The motor is an important part of an electric bike. What parameters are most important for a motor? Is the bigger power the bigger speed and torque?
Here we would like to share some of our knowledge about the motor parameters.
Generally, there are 3 positions for an ebike, front-front wheel, rear- rear wheel, middle- crank. In different positions, the motor’s installation width is different. For example, many front motors with 100mmm, rear motors with 135mm or 142mm, middle motors with 68mm or 73mm.
- Rated power (W):
It means the power that motors are designed to work under normal conditions. The motor can handle less than this amount of power and might be able to handle a little more power, but it should handle any amount up to the rated power and perform as expected.
Power is in general rated in watt (W) or horsepower (hp), it is the measure of how much work a motor can be expected to do. This value is based on the motor’s full-load torque and full-load speed ratings and is calculated as follows:
Power (hp)=[ Motor Speed(rpm) × Torque (lb-ft) ]÷5252
1 hp = 746 watts = 0.746 KW
1 lb-ft = 1.356 Nm
Power(W)= U × I = I2 × R
- Rated Voltage (V):
A specific voltage level that motors are designed to yield optimal performance when operating at. Generally, motors are designed with a 10% tolerance for voltage above and below the rated value due to changing load conditions.
It is the measure of the energy required to rotate a shaft.
- Speed (Rpm):
This rotating speed is the rate of rotation around an axis, measured in revolutions per minute (RPM) and is one of the key information found in every motor’s parameters.
It means the ratio of the mechanical power available at the shaft end ”output” (Pu) and the electric energy absorbed by the motor from the supply “ input”( Pa ).
Is the bigger power the bigger speed and torque?
From the equation
Power (hp)=[ Motor Speed(rpm) × Torque (lb-ft) ] ÷ 5252
We could learn that a bigger power could have a bigger motor speed and torque, but a bigger motor speed not implies a bigger torque.
Here is the specific Torque/Speed curve and Power curve.