Fan Current

When selecting a fan as a component of a device, you must be aware of changes in current during startup and shutdown to prevent the device from burning out. In the fifth lesson, we will explain fan current in detail.

Fans are driven by motors, so one of the characteristics of motors is that the current changes. The current value gradually decreases from the moment the fan is turned on, and during steady operation, it shows an almost constant stable value. Here, we will explain the process of current change in three stages: "When powering on," "At startup," and "During steady operation."

Fan current: power on

The fan drive circuit is equipped with components such as smoothing capacitors. Therefore, the moment the power is turned on, all of them start charging at once, and a large current flows instantaneously. This large current is called "inrush current."
The peak current value of the inrush current is determined by the fan's drive circuit, and is often the highest current value during the current change, but at the same time it has the characteristic of converging in a short period of time.

Fan current: at start

After power is applied to a fan, a high peak current is generated even when the fan speed is low. We call this the starting current. The starting current is at its maximum immediately after startup and gradually decreases as the fan speed increases.
This is because when the fan rotates, it also acts as a generator, generating a power called "back electromotive force" that is proportional to the rotation speed.
The duration of the starting current varies depending on the fan, but in most of our products it is less than 10 seconds.

Fan current: during steady operation

The current when the fan's rotation speed is stable is called the "rated current." The current during steady operation contains ripple due to the effects of back electromotive force, etc. The rated current value listed in the catalog is the average value including ripple, but since approximately the same amount of ripple occurs periodically, it does not have a significant effect on the average value.

Fan current: When the blades are restrained

In the unlikely event that the fan blades are constrained by some external factor, a high current similar to the starting current will be generated. To prevent the fan from burning out due to a continuous high peak current, our products are equipped with a "burnout protection function when the fan is constrained." For example, in the case of DC Cooling Fan, the "current cut method" automatically turns the fan on and off periodically to prevent the fan from burning out.

Supervised by: SANYO DENKI CO., LTD. Cooling System Design Department

release date: 2018-05-07 00:00