An inverter is a device used to obtain ac power of desired voltage and frequency from a dc power. The inverters achieve this by using thyristors with forced commutation or other semiconductor devices like BJT, MOSFET, IGBT, etc. According to circuit configuration, inverters are classified into three broad categories,

- Series Inverters,
- Parallel Inverters, and
- Bridge Inverters.

In this article, let us learn about the circuit diagram and operation of a series inverter.

## What is Series Inverter?

A series inverter is a type of inverter in which the commutating components are connected in series with the load. A series inverter employs class-A commutation or resonant commutation since the current decays to zero naturally by load commutation but not by forced commutation. Class-A commutation exists in circuits supplied from a dc source only.

## Circuit of Series Inverter :

The basic circuit of a series inverter is shown below. In the below figure inductor (L) and capacitor (C) are commutating components, T_{1} and T_{2} are two thyristors that conduct for positive and negative half-cycles of load current.

In a series inverter, values of the inductor (L) and capacitor (C) are chosen in such a way that the series RLC circuit should be underdamped since oscillations are possible in an underdamped circuit only. For an RLC circuit to be underdamped, the condition R^{2} < (4L/C) must be satisfied.

Series inverters are also known as self-commutated inverters or load-commutated inverters or resonant inverters because they employ class-A commutation. Series inverters are capable of producing the output waveform ranging from 200 Hz to 100 kHz frequencies.

## Working of Series Inverter :

#### The operation of a series inverter can be understood through three modes.

### Mode I :

Mode I starts when thyristor T_{1} is triggered at instant t = t_{0} by applying gate pulses to it. As T_{1} is triggered, it starts conducting and the load current flows through the path V_{dc}^{+} → T_{1} → L → C → R → V_{dc}^{–} as shown below.

Initially, the capacitor is charged to a negative voltage -V_{C}, but once T_{1} is triggered capacitor starts charging to positive voltage with upper plate positive and lower plate negative as shown above. As the current increases and reaches its positive maximum value, the voltage across the capacitor becomes equal to supply voltage V_{dc}.

Now, the current starts decreasing after reaching its positive maximum value but the voltage across the capacitor does not decrease. Instead of decreasing it increases further and reaches a value higher than V_{dc}, and the capacitor retains this voltage for some time.

At t = t_{2}, thyristor T_{1} is turned OFF when the current reaches zero by natural commutation, but still, the capacitor holds the voltage (V_{C} + V_{dc}) in it.

### Mode II :

This mode starts from instant t_{2} when thyristor T_{1} is commutated and it remains in OFF state for a sufficient period of time (t_{2} to t_{3}) as shown in the below waveforms. Hence, in this mode, both the thyristors T_{1} and T_{2} are in OFF-state and the capacitor voltage is maintained at a constant value of (V_{C} + V_{dc}), and the load current I_{L} remains zero in this mode i.e., from t_{2} to t_{3}.

### Mode III :

In this mode of operation, thyristor T_{2} is triggered at instant t_{3} since the positive polarity of the capacitor appears across the anode of T_{2} and it starts conducting. As thyristor T_{2} conducts, the load current starts flowing in the negative direction through the path C^{+} → L → T_{2} → R → C^{–} as shown below.

Now, the capacitor starts discharging and the load current I_{L} flows in the reverse direction and reaches its negative maximum value. Then, load current starts decreasing and becomes zero at t_{3}, due to this thyristor T_{2} gets turned OFF at t_{3}. Then after capacitor again charges to negative voltage -V_{C} as shown in the waveform. Again, after maintaining a certain amount of time delay, thyristor T_{1} is triggered and the cycle repeats.

In the above waveforms, we can observe that the positive half-cycle of the load current is exactly equal to the negative half-cycle of load current since the capacitor stores charge during one half-cycle and releases the equal charge in the next half-cycle. But in practice, the output voltage wave of a series inverter is not a pure sine wave and distortions are present in it.

## Disadvantages of Series Inverter :

#### The following are the limitations of a series inverter,

- Load voltage waveform has distortions and harmonic contents because of time delay in turning ON of the thyristors and the distortion increases if the operating frequency of the inverter is less than the resonance frequency. Hence, the maximum operating frequency is limited in the inverter as it should be less than the resonance frequency.
- If the maximum operating frequency of the inverter is exceeded by the circuit firing frequency, then the dc input source will be short-circuited.
- The current flow from the supply is discontinuous because in one half of the cycle, the load current is drawn from the supply, and in another half of the cycle, the load current is supplied to the load by the charging of the capacitor. Hence, the peak current required is high.
- As the load current is carried by the components L and C in both the cycles, high ratings are required for the components.
- Since the load current is discontinuous, ripples are present in it.
- Since load current depends upon components L and C, load regulation will be poor.

The above limitations of a series inverter can be overcome by doing some modifications to the circuit. Let us see the circuit and working of a modified series inverter.

## Modified Series Inverter :

In the modified series inverter circuit two inductors L_{1} and L_{2} of same inductances which are closely coupled are used. Due to these modifications, the conduction period of two thyristors T_{1} and T_{2} can be overlapped i.e., thyristor T_{2} can be triggered while T_{1} is ON. The below shows the circuit of the modified series inverter.

When T_{1} is triggered, an equal voltage will be induced in both the inductors L_{1} and L_{2} being closely coupled and capacitor charges. The capacitor voltage will add up to the voltage induced in L_{2} and maintain reverse bias of the thyristor T_{2}.

If suppose T_{2} is triggered during conduction of T_{1}, a reverse voltage will be induced in inductor L_{1}. Due to this reverse voltage, T_{1} gets reverse bais and is turned OFF. Thus in a modified series inverter the output frequency range of the inverter can be increased higher than the oscillating frequency of the RLC circuit.