Power Electronics and Drives MCQ Quiz - Objective Question with Answer for Power Electronics and Drives - Download Free PDF

Explanation:

Thyristor or SCR (Silicon Controlled Rectifier) Behavior After Gate Pulse Removal

Definition: A thyristor, commonly known as an SCR (Silicon Controlled Rectifier), is a four-layered semiconductor device used in power electronics to control high-power applications. It operates as a bistable device, meaning it can switch between ON and OFF states based on the triggering of its gate terminal and the conditions of its anode current.

Working Principle: The SCR remains in its OFF state (blocking mode) until a gate pulse is applied. Once a sufficient gate pulse is provided to trigger the SCR, it enters the ON state (conducting mode). In the conducting mode, the SCR continues to conduct current even after the gate pulse is removed, as long as the anode current remains above a certain threshold, known as the latching current. If the anode current drops below a critical level called the holding current, the SCR turns OFF.

Correct Option Analysis:

The correct option is:

Option 1: Drops to zero

When the gate pulse is removed after firing the SCR, the current through the SCR does not immediately drop to zero. The SCR remains in the conducting state as long as the current flowing through it is above the holding current. However, if the anode current drops below the holding current level (e.g., due to circuit conditions such as a reduction in the load current or the AC supply reaching its zero-crossing point), the SCR will turn OFF, and its current will drop to zero. Hence, the option "Drops to zero" is correct but must be understood in the context of the current falling below the holding current.

Important Concepts Supporting This Behavior:

• Holding Current: This is the minimum current that must flow through the SCR to keep it in the ON state. If the current falls below this value, the SCR will turn OFF.
• Latching Current: This is the minimum current required to latch the SCR into the ON state immediately after it is triggered by the gate pulse. Once the SCR is latched, the gate pulse is no longer needed to sustain conduction.
• Gate Pulse Role: The gate pulse is only required to trigger the SCR into conduction. After firing, the gate signal has no effect on the SCR's conduction state.
• AC Circuit Operation: In AC circuits, the SCR naturally turns OFF at the zero-crossing point of the AC waveform because the current falls to zero, which is below the holding current.

Additional Information

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

Option 2: Increases

This option is incorrect. After the gate pulse is removed, the SCR's current does not increase. The current through the SCR is determined by the external circuit parameters, such as the load and supply voltage. The gate pulse only serves to trigger the SCR into conduction, and once the SCR is ON, the gate pulse has no impact on the current. An increase in current would require a change in the circuit's external conditions, not the removal of the gate pulse.

Option 3: Decreases

This option is partially correct but not entirely accurate in all scenarios. While the current through the SCR may decrease due to external circuit conditions (e.g., reduced load or the supply reaching a lower voltage), the mere removal of the gate pulse does not cause the current to decrease. The SCR remains in conduction as long as the anode current stays above the holding current. If the anode current falls below the holding current, the SCR turns OFF, and its current drops to zero rather than just decreasing.

Option 4: Remains the same

This option is incorrect in the long term. While the current through the SCR may remain the same immediately after the gate pulse is removed, it is not guaranteed to stay constant indefinitely. The current in the SCR depends on the external circuit conditions. In an AC circuit, for example, the SCR's current will naturally fall to zero during the AC waveform's zero-crossing point, causing the SCR to turn OFF.

Conclusion:

The behavior of an SCR after the gate pulse is removed is primarily governed by the anode current and its relationship to the holding current. Once the gate pulse has triggered the SCR into conduction, the gate signal has no further effect. The SCR will remain in the ON state as long as the anode current remains above the holding current. If the current drops below the holding current (e.g., due to external circuit conditions), the SCR turns OFF, and its current drops to zero. Therefore, the correct answer is "Drops to zero."

The correct answer is: 2) Forced commutation

Explanation:

In a chopper circuit, the thyristor (SCR) is used to control the output voltage by rapidly switching ON and OFF. However, unlike transistors or MOSFETs, thyristors cannot turn OFF on their own once triggered; they must be commutated (i.e., brought back to the OFF state).

• In DC circuits like choppers, there is no natural zero crossing of current (as in AC circuits) to help turn OFF the thyristor.

• Therefore, natural commutation (which relies on the current going to zero naturally) cannot be used.

• Instead, a forced commutation technique is used, where an auxiliary circuit forces the current through the thyristor to zero, turning it OFF.

