What logic is speed control based on?

Find out how the speed control works in the 7-Speed KUBE series controllers.

The KUBE line is enriched with a new family of temperature and speed controllers: the K_7 SPEED family. This series of controllers, in addition to the features already present on the Kube, also incorporates the speed control for 24 V DC brushed motors. The peculiarity of this control consists in keeping the speed of an engine constant as the load applied varies, without the need for an additional speed sensor.

WHAT LOGIC IS SPEED CONTROL BASED ON ?

The solution is based on 2 considerations:

First consideration

A brushed DC motor with permanent magnets, when mechanically rotated (without giving it voltage), behaves like a dynamo. As many will know, a dynamo produces a voltage directly proportional to the number of revolutions.

Second consideration

To adjust the speed of a DC motor, simply adjust the supply voltage. The most efficient way to regulate the voltage of a DC motor is the so-called PWM. [pulse-width modulation, is a type of digital modulation that allows to obtain a variable average voltage, depending on the relationship between the duration of the positive and negative pulse (duty cycle)].

In practice, we apply the entire power supply voltage to the motor for a time T1 and then remove voltage for a time T2 (T1 + T2 = cycle time).

Let’s take an example. For a moment, think of a DC light bulb. If the cycle time is large (seconds) we will see the light bulb turn on and off, but if the cycle time becomes small (e.g. 20 ms) the light bulb (thanks to its thermal inertia) does not reach maximum brightness or go out and the perceived result is that the brightness is lowered. The same thing happens with a motor (thanks to its mechanical inertia).

By adding the two considerations, we will therefore obtain that: when I energize the motor (T1) I produce mechanical energy, when I remove the voltage from the motor (T2) I read the voltage produced by the motor (dynamo) and I get the current speed. As already said, the cycle time must be fast, therefore also the speed measurement will be frequent and the reaction to load changes will be fast and effective.