Electric Bicycle: Motor Specifications (II)｜Square vocus

This article discusses motor-related issues in electric bicycles, focusing on the motor torque.

Generally speaking, in the introduction of the motor specifications of electric bicycles, several pieces of information will be obtained. Among them, the torque item also makes the author feel abnormal, and I am a little unsure of the mentality of the label.basicallyhub motorBoth are marked with more than 30Nm, while the mid-mounted motor is marked with more than 50Nm.

Of course, it is a good thing to have a marked torque, so that the rider can use the torque value to calculate the acceleration performance. The mathematical formula is as follows, where T is the torque, J is the inertia, and α is the angular acceleration. Therefore, it is not wrong to indicate the torque, but when I saw the configuration of the entire electric bicycle, I was deeply confused.

From the motor characteristic specification table in the figure below, it can be found that to achieve the output torque of 30Nm, the input current needs to be 22A; if a higher current value can be input, the motor torque can continue to increase. Therefore, the author has doubts. If the motor capacity is to be described separately, the torque value should be marked with a higher value; the stall torque of this motor can even be as high as 83.16Nm. Stall refers to the zero speed of the motor. maximum torque value. If the current limit is considered, it should be clearly marked. This torque specification should be matched with the battery and driver that can provide more than 22A to meet the torque value of 30Nm. In fact, it was found that the driver matched with this motor has a current upper limit of 15A, which means that the maximum output torque can only reach 20Nm, but it is still marked as 30Nm. I feel that on the label of the product, the semantics are very unclear, and there are many gray areas, so that the industry can explain it by themselves.

And this characteristic specification table is another mystery; most electric bicycles do not actually provide this table, so it is difficult for ordinary users to confirm whether the efficiency and torque of this power system are in line with the product. marked. Of course, this form may also be too complicated for ordinary users; its practitioners can also provide some more simplified information for users to understand.At present, the electric bicycles used are allPermanent magnet motorthis type of motor characteristic hasconstantThe advantage of , that is, the fixed proportional relationship between the current and the torque, is called the torque constant, and the voltage and the speed are also in a fixed proportional relationship, called the back EMF constant.

When the rider obtains the torque constant, the torque information can be obtained by directly multiplying the current information provided by the electric bicycle. The equations of the motor torque corresponding to the current are listed below, and Kt is the torque constant.

It can be seen from the above that if the rider wants to confirm the status of the electric bicycle, in addition to the speed indication, the torque indication is also important; when the rider cannot directly read the torque information, querying the current information will also have the same effect. The effect is due to only a fixed constant relationship between the current and the torque. However, the current is limited by the battery and the motor driver. When the current supply is insufficient, the output torque of the motor will definitely drop. However, what is interesting is that the back EMF constant tells us that the speed is related to the voltage value, so when the rider encounters the current limit, he will not find any abnormality in the speed, only the difference in acceleration; so most of the It is difficult for the rider to notice. After all, it takes 3 seconds or 3.5 seconds to reach the maximum speed, which is too difficult for people to detect, which also leaves room for manufacturers to cut corners.

Highlights:
Current is important information and can correspond to torque.