Inverters MCQ Quiz - Objective Question with Answer for Inverters - Download Free PDF

Explanation:

Single-Phase Full Bridge Inverter:

Definition: A single-phase full bridge inverter is an electronic device that converts direct current (DC) into alternating current (AC). It consists of four switches that control the direction of the current flow through the load, allowing the output voltage to alternate its polarity. The switches are typically implemented using transistors or thyristors.

Working Principle: The operation of a full bridge inverter involves turning on pairs of switches in a specific sequence to generate a square wave AC output. The switches are arranged in an H-bridge configuration, where each leg of the bridge has two switches. By controlling which switches are on and off, the inverter can produce positive, negative, or zero voltage across the load.

Switching States: The four switches in the inverter are denoted as S1, S2, S3, and S4. The output voltage across the load depends on the combination of switches that are turned on. The possible states are:

• S1 and S2 ON: Positive voltage (+V_dc) across the load.
• S3 and S4 ON: Negative voltage (-V_dc) across the load.
• S1 and S3 ON: Zero voltage (0 V) across the load (both ends connected to +V_dc).
• S2 and S4 ON: Zero voltage (0 V) across the load (both ends connected to ground).

Correct Option Analysis:

The correct option is:

Option 4: -V_dc

When switches S2 and S3 are turned on, the output voltage across the load is determined by the connections of these switches. Here's the detailed analysis:

• S2 ON: This switch connects the positive terminal of the DC supply to the left side of the load.
• S3 ON: This switch connects the right side of the load to the negative terminal of the DC supply.

With S2 and S3 turned on, the current flows from the positive terminal of the DC supply through S2, then through the load from left to right, and finally through S3 to the negative terminal of the DC supply. This results in a negative voltage (-V_dc) across the load, as the positive terminal of the load is connected to the negative terminal of the supply and vice versa.

Conclusion:

The correct output voltage when switches S2 and S3 are turned on is -V_dc, making option 4 the correct answer.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: -2V_dc

This option is incorrect because the output voltage of a single-phase full bridge inverter is either +V_dc, -V_dc, or 0 V. The inverter cannot produce an output voltage of -2V_dc as it is beyond the voltage range of the DC supply.

Option 2: V_dc

This option is incorrect in the context of switches S2 and S3 being turned on. The output voltage V_dc is produced when switches S1 and S2 are turned on, allowing current to flow from the positive terminal of the supply through the load to the negative terminal.

Option 3: 2V_dc

This option is incorrect because, similar to option 1, the inverter cannot produce an output voltage of 2V_dc. The maximum output voltage is limited to the magnitude of the DC supply voltage.

Option 4: -V_dc

This option is correct, as explained in the detailed analysis above. When switches S2 and S3 are turned on, the output voltage across the load is -V_dc.

Conclusion:

Understanding the switching states and the resulting output voltages of a single-phase full bridge inverter is essential for analyzing its operation. The correct output voltage when switches S2 and S3 are turned on is -V_dc, as this configuration connects the positive terminal of the load to the negative terminal of the DC supply and vice versa.

In a single-phase half wave inverter, only one Silicon Controlled Rectifier (SCR) is gated at a time. This is because a half wave inverter typically uses a single SCR to control the output waveform. When the SCR is triggered, it allows current to flow during the positive half cycle of the AC input, producing a half-wave rectified output.

Line-commutated inverters are devices that convert DC power into AC power using the AC supply line for commutation. This type of inverter relies on the AC supply for turning off the thyristors used in the circuit. Therefore, it has AC on both the supply side and the load side.

A single phase half bridge inverter requires a three-wire DC supply. This is because the half-bridge inverter configuration involves two power switches and splits the DC supply into two equal parts using a center-tap or a neutral wire, creating the three-wire system.

The output of a single-phase half bridge inverter on an R load is ideally a square wave. This is because the inverter switches the DC input voltage between two levels, resulting in a square wave output.

A voltage source inverter (VSI) is employed when source inductance is small and load inductance is large because higher value of source inductance will increase the overlap angle and cause commutation issues.

