What Is the Role of a Varistor in a Motor?

In modern electrical and electronic systems, especially those involving motors, managing voltage spikes and protecting components is crucial for reliable and long-lasting performance. One of the key components used for this purpose is a varistor. But what exactly does a varistor do in a motor, and why is it important?

 1. What Is a Varistor?

A varistor (short for “voltage-dependent resistor”) is an electronic component that protects circuits against excessive transient voltages, such as those caused by switching operations, lightning strikes, or inductive load interruptions.

When the voltage across a varistor remains within its normal operating range, it behaves like a high-resistance element and allows only minimal current to flow. However, when a voltage spike exceeds a certain threshold, the varistor's resistance rapidly drops, allowing it to divert the excess energy away from sensitive components. Once the spike passes, it returns to its high-resistance state.

 2. Why Use a Varistor in a Motor?

Electric Motors—especially brushed DC motors—often generate voltage spikes during switching, reversing, or sudden stops. These spikes are caused by the inductive nature of the motor windings and can lead to:

- Electromagnetic interference (EMI) affecting nearby electronics  
- Damage to driver circuits or controllers  
- Accelerated wear of switches or relays  
- Increased electrical noise in the system

By placing a varistor across the motor terminals, these spikes can be effectively absorbed and clamped, reducing EMI and protecting surrounding components. This is especially important in sensitive applications such as medical devices, industrial automation, or consumer electronics.

 3. Internal vs External Varistors

Some motors come with internally integrated varistors, offering built-in protection. Others may require external suppression, where a varistor is added outside the motor casing. Both methods are effective, though internal suppression typically offers more compact and optimized protection.

A varistor plays a vital role in protecting motors and the systems around them from harmful voltage spikes. Whether built-in or externally added, this small component can significantly enhance motor reliability, reduce interference, and extend the lifespan of both the motor and its control circuitry.