How to Design a FOC-Controlled PMSM Water Pump Driver

1. Understanding of Motor Characteristics

• Motor Parameters:

  • First of all, it is necessary to clearly understand the various parameters of the PMSM motor used, including rated power, rated speed, rated torque, stator resistance, stator inductance, permanent magnet flux linkage, pole pairs, etc. These parameters are crucial for subsequent drive system design and the selection of control algorithms.

  • For example, appropriate power devices can be selected based on the motor's rated power and speed, and the electrical frequency of the motor can be calculated based on the pole pairs and rated speed of the motor, so as to select a suitable control chip and drive frequency.

2. Selection of Power Devices

• Switch Tube Types:

  • IGBT (Insulated Gate Bipolar Transistor) or MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is usually used as the main switch tube. For the drive of small and medium-power water pump PMSM motors, MOSFET is more popular due to its fast switching speed and low on-resistance; for high-power applications, IGBT may be more suitable because it can withstand higher voltages and currents, but the switching speed is relatively slower.

  • The power devices should be selected according to the rated voltage and current of the motor, and a certain margin should be considered. Generally, the rated voltage of the selected power device should be 1.5 - 2 times the rated voltage of the motor, and the rated current should be 1.2 - 1.5 times the rated current of the motor.

• Heat Dissipation Design:

  • A large amount of heat will be generated during the operation of power devices, and good heat dissipation design is required. Heat sinks, air cooling or water cooling can be used. For high power density drive systems, water cooling may be required to ensure that the junction temperature of power devices does not exceed their rated temperature, avoiding performance degradation or even damage due to overheating.

3. Drive Circuit Design

• Drive Voltage and Current:

  • The drive circuit needs to provide sufficient voltage and current to ensure the fast and reliable conduction and turn-off of power devices. Generally, IGBT requires a drive voltage of about 15V - 20V, and MOSFET may require a drive voltage of 5V - 15V depending on different models.

  • The drive circuit should be able to provide a sufficiently large peak current to ensure that the power devices are fully turned on or off in a short time to reduce switching losses.

• Isolation:

  • To ensure electrical isolation between the control circuit and the power circuit and prevent interference and ensure safety, the drive circuit usually needs to adopt isolation techniques, such as optocoupler isolation or magnetic isolation. The optocoupler isolation circuit is simple, but there are transmission delays and bandwidth limitations; magnetic isolation (such as transformer isolation) can achieve higher transmission speed and frequency response, but the cost may be higher.

4. Control Strategy

• Field Oriented Control (FOC):

  • FOC is one of the most commonly used control strategies for PMSM motors. It decomposes the stator current into a torque current component and a field current component, and realizes precise control of the motor by controlling these two components. In FOC, rotation transformations (such as Clark transformation and Park transformation) are used to convert the current in the three-phase stationary coordinate system into the current in the rotating coordinate system for convenient control.

  • The coordinate transformation should be accurately implemented according to the motor parameters, and the torque current and field current should be controlled by PI controllers through closed-loop control to achieve accurate speed and torque control.

  • For example, in the constant torque region, the torque output of the motor can be precisely controlled by adjusting the torque current component; in the weak magnetic region, the high-speed operation of the motor can be realized by adjusting the field current component.

• Sensorless Control:

  • For some application scenarios, in order to reduce costs and improve system reliability, sensorless control techniques can be used. Common sensorless control methods include back electromotive force method, sliding mode observer method, model reference adaptive method, etc.

  • The back electromotive force method is based on the relationship between the back electromotive force of the motor and the speed, but the back electromotive force is weak at low speeds, which will affect the measurement accuracy; the sliding mode observer method constructs a state observer of the motor to estimate the rotor position and speed, and has certain robustness to parameter changes, but may cause system jitter; the model reference adaptive method estimates the rotor position and speed through an adaptive algorithm based on the mathematical model of the motor, the algorithm is relatively complex, but has good performance at medium and low speeds.

5. Protection Functions

• Overcurrent Protection:

  • An overcurrent protection circuit should be set in the drive circuit. When the detected current exceeds the set threshold, the power device should be turned off in time to prevent damage to the motor and power devices due to overcurrent. Hall current sensors or shunt resistors can be used to detect the current, and the overcurrent protection function can be realized through comparators and logic circuits.

  • For example, when the current exceeds 1.5 - 2 times the rated current, the protection mechanism should be triggered immediately, the drive signal should be turned off, and an alarm signal should be issued.

• Overvoltage Protection:

  • It includes the protection of power supply overvoltage and motor back electromotive force overvoltage. For power supply overvoltage, a voltage comparator can be used to monitor the input voltage; for motor back electromotive force overvoltage, it can be judged by monitoring the DC bus voltage and motor phase voltage.

  • When the motor decelerates or brakes rapidly, excessive back electromotive force may be generated, and corresponding protection measures should be taken at this time, such as using active or passive clamping circuits to limit the back electromotive force within a safe range.

• Undervoltage Protection:

  • When the input power supply voltage is too low, the normal operation of the drive system may not be guaranteed, which will affect the performance of the motor and even cause the motor to lose steps. Therefore, undervoltage protection needs to be set. When the power supply voltage is lower than the set lower limit value, the motor should be stopped and an alarm should be issued.

6. Electromagnetic Compatibility (EMC) Design

• Filter Circuits:

  • Appropriate filters should be added at the input and output ends, such as using common mode and differential mode filters at the input power supply end to suppress interference introduced by the power supply and suppress the interference of the drive system on the power grid; adding filter inductors and capacitors at the output end to suppress the high-frequency harmonics generated by PWM chopping and reduce electromagnetic interference to the motor.

  • LC or LCL filters can be used, and appropriate inductor and capacitor parameters should be selected according to the rated power of the motor and the drive frequency.

• Shielding and Wiring:

  • The drive circuit board should be shielded, and the power part and the control part should be laid out separately to reduce the influence of electromagnetic interference on the control circuit.

  • When wiring, attention should be paid to keeping the current loop as small as possible, avoiding the intersection of high-current loops and signal loops, and using shielded wires for high-frequency signals and high-current conductors to reduce electromagnetic radiation and coupling.

When designing the PMSM motor drive for water pumps, it is necessary to consider the above aspects comprehensively, and optimize the design of each link according to specific application scenarios and performance requirements to achieve a high-performance, high-reliability and cost-effective motor drive system.