Important Points:

Voltage Source Inverter	Current Source Inverter
It is fed from a DC voltage source having small impedance	It is fed with adjustable current from a DC voltage source of high impedance
Input voltage is maintained constant	Input current is constant but adjustable
Output voltage does not dependent on the load	The amplitude of output current is independent of the load
The waveform of the load current as well as its magnitude depends upon the nature of load impedance	The magnitude of output voltage and its waveform depends upon the nature of the load impedance
It requires feedback diodes	It does not require any feedback diodes
The commutation circuit is complicated	Commutation circuit is simple as it contains only capacitors.
Power BJT, Power MOSFET, IGBT, GTO with self-commutation can be used in the circuit.	They cannot be used as these devices have to withstand reverse voltage.

The correct answer is option 4.

Switched Mode Power Supply (SMPS)

• A switched-mode power supply (SMPS) is an electronic circuit that converts power using switching devices that are turned on and off at high frequencies, and storage components such as inductors or capacitors to supply power when the switching device is in its non-conduction state.
• Switching power supplies have high efficiency and are widely used in a variety of electronic equipment, including computers and other sensitive equipment requiring stable and efficient power supply.
• SMPS takes AC mains input and provides DC output (3.3V to 12V).
• The input DC supply from a rectifier or battery is fed to the inverter where it is turned on and off at high frequencies of between 20 KHz and 200 KHz by switching MOSFET or power transistors.
• The high-frequency voltage pulses from the inverter are fed to the transformer's primary winding, and the secondary AC output is rectified and smoothed to produce the required DC voltages.
• A feedback circuit monitors the output voltage and instructs the control circuit to adjust the duty cycle to maintain the output at the desired level.

Power electronic circuits can be classified as follows.

1. Diode rectifiers:

• A diode rectifier circuit converts ac input voltage into a fixed dc voltage.
• The input voltage may be single phase or three phase.
• They find use in electric traction, battery charging, electroplating, electrochemical processing, power supplies, welding and UPS systems.

 

2. AC to DC converters (Phase controlled rectifiers):

• These convert ac voltage to variable dc output voltage.
• They may be fed from single phase or three phase.
• These are used in dc drives, metallurgical and chemical industries, excitation systems for synchronous machines.

 

3. DC to DC converters (DC Choppers):

• A dc chopper converts dc input voltage to a controllable dc output voltage.
• For lower power circuits, thyristors are replaced by power transistors.
• Choppers find wide applications in dc drives, subway cars, trolley trucks, battery driven vehicles, etc.

 

4. DC to AC converters (Inverters):

• An inverter converts fixed dc voltage to a variable ac voltage. The output may be a variable voltage and variable frequency.
• In inverter circuits, we would like the inverter output to be sinusoidal with magnitude and frequency controllable. In order to produce a sinusoidal output voltage waveform at a desired frequency, a sinusoidal control signal at the desired frequency is compared with a triangular waveform.
• These find wide use in induction motor and synchronous motor drives, induction heating, UPS, HVDC transmission etc.

 

5. AC to AC converters: These convert fixed ac input voltage into variable ac output voltage. These are two types as given below.

i. AC voltage controllers:

• These converter circuits convert fixed ac voltage directly to a variable ac voltage at the same frequency.
• These are widely used for lighting control, speed control of fans, pumps, etc.

 

ii. Cycloconverters:

• These circuits convert input power at one frequency to output power at a different frequency through a one stage conversion.
• These are primarily used for slow speed large ac drives like rotary kiln etc.

 

6. Static switches:

• The power semiconductor devices can operate as static switches or contactors.
• Depending upon the input supply, the static switches are called ac static switches or dc static switches.

Pulse Width Modulation control (PWM):

In this method, a fixed dc input voltage is given to the inverter and a controlled ac output voltage is obtained by adjusting the on and off periods of the inverter components.

