Typical Applications of Metal Oxide Varistors (MOVs) in Motor Systems

1. Introduction
    In motor control systems, brushed DC motors are widely adopted in compact devices, household tools, and automotive electronics due to their simple structure, flexible speed control, and cost advantages. These motors operate based on mechanical contact between brushes and commutators, enabling continuous rotation through periodic current switching.
    Despite advancements in brushless motor technology, brushed motors continue to play a crucial role in cost-sensitive and response-critical applications.

2. Electromagnetic Interference and Mitigation Strategies
    Sparking and pulse surges during carbon brush switching are the primary sources of electromagnetic interference (EMI), which may cause:

    Surge voltage intrusion, reducing component reliability;

    Excessive EMI levels, resulting in non-compliance with regulatory standards.

    While BDL filter circuits are effective at suppressing certain high-frequency interferences, additional mitigation measures are often necessary in high-disturbance or complex environments. This application note recommends the integration of Metal Oxide Varistors (MOVs) to enhance surge protection performance.

3. MOV Operation Mechanism
    MOVs are ceramic components based on zinc oxide materials with characteristic nonlinear voltage-current behavior. Under normal voltage, the device remains in a high-impedance state. Once the threshold voltage is exceeded, its impedance drops sharply, allowing a large current to pass through and clamping the voltage, thereby effectively protecting the system from surge damage.

4. Comparison with Other Protection Devices
    MOVs and TVS diodes differ in both structure and application. TVS diodes are semiconductor devices based on PN junctions, ideal for high-speed signal protection. In contrast, MOVs are better suited for power lines and high-current circuits due to their superior surge current capacity and energy absorption capabilities.
    Additionally, MOVs exhibit certain capacitance characteristics, allowing them to replace multiple parallel filter capacitors in circuit design, thereby simplifying the layout.

5. Key Electrical Parameters

    Vrms / Vw: Maximum continuous operating voltage.

    IL: Leakage current under maximum rated voltage.

    V1mA: Voltage across the device when 1 mA current is applied.

    Vc: Maximum clamping voltage under pulse current.

    IPP: Peak pulse current capacity, tested under 8/20 μs waveform conditions.

6. Applications of SMD-Type MOVs
    SMD MOVs feature compact dimensions and convenient mounting, making them suitable for space-constrained, low-power circuits, such as appliance control boards, toy motors, and electric toothbrushes in consumer electronics.

7. Applications of DIP-Type MOVs
    For high-power applications like industrial equipment and electric vehicle drives, DIP-type MOVs offer higher surge endurance. These devices can effectively suppress large voltage spikes, ensuring system stability under harsh conditions.

8. Product Selection Guidelines
    MOV selection should consider rated voltage, motor size, surge intensity, and PCB layout.

    Small motors or modular designs → SMD MOVs

    Industrial-grade or high-surge conditions → DIP MOVs
    For optimal EMI suppression, the combination of MOVs with filtering components is recommended.

9. Comparative Test Data

    Without protection: The circuit is vulnerable to surge breakdown, with EMI tests failing significantly.

    With MOV + BDL solution: Surge responses are effectively suppressed, EMI levels are controlled, and system reliability is notably improved.

10. MOV Advantages and Limitations
  Advantages:

    Low cost with high current capacity

    Fast response time and wide protection range

    Reduces reliance on additional filter capacitors, simplifying system design

  Limitations:

    Larger size, not suitable for compact or high-speed signal circuits

    High parasitic capacitance may adversely affect certain designs

  Precautions:

    Avoid use in high-humidity, high-temperature, or solvent-rich environments

    Do not apply external force or damage the device surface during handling

    Maintain a minimum 2 mm distance between the leads and the component body during bending

11. Conclusion
    MOVs offer a cost-effective solution for mitigating surge and EMI issues in motor systems. With proper selection and circuit integration, system-level EMC performance and operational stability can be significantly improved, providing engineers with reliable technical support in complex application scenarios.