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Inverter control

Recently it becomes common technology to use a microcontroller to do Inverter Control as a main controller. Toshiba has produced microcontrollers especially for the Inverter Control usage and expands strategically the lineup of the products to satisfy customers’ strong demand. The purpose of this document is to introduce the Inverter Control technology for non-professional engineers to easily understand the brief knowledge of the technology.

Inverter control

What is Inverter Control?

The primitive definition of “Inverter Control” is conversion from DC (Direct Current) to AC (Alternate Current). As known well, DC is the current whose voltage has a time-independent constant value, while AC voltage has time dependency. One of the most popular example of DC is the output voltage of a dry cell battery, and, AC, 60 Hz power supply which is available at home. 
The Inverter Control is widely used in several kinds of energy conversion, for example, a motor control (electric energy to motive power) for an air conditioning system or washing machines, and so on, IH cooking machines (electricity to heat), and power conditioners which convert solar-generated electric power to home AC power supply (electric to electric).

Inverter Control Usage
Inverter Control Usage

How is the Inverter Control achieved?

The system of the Inverter Control consists of two function circuitries. One of them is “Origin Wave Generator” for AC voltage, and the other is “AC generator” which produces a target AC voltage wave. The Origin Wave Generator makes a series of pulses whose heights are identical but widths are selected by the generator; the series of the pulses is “Origin Wave” for the target AC wave. The width of each pulse is decided by a special calculation which will be illustrated later.
And the AC generator modifies the Origin Wave to the AC wave. This circuit has several pairs of switches inside. For simple explanation, consider the case in which only one pair of two switches is existing in the function circuit. One of the terminals of one switch is tied to DC voltage source (V+) and the other switch, Ground level. Other terminals of both switches are connected each other, which makes output terminal of the AC Generator. Each switch is controlled by modification waves of the Origin Wave. This configuration can produce three voltage levels as the DC voltage level (V+), Ground level, and an intermediate level between V+ and Ground.
This explanation is dedicated to only two switches, but it is clear that more switches and sophisticated switch control will create more complicated AC waves from the simple DC and GND levels.

Now the topic should be changed to the Origin Wave Generator because it is the aim of this article.

Inverter control system
Inverter control system

1-2) Sine Curve

In many cases the target AC Waves would be Sine Curves. For example, a motor control system will require a sine wave to drive a motor because an ideal sine curves should give the most quiet rotation or the least power consumption. Another example is a power conditioner which will generate 60Hz sine wave on the commercial usage power lines.

Now, it is discussed how the Origin Wave generator produces the Origin Wave which will be transferred to a sine curve through the AC generator.

Sine Curve
Sine Curve

The Origin Wave of the sine curve is generated as follows.
At first, some definition should be done. The maximum output level and minimum one of the AC Generator are +V and –V, respectively. And the amplitude of the sine curve as the output is smaller than the value 2 x V.

Next, an isosceles triangle is prepared. The height of the triangle is 2 x V and it repeats along horizontal axis (time axis) and the base is a fixed time interval. The sine curve is put on the chart along with the triangles back ground.

Sine Curve and Isosceles Triangle back ground
Sine Curve and Isosceles Triangle back ground

Comparing the values of the triangles to those of the sine curve, define ‘one’ if the sine curve is bigger than the triangle and ‘zero’ if, not. That would get a sequential of unit-height pulses, which is the Origin Wave of the sine curve.

Generation of “Origin Wave S”
Generation of “Origin Wave S”

The Origin Wave (Signal S) features that the wider pulse appears at the bigger value in the sine curve. For better understanding if the pulses will be modified to fill in neighboring space without changing the area of the pulse, a shape of a sine curve looms (Signal Sa). As easily imagined, the shape is closer to a sine curve when the isosceles triangle becomes steeper (the base is smaller). Note that the Signal Sa is not a real wave but a conceptual wave.

Signal Sa modified by Origin Wave Signal S
Signal Sa modified by Origin Wave Signal S

The technology to generate a wave like the Origin Wave, which consists of pulses of constant height and variable width, is called PWM (Pulse Width Modulation). Inverter control is realized with PWM technology.

1-3) Feedback Control

The fundamental function of Inverter control is that Origin Wave generator produces PWM Origin wave and AC generator will generate a sine wave transformed by the Origin Wave. This is not all in actual implementation. The control system has a motor or another device inside which would be called “a load” in electric world. When the load is operating, it distorts the sine wave of the output of the AC generator; the amplitude of the sine wave may decrease, the phase may slightly change, or the frequency may be unstable, and so on.
There should be more several functions in the system to acquire ideal curve of the sine wave. A monitor function of the output wave of the AC generator (it is the input of the load). Next, the monitored signal should be compared with the ideal waveform. As the result, if the amplitude of the monitored signal is smaller, the output of Origin Wave Generator, PWM pulses, should be longer, and, vice versa. After repeating this process the output wave is quite close to the ideal wave and try to keep the waveform in the same shape.

Such loop as described above, generally speaking, is known well as “a feedback control” system. Owing to the feedback control the Inverter Control can be applied to a variety of different load values.

Feedback Control
Feedback Control

1-4) Circuits necessary for Inverter Control

The brief idea of Inverter Control has been given above, and next question will be what circuits are available to realize the control.
The answer is; a monitor circuit to monitor AC generator output, an isosceles triangle generator, a comparing circuit of the monitored signal and the isosceles triangle (a generator of signal S), a comparator of signal S of the monitored signal and the ideal signal S of the ideal sine wave, a storage of the ideal signal S, PWM pulse generator, and of course, AC generator itself.
Let's see more detail of each circuit.
The monitor circuit for the AC generator output would be an AD Converter which converts monitored analog signal to digital values. This conversion makes it easier to compare magnitudes between the conversion values and the isosceles triangle values (digital values).
A counter circuit is used to make the isosceles triangle. The counter should count pulses with a rather quick frequency clock, and it increments till some predefined count value and decrements after hitting the count value; which generates the isosceles triangle.
The comparison would be done by a digital calculator circuit.
The signal S of the ideal sine curve is stored in a storage memory.
And PWM pulses will be generated by a special circuit which is dedicated to control a series of PWM pulses.

Inverter Control Circuits
Inverter Control Circuits

As easily understood, almost all circuits for the Inverter Control function are integrated on a microcontroller chip. Especially, a microcontroller with PWM control IP (Intellectual Property) is one of the best solutions to do Inverter Control.

This is the end of the brief explanation of Inverter Control. It may be too simplified example to understand full of the technology. Please refer to professional books which are found in the shelves of bookstores or libraries to study more.


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