Advantages:

• The output voltage control with this method can be obtained without any additional components.
• Lower order harmonics can be eliminated or minimized along with its output voltage control. As higher-order harmonics can be filtered easily, the filtering requirements are minimized.

 

Disadvantage:

The SCRs are expensive as they must possess low turn-on and turn-off times.

Key differences between Linear Power Supply and Switch Mode Power Supply:

1. The main difference between the linear power supply and SMPS is that linear power supply converts the high voltage of AC into low voltage AC first then the rectification procedure takes place.

On the contrary, the SMPS converts the AC signal into DC signal first then the stepping down of voltage signal takes place.

2. The linear power supply uses a voltage regulator for voltage regulation of the output voltage while SMPS uses a feedback circuit for voltage regulation.
3. Power dissipation also plays a key role in differentiating linear power supply and SMPS.

The linear power supply also dissipates power and thus requires a heat sink, but SMPS does not require heat sink as there is no power dissipation.

4. The step-down transformer used in linear supply is bulky while in SMPS the step-down transformer is light in weight.
5. The noise disturbance is more in SMPS due to switching action; this makes SMPS inappropriate for audio and radiofrequency applications.

The linear power supply is immune to noise disturbance and thus used in audio and radiofrequency applications.

6. There is a major difference between the efficiency of the linear power supply and SMPS.

The efficiency of the linear power supply is low about 20-25% due to ohmic losses while that of SMPS is high, i.e. about 65-75%.

PARAMETERS	LINEAR POWER SUPPLY	SWITCH MODE POWER SUPPLY (SMPS)
Definition	It completes the stepping down of AC voltage first then it converts it into DC.	It converts the AC input signal into DC first then it steps down the voltage up to the desired level.
Efficiency	Low efficiency i.e. about 20-25%	High Efficiency i.e. about 60-65%
Voltage Regulation	Voltage regulation is done by voltage regulator.	Voltage regulation is done by the feedback circuit.
Magnetic material used	CRGO core is used	The ferrite core is used
Weight	It is bulky.	It is less bulky in comparison to the linear power supply.
Reliability	More reliable in comparison to SMPS.	Its reliability depends on the transistors used for switching
Complexity	Less complex than SMPS.	More complex than the Linear power supply.
Transient response	It possess faster response.	It possesses a slower response.
RF interference	No RF interference	RF shielding is required as switching produces more RF interference.
Noise and Electromagnetic interference	It is immune to noise and electromagnetic interference.	Effect of noise and electromagnetic interference is quite significant, thus EMI filters are required.
Applications	Used in Audio frequency applications and RF applications.	Used in chargers of mobile phones, DC motors, etc.

Single-pulse modulation:

• It consists of a pulse width of 2d located symmetrically about π/2 and another pulse located at symmetrically about 3π/2. The range of pulse width varies from 0 to π.
• The output voltage is controlled by varying pulse width.
• To eliminate nth harmonic, ‘nd’ should be made equal to π.
• This method of voltage control, a great deal of harmonic content is introduced in the output voltage, particularly at low output voltage levels.

 

The output voltage waveform of a single-phase full-bridge inverter is represented as below.

\({v_0} = \mathop \sum \limits_{n = 1,3,5}^\infty \frac{{4{V_s}}}{{n\pi }}\sin \frac{{n\pi }}{2}\sin nd\sin n\omega t\)

\({v_0} = \frac{{4{V_s}}}{\pi }\left[ {\sin d\sin \omega t - \frac{1}{3}\sin 3d\sin 3\omega t + \frac{1}{5}\sin 5d\sin 5\omega t} \right]\)

If nd is equal to π or \(d = \frac{\pi }{n}\) or if pulse width is made equal to \(2d = \frac{{2\pi }}{n}\), it shows that nth harmonic is eliminated from the inverter output voltage.

Calculation:

Given that, harmonic component to eliminate is fifth harmonic.

n = 5

Pulse width \( = \frac{{2 \times 180}}{5} = 72^\circ \)

Total harmonic distortion

The presence of AC harmonic components in DC output is known as total harmonic distortion.

The output current is the combination of fundamental (DC) + AC harmonics components.

\(I_{or}=I_{or1}+I_{or2}+I_{or3}........\)

\(I_{or}-I_{or1}=I_{or2}+I_{or3}........\)

where, I_or1 = RMS value of the fundamental component

\(I_{or2}+I_{or3}........\)= RMS value of net harmonic current

\(THD=\sqrt{({1\over CDF})^2-1}\)

\(THD=\sqrt{({I_{or}\over I_{or1}})^2-1}\)

\(THD=\sqrt{I_{or}^2-I_{or1}^2\over I_{or1}^2}\)

\(THD=\sqrt{({I_{or2}+I_{or3}........\over I_{or1}})^2}\)

\(THD={I_{or2}+I_{or3}........\over I_{or1}}\)

\({I_{or2}+I_{or3}........}=THD\times I_{or1}\)

\({I_{or2}+I_{or3}........}= 0.04 \times \frac{4}{{\sqrt 2 }} = 0.08\sqrt 2 \;A\)

1ϕ half-bridge inverter:

Case 1: 0 < t < T/2:

Thyristor T₁ conducts.

 \(V_o = {{V} \over 2}\)

Case 2:  T/2 < t < T:

Thyristor T₂ conducts.

 \(V_o = -{{V} \over 2}\)

The RMS output voltage is given by:

\((V_o)_{RMS} = {V \over 2}\)

Given, V = 100

\((V_o)_{RMS} = {100 \over 2}\)

\((V_o)_{RMS} = 50\space V\)

Single-phase full-bridge inverter:

The circuit diagram of a single-phase full-bridge inverter is shown below.

Response (or) Output waveforms for various types of load:

RLC Underdamped: ζ < 1

Where \({\rm{\zeta }} = \frac{R}{2}\sqrt {\frac{C}{L}}\) for RLC circuit

To turn OFF the switches S₁, S₂ at T/2, the anode current should come to zero. Due to this type of load, it is possible as the current becomes zero before the voltage becomes zero.

Therefore, the forced commutation is not required for this kind of load to make the anode current zero

In this case, a leading current will flow in the circuit and it will become zero. So, the thyristor will be load commutated.

Load commutation occurs due to an underdamped response.

RLC Overdamped: ζ > 1

Where \({\rm{\zeta }} = \frac{R}{2}\sqrt {\frac{C}{L}}\) for RLC circuit

To turn OFF the switches S₁, S₂ at T/2, the anode current should come to zero. Due to this type of load, it is possible as the current becomes zero after the voltage becomes zero.

Therefore, the forced commutation is required for this kind of load to make the anode current zero.

The load commutation will not occur due to an overdamped system. So, forced commutation is required.

• A voltage source inverter (VSI) is employed when source inductance is small and load inductance is large because higher value of source inductance will increase the overlap angle and cause commutation issues.
• IGBT is a bipolar device, it is a three-terminal device (emitter collector and gate). MOSFET is a unipolar device.
• A voltage source inverter (VSI) is fed by a stiff DC voltage, whereas a current source inverter is fed by a stiff current source. A voltage source inverter can be converted to a current source inverter by connecting a series inductance and then varying the voltage to obtain the desired current.

Important Points:

Voltage Source Inverter	Current Source Inverter
It is fed from a DC voltage source having small impedance	It is fed with adjustable current from a DC voltage source of high impedance
Input voltage is maintained constant	Input current is constant but adjustable
Output voltage does not dependent on the load	The amplitude of output current is independent of the load
The waveform of the load current as well as its magnitude depends upon the nature of load impedance	The magnitude of output voltage and its waveform depends upon the nature of the load impedance
It requires feedback diodes	It does not require any feedback diodes
The commutation circuit is complicated	Commutation circuit is simple as it contains only capacitors.
Power BJT, Power MOSFET, IGBT, GTO with self-commutation can be used in the circuit.	They cannot be used as these devices have to withstand reverse voltage.