The correct answer is: 4) All of the above

Explanation:

DC/DC converter topologies are used to convert one DC voltage level to another. The major types include:

1. Buck Converter

   • Steps down the voltage

   • The output voltage is less than the input voltage

2. Boost Converter

   • Steps up the voltage

   • The output voltage is greater than input voltage

3. Flyback Converter

   • An isolated converter

   • Can step up or step down voltage

   • Often used in low-power applications and provides galvanic isolation using a transformer

These are all commonly used topologies in DC/DC converter designs.

Hence, Option 4: All of the above is correct.

Explanation:

Comparison of Linear Power Supplies and SMPS

Definition: A Switch Mode Power Supply (SMPS) is a type of power supply that uses high-frequency switching to convert electrical power efficiently. It is widely used in modern electronic devices due to its compact size, lightweight nature, and high efficiency. On the other hand, a Linear Power Supply uses a transformer to step down the input voltage and regulate the output voltage using linear regulation, which results in a less efficient energy conversion process.

The statement in the CSV provided highlights that compared to Linear Power Supplies, SMPS offers higher efficiency. Let us analyze why this is the correct option and further evaluate the other given options.

Advantages of SMPS over Linear Power Supplies:

• Higher Efficiency: SMPS achieves higher efficiency by minimizing energy loss during power conversion. Unlike linear power supplies, where excess energy is dissipated as heat, SMPS uses switching techniques to reduce power wastage.
• Compact Size and Lightweight: Due to the high operating frequency, the size of transformers and other components is significantly reduced in SMPS.
• Wide Input Voltage Range: SMPS can handle a wide range of input voltages, making it suitable for various applications, including international standards.
• Better Thermal Management: The reduced energy dissipation in SMPS results in less heat generation, leading to better thermal performance.

Hence the correct option is 1

Electrical Drives

Based on their assembly, electric drives can be of the following three types.

Group Drive:

• A group drive system uses a high-powered motor to drive a common shaft. When several machines are organized on a single shaft and are driven by a single large motor, this system is known as a group drive.
• A key characteristic of a group drive system is its lower capital cost compared to other drives. This is because a single, larger motor and associated control gear can power multiple machines connected to a common shaft, rather than requiring a separate motor for each machine. 
• Examples: Food grinding mills, paper mills, etc.
   

Multi-motor Drive:

• A multi-motor drive system uses multiple electric motors to power different parts of a machine, each motor operating its own mechanism.
• This allows for independent control and operation of various machine functions, as seen in traveling cranes where separate motors handle hoisting, long travel, and cross-travel. Example: Cranes
   

Individual Drive:

• In an electric drive system, if an individual machine is fitted with its motor and each operator has complete control over their machine, then it is termed as an individual electric drive.
• Examples: Drill machine, lath machine, etc.

In majority carrier devices conduction is only because of majority carriers whereas in minority carrier devices conduction is due to both majority and minority carriers.

1. MOSFET is a majority carrier device.

2. Diode is both majority and minority carrier device.

3. Thyristor is minority carrier device

Concept:

Ripple frequency at the output = m × supply frequency

fo = m × fs

Where m = types of the pulse converter

Calculation:

A three-phase full-wave AC to DC converter is a 6-pulse converter

Number of pulses (m) = 6

fo = 6 × supply voltage frequency

∴ f₀ = 6 x 50

f₀ = 300 Hz

Formula:

$V_o=V_{in}(\frac{T}{T-T_{ON}})$     ---(1)

Where, Vo is the output voltage

V_in is the input voltage

T_ON is the pulse width

Application:

Given,

V_in = 200 volts

T_ON = 200 µs

V₀ = 600 V

From equation (1),

 $\frac{V_o}{V_{in}}=(\frac{T}{T-T_{ON}})$

or, $3=\frac{T}{T-200}$

or, 3T - 600 = T

Hence, T = 300 µs

If the Pulse width is half then, the new value of pulse width (T_ON') will be,

$T_{O{N}^{\prime }}=\frac{T_{ON}}{2}=\frac{200}{2}=100\mu s$

Hence,

Hence, the new value of output voltage (V_0') will be,

A chopper is a static device which is used to obtain a variable dc voltage from a constant dc voltage source. Also known as a dc-to-dc converter.

They can step up the DC voltage or step down the DC voltage levels.

Types of Choppers:

• Type A Chopper or First-Quadrant Chopper
• Type B Chopper or Second-Quadrant Chopper
• Type C Chopper or Two-quadrant type-A Chopper
• Type D Chopper or Two-quadrant type-B Chopper
• Type E Chopper or fourth-quadrant Chopper

Note:

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.

Construction:

• The SCR is a four-layer and three-terminal device.
• The four layers made of P and N layers are arranged alternately such that they form three junctions J₁, J_2, and J₃.
• These junctions are either alloyed or diffused based on the type of construction.

Doping level:

• The level of doping varies between the different layers of the thyristor.
• Out of these four layers, the first layer (P1 or P+) and Last layer (N2 or N+) are heavily doped layers.
• The second layer (N1 or N-) is a lightly doped layer and the third layer (P2 or P+) is a moderately doped layer.
• The junction J1 is formed by the P+ layer and N- layer.
• Junction J2 is formed by the N- layer and P+ layer
• Junction J3 is formed by P+ layer and N+ layer.
• Thinner layers would mean that the device would break down at lower voltages.

Concept:

Center tapped full wave rectifier:

• The Center tapped full-wave rectifier is a device used to convert the AC input voltage into DC voltage at the output terminals.
• It employs a transformer with the secondary winding tapped at the center point. And it uses only two diodes, which are connected to the opposite ends of a center-tapped transformer as shown in the figure below.
• The center tap is usually considered as the ground point or the zero voltage reference point.

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Analysis:

The DC output voltage or average output voltage can be calculated as follows,

${\mathrm{V}}_0={\mathrm{V}}_{\mathrm{d}\mathrm{c}}=\frac{1}{\pi }\underset{0}{\overset{\pi }{\int }}{\mathrm{V}}_{\mathrm{m}}sin\omega td\omega t$

$=\frac{V_m}{\pi }\left(-\cos \omega t\right)|\begin{array}{c}\pi \\ 0\end{array}$

$=\frac{V_m}{\pi }\left(-cos\pi -\left(-cos{0}^{\circ }\right)\right)$

$=\frac{V_m}{\pi }\left(-\left(-1\right)+1\right)$

V₀ = 2V_m / π 

Now we can calculate the average or mean current of load by dividing the average load voltage by load resistance R_L. Therefore mean load current is given by

I₀ = V₀ / R_L

If the internal resistance of the diode is given in that case mean load current I₀ = V₀ / (R_L + r)

Where r = internal resistance of the diode.

Calculation:

Given that 

Rms value of supply voltage V = 50 V

The internal resistance of diode r = 20 Ω 

The load resistance R_L = 980 Ω 

Maximum voltage on the secondary side V_m = √2 V = √2 × 50 = 70.7 V

Average or DC output voltage V₀ = $\frac{(2\times 70.7)}{\pi }=45V$

Average or mean load current is

I0 = V0 / (RL + r) = $\frac{45}{(980+20)}$ = 45 mA​

Full wave rectifier

Case 1: During +ve half

D_o ON and D₁ OFF

V_o = V_s

Case 2: During -ve half

Do OFF and D1 ON

Vo = -Vs

The output waveform is:

The rectification efficiency is the ratio of the DC output power to the AC input power. 

$V_{o(avg)}=\frac{2V_m}{\pi }$ and $I_{o(avg)}=\frac{2V_m}{\pi R}$

$V_{o(rms)}=\frac{V_m}{\sqrt{2}}$ and $I_{o(rms)}=\frac{V_m}{\sqrt{2}R}$

% η = $\frac{V_{o(avg)}\times I_{o(avg)}}{V_{o(rms)\times I_{o(rms)}}}$

% η = $\frac{\frac{2V_m}{\pi }\times \frac{2V_m}{\pi R}}{\frac{V_m}{\sqrt{2}}\times \frac{V_m}{\sqrt{2}R}}$

% η = 81.2%

Mistake Points The rectification efficiency of half wave rectifier is 40.6%

Concept:

In a three-phase semi-converter,

The conduction period of each thyristor = π – α

The conduction period of freewheeling diode = β – 60°

Where α is the firing angle

β is the extinction angle

Calculation:

Given that, firing angle (α) = 120°

Extinction angle (β) = 110°

The conduction period of each thyristor = π – α = 180 – 120 = 60°

The conduction period of freewheeling diode = β – 60° = 110 – 60 = 50°

Key Points

Holding Current: It is the minimum anode current to maintain the thyristor in the on-state. 

Latching current is always greater than holding current.

Additional Information The thyristor or SCR is a power semiconductor device which is used in power electronic circuits.

They work like a bistable switch and it operates from nonconducting to conducting.

The designing of thyristors can be done with 3-PN junctions and 4 layers.

It includes three terminals namely anode, gate, and cathode.