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What is four quadrant control?

07 October 2019

The principles of four quadrant control are fundamental to process control, especially continuous process and are present in all our traditional industries such as Paper, Film, Extrusion, Steel, Printing and Wire and Cable to name but a few.

In a bid to explain the basic premise we need to look at motor performance in the real world and how a motor can deliver power and torque as well as being subjected to external loads and forces. So, to start with the basics. What are the four quadrants? If a motor is connected to a load then it can be driven (or accelerated) in a forward direction. This is quadrant 1. If the load is then slowed down or braked in this direction (decelerated) then this is quadrant 2. If the motor is decelerated or braked past zero speed and into reverse then this is quadrant 3. Finally, if the motor is accelerated in reverse then this is quadrant 4 (see Figure 1). 

To simplify this slightly, if you look at acceleration and deceleration as positive torque and negative torque it is sometimes easier to apply the 4Q model to an application (also shown in Figure 1).

This all looks very straight forward until you look at controlling a motor with a variable speed controller and here, we find one of the main differences between AC inverters and DC controllers. The ability to control in all four quadrants requires some advanced electronics in the AC world.

Read the full article in the October issue of DPA.

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