2022-04-22

The brushless DC motor consists of a motor body and a driver, and is a typical mechatronics product. Since the brushless DC motor is operated in an automatic control mode, it will not add a starting winding to the rotor like the synchronous motor that starts under heavy load under variable frequency speed regulation, nor will it produce oscillation and loss of step when the load changes suddenly.

The brushless motor brake is divided into high-level brake and low-level brake.
1. High-level brake causes the DC brushless motor current to reverse instantly.
2. Low-level brake, first cut off the brushless motor drive power supply, and then short-circuit the lower bridge MOS to make the three phases short-circuited.
Both have certain advantages and disadvantages. High-level brake is fast and the circuit is more complicated. Low-level brake causes excessive phase current and may burn the motor.

The state of free sliding of the motor without driving voltage does not actually exist. One is that the motor has a tooth slot positioning torque, that is, when the motor is in an open circuit state, the shaft of the brushless motor can feel a stagnant resistance. It is formed by the closure of the rotor permanent magnet and the stator magnetic circuit. Therefore, even if the rotor is in a free state, it is still in a specific position.

In addition, since the motor is in the power generation state at this time, although the switch tube is in the off state, there is a reverse diode in parallel with the switch tube, which is in the forward conduction state. It can feed back the energy generated in the power generation state to the power supply, which will inevitably be converted into braking torque. If the rotor speed is relatively high, the discharge capacity of the power supply should also be considered. The general rotation speed does not need to be considered. Therefore, in the initial deceleration stage of the motor, the above braking force can be used to reduce the motor speed before considering other rotation measures.

There are usually two simple ways to use the motor itself for rapid braking, one is energy consumption braking, and the other is short-circuit braking. Energy consumption braking consumes the kinetic energy of the motor on the external braking resistor, and short-circuit braking consumes the kinetic energy of the motor on the stator winding of the motor. Obviously, energy consumption braking is more beneficial to reducing motor heating. However, short-circuit braking does not require any changes to the hardware. Its simplicity and convenience are its outstanding advantages, so we focus on short-circuit braking.

The so-called short-circuit braking means that when braking, the upper bridge arm (or lower bridge arm) of the motor's driving MOS tube can be fully turned on while the lower bridge arm (or upper bridge arm) is turned off, and the three-phase stator windings of the motor are all short-circuited. The motor in the power generation state is equivalent to the power supply being short-circuited. Because the resistance of the winding is relatively small, a large short-circuit current can be generated, and the kinetic energy of the motor is quickly released, so that the motor instantly generates a large braking torque, which can achieve the effect of rapid braking. The higher the motor speed, the greater the short-circuit current and the greater the braking force.

However, it must be considered that it cannot exceed the tolerance of the MOS tube, so it is generally necessary to wait for the motor speed to drop to a certain level before using short-circuit braking. Specifically, our current hardware circuit has PWM control in its lower bridge arm. Therefore, the three-phase wiring of the lower bridge arm is short-circuited, so that the braking force can be appropriately adjusted. In order to avoid excessive short-circuit current at high speed of the motor, which exceeds the tolerance of the MOS tube, the duty cycle of PWM control should generally not exceed 30% when short-circuiting braking begins.