3 Stages of Fan Current Curve Change | Fan Basics | Cooling Tech

When choosing fans as components for a device, it is essential to implement safeguards that prevent device burnout in cases where the fans become stuck or when current fluctuations occur during startup. This paper focuses on analyzing the fan's current behavior.

Since fans are powered by motors, they inherently undergo current fluctuations—a characteristic of motor-driven systems. The current initially decreases gradually after the fan is powered on and stabilizes at a nearly constant level during normal operation. This paper outlines the fan's current variation in three distinct phases: power-on, startup, and steady-state operation.

Power On

The fan drive circuit includes smoothing capacitors and other components that begin charging immediately when powered on, causing a large surge of instantaneous current known as inrush current.

The peak of the inrush current is determined by the fan's drive circuit. This peak is the highest point of current flow but lasts only briefly.

Start-Up

When the fan starts, even at low speeds, it generates a peak current called startup current. This current spikes immediately after startup and gradually decreases as the fan's rotational speed increases.

This reduction occurs because spinning fans generate a counter-electromotive force (CEMF), which is proportional to their speed.

The startup current typically lasts less than 10 seconds, though this varies by fan model.

Normal Operation

Once the fan reaches a stable speed, it operates at its rated current. Due to CEMF and other factors, minor ripple currents occur during operation. However, since these ripples are periodic and consistent, they have little impact on the average rated current listed in catalogs.

Locked Rotor Burnout Protection

If the fan blades are obstructed, a high current similar to the startup current can occur. To prevent damage from this peak current, fans can be equipped with a "locked rotor burnout protection function." For example, DC fans use a "current cut-off system" that automatically cycles the current on and off to protect against burnout.