D
Deleted member 4366
Guest
I started this thread to answer Selrahc 1992 from another thread. It necer does any harm to talk about how motors work. All the statements below are generalisations and should be considered at maximum throttle/PAS or whatever.
Basic principles:
Torque is proportional to current. The more current, the more torque. Torque is what pushes you along. We feel torque as power.
As soon as a motor starts to turn, it becomes a generator. The faster it turns, the more volts it generates. Unfortunately, the power generated is in the opposite direction of what you give it, so as the motor speeds up, the net power becomes less and less until it becomes zero at a certain speed. It actually generates volts, so when the voltage reaches the same as your battery, you can't go any faster. That is why getting a higher voltage battery increases your speed.
Current depends on speed. When the motor starts from stationary, it doesn't generate anything, so all the battery voltage can push the power through the motor, which would mean a lot of current. As the motor speeds up, the net voltage would go down, so the current goes down in proportion until it becoes zero at the maximum speed of the motor. The current at low speed would be too high for the motor, so we have to have a controller that limits it. This means that up to a certain speed the current will be constant and equal to the maximum allowed by the controller, then it will ramp down to zero at maximum speed. Therefore, the torque from the motor does the same. You get maximum torque up to the speed where the net voltage is not enough to push the maximum current through the motor anymore. The cross-over point depends on the controller setting. The higher the allowed current, the sooner you reach the cross-over point.
Efficiency also has a bearing on power. Maximum efficiency happens somewhere about 3/4 maximum rpm. It drops down a lot once you are below 50% of maximum rpm, so that maybe 50% of the power that comes from your battery is wasted, and you get reducing power and torque as you slow down until the motor stalls out.The wasted power is converted to heat, so your motor can burn if you overdo things.
Maximum power happens at about 2/3 maximum rpm. It drops off after that because net voltage and current are reducing. The cross-over point mentioned above has a bearing on the speed at which maximum power occurs. When we talk about maximum speed, we normally mean the maximum speed the motor will spin. You don't get this speed on the road because the power reducest before you get there.
The same rules apply to crank motors as hub-motors, but speed means crank speed for them, so ideally, you want to get peak efficiency at your own personal natural cadence, but at that point, power is already reducing. If you had peak power at your natural cadence, you'd suffer a slight efficiency loss. You can get more torque by slowing down your cadence, but then also less efficiency. If the motor speed doesn't match your pedal speed, efficiency will suffer too, either for you or the bike or both.
Basic principles:
Torque is proportional to current. The more current, the more torque. Torque is what pushes you along. We feel torque as power.
As soon as a motor starts to turn, it becomes a generator. The faster it turns, the more volts it generates. Unfortunately, the power generated is in the opposite direction of what you give it, so as the motor speeds up, the net power becomes less and less until it becomes zero at a certain speed. It actually generates volts, so when the voltage reaches the same as your battery, you can't go any faster. That is why getting a higher voltage battery increases your speed.
Current depends on speed. When the motor starts from stationary, it doesn't generate anything, so all the battery voltage can push the power through the motor, which would mean a lot of current. As the motor speeds up, the net voltage would go down, so the current goes down in proportion until it becoes zero at the maximum speed of the motor. The current at low speed would be too high for the motor, so we have to have a controller that limits it. This means that up to a certain speed the current will be constant and equal to the maximum allowed by the controller, then it will ramp down to zero at maximum speed. Therefore, the torque from the motor does the same. You get maximum torque up to the speed where the net voltage is not enough to push the maximum current through the motor anymore. The cross-over point depends on the controller setting. The higher the allowed current, the sooner you reach the cross-over point.
Efficiency also has a bearing on power. Maximum efficiency happens somewhere about 3/4 maximum rpm. It drops down a lot once you are below 50% of maximum rpm, so that maybe 50% of the power that comes from your battery is wasted, and you get reducing power and torque as you slow down until the motor stalls out.The wasted power is converted to heat, so your motor can burn if you overdo things.
Maximum power happens at about 2/3 maximum rpm. It drops off after that because net voltage and current are reducing. The cross-over point mentioned above has a bearing on the speed at which maximum power occurs. When we talk about maximum speed, we normally mean the maximum speed the motor will spin. You don't get this speed on the road because the power reducest before you get there.
The same rules apply to crank motors as hub-motors, but speed means crank speed for them, so ideally, you want to get peak efficiency at your own personal natural cadence, but at that point, power is already reducing. If you had peak power at your natural cadence, you'd suffer a slight efficiency loss. You can get more torque by slowing down your cadence, but then also less efficiency. If the motor speed doesn't match your pedal speed, efficiency will suffer too, either for you or the bike or both.
Last edited by a moderator:
