get accurate technical information from.
.

 

[Tiberius]

Yes, but,

There is the question of whether that 250 W is the electrical input power to the motor or the mechanical output power.

Also, when a motor is run at lower than its optimum speed/voltage combination it takes more current and gives less power out. The efficiency falls right off. So suppose you had a motor that was 250 W input, 200 W output, ie 80% efficiency, at 36 V and 12 mph, it would draw 6.94 A as you say. But at 6 mph it would draw much more, say 12 A, unless the controller current limit cut in.

Nick