# Voltage Control Methods of Inverter – PWM Technique

In practice, the waveform of the output voltage obtained from a single-phase inverter is rectangular in nature with an amplitude approximately equal to the input dc voltage. However in many applications, the output voltage of the inverter needs to be controlled due to the following reasons,

## External Control of AC Output Voltage :

In this method of control, an ac voltage controller is connected at the output of the inverter to obtain the required (controlled) output ac voltage. The block diagram representation of this method is shown in the below figure.

The voltage control is primarily achieved by varying the firing angle of the ac voltage controller that feeds the ac load. In this method, there is a high level of harmonic content when the output voltage from the controller is at a low level. This method is limited to low-power applications only.

## External Control of DC Input Voltage :

The external control of dc input voltage is a technique that is adapted to control the dc voltage at the input side of the inverter itself to get a desired ac output voltage at the load side. This method is further classified into two categories based on the type of source.

### When Source is a Constant AC Voltage Source :

In this case, the constant ac voltage from the source is rectified and then fed to the inverter. The rectification of input ac voltage can be done by using a controlled rectifier or an uncontrolled rectifier or an ac voltage controller.

### When Source is a Constant DC Voltage Source :

When the available input voltage source is dc, the inverter’s input voltage can be controlled by using a chopper. The block diagram for controlling the output voltage of the inverter when the input voltage available is constant is of constant DC type is shown below.

The above discussed voltage control methods can be implemented in practice, but however, it requires additional filters in order to reduce dc voltage ripple that increases the cost and weight of the inverter. Also, the use of above methods decreases the overall efficiency of the equipment due to increased power stages.

## Internal Control of Inverter :

The output voltage of an inverter can be adjusted by employing the control technique within the inverter itself. This control technique can be accomplished by the following two control methods.

### Series Inverter Control :

The below figure shows the typical arrangement explaining the method which consists of one or more inverters connected in series. In the below figure shows it is also seen that the load voltage of the dual connected inverters is added up with the help of two transformers TF1 and TF2 in order to maintain an adjustable output voltage.

Each of the inverter’s outputs is connected to the primary winding of the transformers with secondaries connected in series. The voltage V1 and V2 are obtained, whose phasor sum gives the resultant voltage VL. From the phasor diagram, the expression for load voltage is given as,

### Pulse Width Modulation (PWM) Control :

The most efficient method for the control of output voltage is to introduce pulse width modulation within the inverters which doesn’t require any extra peripheral components. It is an efficient and economical way for output voltage control of inverters compared to other methods. The commonly used pulse width modulation techniques are,

#### Single pulse width modulation.Multiple pulse width modulation.Sinusoidal pulse width modulation.

The above three methods of PWM techniques differ only by harmonic content in their output voltages. In this, the inverter circuit is fed from a constant dc voltage source and a controlled ac voltage is obtained at the output terminals by turning ON and OFF the switching components in the inverter circuit. The main drawback of this method is that it requires very low turn-ON and turn-OFF time thyristors which are very expensive.