Brushless Dc Motor Starting Strategies And Problems

Due to the operating characteristics of brushless DC motors, if the starting strategy is improper, the starting current may be much higher than the normal value, thereby damaging the hardware circuit; on the other hand, starting current that is too small may also lead to insufficient electromagnetic torque, making it difficult to start the motor. And the startup time is longer.

Brushless DC motor speed single closed-loop speed regulation system
In a brushless DC motor speed single closed-loop speed regulation system, it is usually impossible to directly measure the armature current. In order to ensure reliable operation of the system, the motor is usually started under no-load or light-load conditions. When starting, the PWM duty cycle gradually increases from a small value enough to overcome the load torque until it reaches the set speed. Then, the control system switches to normal speed closed-loop control. Gradually increasing the PWM duty cycle is equivalent to gradually increasing the voltage applied to the motor armature, thereby limiting the starting current to a suitable level. Usually, controlling the start-up time within 50 to several hundred milliseconds can effectively limit the impact of the start-up current.

The Starting And Speed Adjustment Process Of Brushless Dc Motor

The starting and speed adjustment of a brushless DC motor can be divided into the following three steps, taking driving a fan as an example:

1. Rotor positioning: 

Select the starting torque according to the load size, and calculate the armature voltage to determine the starting PWM duty cycle. The commutation circuit outputs a specific phase voltage until the rotor rotates to a specific position.

2.Ramp up speed:

Start commutation based on the detected position signal and start measuring the speed. Calculate the load torque based on the measured speed, and determine the required electromagnetic torque combined with the required speed-up time. Then calculate the corresponding armature current. Finally, the new PWM duty cycle value is calculated and output. Repeat this process until startup is complete.

3.Normal operation:

The system enters the closed-loop operation state, and the PWM duty cycle is determined by the speed PID operation.

System Advantages And Limitations

In general, the speed single closed-loop system has the advantages of simple system and low cost. However, since the current cannot be directly controlled, the dynamic performance of the system may not be ideal, making it unsuitable for applications that require frequent starts and stops.

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