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Class 12 Electronics Chapter 7 AC Drives and Soft Starters
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AC Drives and Soft Starters
Chapter: 7
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A. Multiple choice questions:
1. Which of these is not the role of an electric drive in an electrical system?
(a) Control speed, position, or torque of different loads.
(b) Control position.
(c) Control torque of different loads.
(d) Circuit breaking.
Ans: (d) Circuit breaking.
2. Which of the following is not the control key on the VFD panel?
(a) Start reverse run.
(b) Stop/Reset.
(c) Start forward run.
(d) Forward-reverse run.
Ans: (d) Forward-reverse run.
3. Which of the following is not a function key on the VFD panel?
(a) Enter.
(b) Escape.
(c) Previous.
(d) Numlock.
Ans: (d) Numlock.
4. In bridge rectifier the output will be in the form of ___________.
(a) AC.
(b) DC.
(c) Pulsating DC.
(d) Square waveform.
Ans: (c) Pulsating DC.
5. VFD stands for ______________.
(a) Variety frequency drive.
(b) Variation fault detection.
(c) Variation frequency detection.
(d) Variable frequency drive.
Ans: (d) Variable frequency drive.
6. Which of the following components is not in the VFD?
(a) Power electronic converters.
(b) Filter.
(c) Central control unit.
(d) RAM.
Ans: (d) RAM.
7. Bridge full wave rectifier contains ______________.
(a) One diode.
(b) Two diodes.
(c) Three diodes.
(d) Four diodes.
Ans: (d) Four diodes.
8. The role of a capacitor filter in the rectifier is to ________.
(a) Convert AC to DC.
(b) Remove the noise.
(c) Remove the DC components in the signal.
(d) Smoothen the pulsating DC into pure DC.
Ans: (d) Smoothen the pulsating DC into pure DC.
9. Which of the following is not the control scheme of the VFD?
(a) Scalar control.
(b) Vector control.
(c) Direct torque control.
(d) Indirect torque control.
Ans: (d) Indirect torque control.
10. Which of the following is not the symbolic representation of LED light?
(a) Trip.
(b) Power.
(c) Run.
(d) Stop.
Ans: (d) Stop.
B. Fill in the blanks:
1. When an additional wire is connected across the middle of the secondary winding of a transformer, it is known as a ______________.
Ans: Tapped.
2. The output of a centre tapped full wave rectifier is ______________.
Ans: Pulsating DC.
3. Filter is made up of ______________ and ______________.
Ans: Capacitor, resistor.
4. In VFD central controller, ______________ are used to control the operation.
Ans: Microcontroller or microprocessor.
5. The schemes of VFD control are ___________, ___________ and ____________.
Ans: Scalar, vector, direct torque.
C. State whether the following statements are True or False:
1. By using AC drives, one can change the rotating speed of the motor.
Ans: True.
2. Power modulator converts energy according to the requirement of the motor.
Ans: True.
3. Centre tapped transformer is used in full wave rectifier.
Ans: True.
4. Rectifier converts AC power into DC power with negligible ripples.
Ans: True.
5. In filter, charging time of the capacitor is less compared to the discharging time of the capacitor.
Ans: False.
D. Short answer questions:
1. What is an AC drive?
Ans: In an electrical network, ‘AC’ stands for ‘alternating current’ and drives represent the ‘controlling’ of the circuit or system. Therefore, AC drives are used to control the operating speed of an electrical system specially AC or DC motors. By using AC drives, one can change the rotating speed of the motor torque, or the direction of rotation.
2. List out the areas where AC drives are used.
Ans: It is used in a large number of industrial and domestic applications like transportation systems, rolling mills, paper machines, textile mills, machine tools, fans, pumps, robots and washing machine.
3. What is silicon control rectifier?
Ans: Silicon Controlled Rectifier (SCR): It is a four-layer solid state current controlling device. It is used to control the power flow in electrical circuits.
4. What is the role of a soft starter in the electrical system?
Ans: In technical terms, a soft starter is any device that reduces the torque applied to an electric motor. It generally consists of solid state devices such as thyristors to control the application of power to the motor.
5. What are the advantages of using AC drives in the electrical system?
Ans: AC drives can minimise power supply requirement, as the input power to the AC motor is properly utilised to generate maximum power through it. By using AC drives, electric energy can be saved.
6. How is variable frequency control used to adjust the frequency of the voltage supply?
Ans: Though there are different types of VFDs (or AC drives), all of them work on same principle, which is converting fixed incoming voltage and frequency into variable voltage and frequency output. The frequency of the drive determines how fast the motor can run. The combination of voltage and frequency decides the amount of torque that the motor can generate.
7. What is rectification? Explain the centre tapped full wave rectifier.
Ans: The process of converting AC power into DC power is called rectification. It can be achieved by using a single diode or a group of diodes.
Definition of Full Wave Rectifier: A full wave rectifier converts both half cycles of the AC signal into a pulsating DC signal.
Centre Tapped Full Wave Rectifier: During positive half cycle the upper diode (D1) conducts and the lower diode (D2) remains reverse biased, hence current flows through the load resistor (RL). During the negative half cycle the lower diode (D2) conducts and the upper diode (D1) remains reverse biased, hence current again flows through the load resistor (RL) in the same direction. Both half cycles of the input AC are thus converted to DC output.
8. What do you understand by the term centre tapped?
Ans: It is the middle point of the secondary winding of the transformer which is connected to the ground or reference point. It provides equal voltages across the two halves of the winding but opposite in polarity.
9. List out the keys that are used in a VFD panel.
Ans: LED Panel of VFD: The indicator and operation of an LED panel are described below to set parameters and monitor the VFD.
Keys used: Run indicator, Reset key, ESC key, Data display (5 digit), LED Indicator, Enter key, Down key.
10. What are the control schemes of the VFD?
Ans: Control Schemes of VFD: There are different speed control techniques implemented for variable frequency drives.
The major classification of control techniques used in modern VFDs is given below:
(i) Scalar Control.
(ii) Vector Control.
(iii) Direct Torque Control.
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