Modulation involves adjusting the on and off duration of inverter switches under constant input DC voltage to achieve controlled inverter output voltage.
The most popular modulation technique used in inverters is pulse width modulation (PWM).
Space vector modulation is often used in inverters due to its ease of implementation.
Inverters are significant in industrial systems, as they are used for feeding AC loads
Inverters are significant in industrial systems, as they are used for feeding AC loads. Depending on the type, the input voltage requirements of AC loads may be constant AC voltage or adjustable AC voltage. To satisfy AC load input requirements, inverters are controlled so that they supply the voltage in the desired magnitude, phase, and frequency.
The inverter output voltage can be controlled in various ways. Modulation is an internal method of controlling an inverter to generate the desired voltage waveform. Compared to other methods of inverter control, modulation requires no additional components. Let’s explore the different types of modulation used to control inverters.
Methods to Control the Inverter Output Voltage
The methods to control the output voltage of an inverter can be external or internal, and includes the following:
External Control of Inverter Output Voltage
In this inverter control, the inverter output AC voltage is converted into the required specification using peripheral components. The external control of inverter output voltage can use either:
AC voltage controllers - The inverter AC output voltage is fed to an AC voltage controller circuit and the controller produces the required voltage waveform for the AC loads.
Series-inverter control - Inverters are connected in series using transformers to achieve the voltage specification of the AC loads.
External Control of Inverter Input Voltage
In this control of inverters, the input DC voltage is varied using circuits—such as a fully controlled rectifier or an uncontrolled rectifier and chopper—so that the inverter output voltage matches the AC load input voltage requirements. The combination of AC voltage controller and uncontrolled rectifier are also used to modify the inverter input to get the required AC output.
Internal Control of the Inverter
The aforementioned methods of inverter control require additional components to generate output voltage or the desired magnitude, phase, and frequency. Modulation involves adjusting the on and off duration of inverter switches under constant input DC voltage to achieve controlled inverter output voltage. In modulated inverters, a control signal is compared with a repetitive switching frequency waveform to generate switching pulses. The duration of the switching pulses controls the turn-on and turn-off of the inverter switches, thus the output AC voltage is controlled.
Next, we will look at the types of modulation used in inverters.
Common Types of Modulation Schemes in Inverters
The most popular modulation technique used in inverters is pulse width modulation (PWM) and the inverters that use this technique are called PWM inverters.
PWM inverters eliminate lower order harmonics and decrease the THD content in the output AC voltage. They also minimize filter requirements. The types of modulation commonly used in inverters are:
- Single pulse modulation - This modulation technique consists of one pulse per half-cycle, and the width of the pulse varies from 0 to 𝜋. By varying the pulse width, the output voltage is controlled.
- Multiple pulse modulation - This modulation scheme is an extension of single pulse modulation, where several equidistant pulses are present per half cycle. The pulse width is equal in multiple pulse modulation.
- Sinusoidal pulse width modulation - This is a type of multiple pulse modulation, where several pulses are present in each half-cycle. The pulse width is the sinusoidal function of the angular position of the pulse in a cycle.
- Space vector modulation - The space vector modulation scheme is commonly used in inverters due to its ease of implementation. It is frequently employed in multilevel inverters.
The type of modulation used in an inverter impacts the THD content and filter requirements, which is why it is so critical to select a suitable technique according to the voltage specification of the AC loads. Luckily, Cadence software can help when designing inverters and their control circuits.