A brushless DC motor is a system consisting of a motor body, an electronic commutation circuit, and a rotor position sensor. The electronic commutation circuit consists of an inverter circuit and a control circuit. The basic working principle of a brushless DC motor is different from that of a brushed DC motor. It does not require brushes and commutator segments for commutation. Instead, it obtains the rotor position information of the brushless DC motor through a rotor position sensor. The controller processes the transmitted rotor position information and generates a logic switch signal to control the on and off of the power switch tube, thereby controlling the operation of the motor [1].
The drive control circuit consists of components such as power transistors, resistors, capacitors, and integrated chips. If any of these components fails, the entire drive control circuit will basically be unable to work properly. In rare cases, the entire drive control circuit can still work when a few components fail, but this often leads to a decrease in motor performance [2]. Therefore, the drive control circuit is a weak link in the reliability of brushless DC motors [3], among which the linear bus capacitor and IGBT have a greater impact on the reliability of the drive control circuit [4]. This paper starts from the fault simulation of brushless DC motors, explores the fault simulation and experimental verification of Hall effect components, and studies the characteristics of motor speed and phase current changes under fault modes.
1. Fault Simulation Model of Brushless DC Motor
A fault simulation model of a brushless DC motor was established using Matlab software, as shown in Figure 1. The model includes a brushless DC motor body module, an inverter module, a PWM signal generation module, a speed and current dual closed-loop control circuit, and a signal feedback circuit.
The parameters used in the simulation of the brushless DC motor are shown in Table 1. The inverter module uses a three-phase inverter bridge composed of six power transistors. The motor adopts a dual closed-loop control strategy for speed and current, with both the current loop and speed loop regulated by PI regulators. The output of the regulator serves as the input to the PWM generator to control the PWM duty cycle, thereby changing the voltage applied to the windings. The feedback circuit collects the speed, current, and Hall sensor signals required for the simulation.
The PWM signal generation module is shown in Figure 2. It performs logic operations based on the three-phase Hall position signals output from the brushless DC motor and the PWM signal output from the DC/DC pulse width modulation generator, thereby outputting control pulses for the six power transistors. This module has a total of four input signals: the three-phase Hall position signals (HA, HB, HC) and the PWM signal generated by the PWM generator.
In the simulation, the brushless DC motor adopts a two-phase, six-state conduction mode, with both switching transistors turned on simultaneously each time. The conduction sequence is T1, T4→T1, T6→T3, T6→T3, T2→T5, T2→T5, T4, for a total of 6 conduction states. The conduction state changes every 60°, switching only one switching transistor each time, with each switching transistor conducting continuously for 120°. The corresponding logic relationship is shown in Table 2.
Figures 3 and 4 show the speed waveform and three-phase current waveform of the brushless DC motor in the model during normal operation. At this time, the motor is running at the rated speed of 3000 r/min, and the output three-phase current corresponds to the rated output torque of the motor of 0.1 N·m.
2. Hall Sensor Fault Simulation
A common Hall sensor failure is a disconnection in the connection between the sensor and the controller. If two or three wires are disconnected, the motor will not rotate. The following analysis focuses on the case of a single-phase Hall sensor disconnection. Figures 5 and 6 show the simulated current and speed waveforms of a disconnected A-phase Hall sensor. After the A-phase Hall sensor disconnects, the corresponding sensor signal remains 1 in all subsequent states. This disrupts the mapping relationship between the Hall signal and the trigger pulses of the six power transistors, causing commutation timing disorder in the windings. This results in phase A remaining non-conductive in the positive direction, while phase C remains non-conductive in the negative direction, reducing the average electromagnetic torque and consequently decreasing the speed. Throughout this process, the speed exhibits significant oscillations. Furthermore, the phase current amplitude increases by more than two times, posing a high risk of motor burnout.
3. Experimental Study on Open Circuit Fault of Winding
This paper presents a fault test on the brushless DC motor under study, and the test platform is shown in Figure 7. The test platform includes the brushless DC motor body and drive controller, a 24 V DC power supply, an air switch, a digital power meter, a handheld tachometer, and a host computer. The 24 V DC power supply powers the brushless DC motor body and drive controller; the on/off state of the air switch is used to simulate various fault modes; the digital power meter measures the relevant voltage and current waveforms and displays them on the host computer software; the handheld tachometer is used to measure the motor speed.
The circuit breaker tripped, simulating a Hall sensor open-circuit fault. During the experiment, regardless of whether the Hall sensor tripped a single phase signal, a two-phase signal, or a three-phase signal, the motor operated normally, and the current waveform, as shown in Figure 8, was essentially the same as the waveform during normal operation.
This is because a sensorless control subroutine based on back-EMF zero-crossing detection has been added to the brushless DC motor control program. When the controller detects an abnormal Hall sensor signal, it switches to the sensorless control subroutine for control. Since the sensorless control program for the brushless DC motor can also effectively control the motor speed, the motor can not only maintain normal operation at 3000 r/min, but also maintain stable voltage and current waveforms. In a typical brushless DC motor, a Hall sensor disconnection fault would prevent normal operation and could potentially burn out the motor. By using the sensorless control subroutine based on back-EMF zero-crossing detection, this potentially fatal fault can be reduced to a minor one, significantly improving the reliability of the brushless DC motor.
4. Conclusion
This paper establishes a brushless DC motor fault simulation model and simulates a Hall sensor disconnection fault. During the simulation, the motor speed exhibits significant oscillations, and the phase current amplitude increases by more than two times, posing a high risk of motor burnout. Then, a brushless DC motor fault simulation test platform is built to conduct Hall sensor disconnection fault tests. In these tests, due to the inclusion of a sensorless control subroutine in the control software, the motor continues to operate normally after the Hall sensor disconnects, significantly improving system reliability.
