Analysis on the difference between start stop control and soft start of Brushless DC motor

Brushless DC motor can start and stop with the power on or off just like the brushless DC motor. This kind of control method is usually completed directly with a contact switch. The disadvantage of this method is that the starting current is relatively large, too large current is easy to cause overheating and demagnetization problems, too fast starting will also cause the impact of load machinery, so it is only suitable for small power motors.

A more common method is to use a contact switch to connect the controller power supply, and then realize the start and stop through the control of the controller, for example:

(1) The controller is equipped with start stop control port (enable control) to control the start and stop of Brushless DC motor with logic level.

(2) The controller with speed regulating function controls the motor speed by the speed control command voltage. The motor often stops when the power is zero. Some controllers have PWM signal interface, when the duty cycle of the signal is zero, the motor stops.

(3) The position sensor is used to excite the power supply.

It is better to use the controller with soft start function for brushless DC motor to reduce the excessive current impact when the motor starts. The speed can rise steadily, reduce the speed overshoot, and reach the predetermined speed in a short time. The common soft start method is PWM. The soft start driver usually makes the PWM duty cycle rise from zero and slowly increase to a larger value until the rated speed. Then, the control system is converted to the normal position, speed or torque (current) closed-loop control. Slowly increasing the PWM duty cycle is equivalent to slowly increasing the voltage applied to the motor, thus limiting the starting current to a suitable level. In general, a soft start ramp of 50 to several hundred milliseconds is sufficient to limit the impact of current. However, some large inertia brushless DC motors need a longer soft start ramp.

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