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,

• The voltage required by ac loads may be constant or adjustable. When inverters are used to feed such ac loads, it is necessary that the inverters provide provision for voltage variations so as to supply the required voltage to ac loads.
• In motor control applications, inverters handle the control of circuit voltage along with frequency so that the saturation of motor magnetic circuits is avoided.
• In the case of variable speed drives, inverters with voltage control help in achieving voltage variation.
• Voltage control of inverters is employed in order to compensate for changes in input dc voltage.

Basically, there are three techniques by which the voltage can be controlled in an inverter. They are,

• External Control of AC Output Voltage
• External Control of DC Input Voltage
• Internal control of Inverter.

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.

• By Using Controlled Rectifier : The block diagram representation of this method is shown below.
  

  The rectifier used in this method is a fully controlled rectifier i.e., either a single-phase or a three-phase fully controlled rectifier (thyristor bridge). The constant ac voltage from the source is fed to the controlled rectifier whose firing angle can be varied to obtain the desired output voltage. The controlled dc voltage obtained from the output of the controlled rectifier is fed to the inverter to get the controlled ac voltage.

• By Using Uncontrolled Rectifier : The block diagram representation of inverter voltage control using uncontrolled rectifier is shown in the below figure.
  

  The constant voltage from the ac source is fed to an uncontrolled rectifier (diode bridge). The constant dc voltage from the rectifier is fed to the chopper in which the variation of firing angle gives the desired dc output voltage. This controlled dc voltage is finally fed to the inverter to obtain controlled ac voltage.

• By Using AC Voltage Controller : The block diagram representation of this method is shown below.
  

  In this case, an ac voltage controller is used along with an uncontrolled rectifier to feed the controlled dc voltage to the inverter. The inverter finally delivers controlled ac voltage to the load.

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,
• Pulse Width Modulation Control.

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 V_L. From the phasor diagram, the expression for load voltage is given as,

It is essential the frequencies of output voltage V₁ and V₂ from the two inverters must be equal.

• For Φ = 0, V_l = V₁ + V₂
• For Φ = π, V_l = 0 (in case V₁ = V₂).

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.