The rated current of brushless DC motors is generally required by users, which involves the power of the DC power supply selected by the user and the amount of discharge (ampere-hour). The smaller the current drawn by the motor from the battery, the longer the battery can be used on a single charge. Especially in electric bicycle motors, users want the motor to have a longer mileage, so the rated current of the motor should be relatively small. The determination of the rated current of the motor is very particular. Therefore, the factors that affect the rated current of brushless DC motors are as follows.

For example:

• (1) The input power of the motor at the rated point is determined, and the efficiency of the motor is basically determined. The higher the rated voltage of the motor, the smaller the rated current.

• (2) The larger the torque constant of the motor, the smaller the rated current of the motor.

• (3) Try to reduce the torque of the motor, that is, reduce the no-load loss of the motor, so that the no-load current of the motor is reduced, so that the rated current of the motor will also be reduced accordingly, but the reduction is not much.

57 Brushless Motor with Built-in Drive

Conclusion

When the rated voltage of the brushless DC motor is increased, the no-load speed of the motor will also increase accordingly. If the rated speed of the motor is determined and the efficiency of the motor is not much different, then the no-load speed of the motor can be calculated. If the rated voltage of the motor is increased, in order to ensure the rated speed of the motor, it means that the back electromotive force constant of the motor must be increased in proportion to the rated voltage. If the shape of the motor remains unchanged, that is, the working magnetic flux of the motor remains unchanged, then the number of effective conductors of the motor must be increased proportionally. For a slot shape of a certain area, this is not a problem. The armature resistance of the motor will also increase accordingly, and the internal resistance of the motor will be large. However, the current of the motor is also reduced accordingly. In the copper loss of the motor, the current decreases in a square relationship, while the resistance of the motor increases in a proportional relationship. Therefore, the copper loss of the motor will not increase, but decrease. Therefore, the efficiency of the same motor made into a high-voltage motor will be higher than that made into a low-voltage motor, mainly because the copper loss of the motor is smaller. Power transmission uses high-voltage power transmission to reduce the loss caused by the current in the line. High voltage can cause personal injury, so the correct selection of the power supply voltage of the brushless DC motor is something that designers need to seriously consider.