Class 12 Electronics Chapter 8 Electrical Machines Solutions English Medium As Per AHSEC New Syllabus to each chapter is provided in the list so that you can easily browse through different chapters Class 12 Electronics Chapter 8 Electrical Machines Question Answer and select need one. Class 12 Electronics Chapter 8 Electrical Machines Notes Download PDF. AHSEC Class 12 Elective Electronics Question Answer English Medium.
Class 12 Electronics Chapter 8 Electrical Machines
Also, you can read the NCERT book online in these sections Solutions by Expert Teachers as per Central Board of Secondary Education (CBSE) Book guidelines. AHSEC Class 12 Elective Electronics Textual Solutions are part of All Subject Solutions. Here we have given AHSEC Class 12 Electronics Textbook Solutions English Medium for All Chapters, You can practice these here.
Electrical Machines
Chapter: 8
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A. Multiple choice questions:
1. The electrical device that converts mechanical energy into electrical energy is ______________.
(a) Motor.
(b) Generator.
(c) Starter.
(d) Transformer.
Ans: (b) Generator.
2. The rotating part of a motor is ______________.
(a) Starter.
(b) Carbon brush.
(c) Armature.
(d) Stator.
Ans: (c) Armature.
3. The slip ring is used in ______________.
(a) AC machines.
(b) DC machines.
(c) Transformer.
(d) Stator.
Ans: (a) AC machines.
4. The electrical device that converts electrical energy into mechanical energy is ______________.
(a) Motor.
(b) Generator.
(c) Starter.
(d) Transformer.
Ans: (a) Motor.
5. In which type of motor is the rotating speed of rotor less than the rotating speed of magnetic field?
(a) Synchronous motor.
(b) Asynchronous motor.
(c) DC motor.
(d) Stepper motor.
Ans: (b) Asynchronous motor.
6. Squirrel cage motor is a type of ______________.
(a) Synchronous motor.
(b) Asynchronous motor.
(c) DC motor.
(d) Stepper motor.
Ans: (b) Asynchronous motor.
7. Which of the following motors overcome the drawback of a squirrel cage motor?
(a) Stepper motor.
(b) DC motor.
(c) Slip ring motor.
(d) Series motor.
Ans: (c) Slip ring motor.
8. Which of the following device is used for safe starting of the induction motor?
(a) Starter.
(b) Relay.
(c) Circuit breaker.
(d) Ground fault detector.
Ans: (c) Circuit breaker.
9. Which of the following is a component of direct on line starter?
(a) Relay.
(b) Contactor.
(c) Input supply.
(d) Energy meter.
Ans: (a) Relay.
10. Which of the following is used to connect the wire winding of a motor into star and delta form?
(a) Thread.
(b) Copper wire.
(c) Rubber.
(d) Metal strip.
Ans: (b) Copper wire.
B. Fill in the blanks:
1. A generator transforms ______________ into ______________.
Ans: Mechanical energy, electrical energy.
2. Fleming’s ______________ rule is used for electric motor.
Ans: Left.
3. Fleming’s ______________ rule is used for generator.
Ans: Right.
4. Electromagnet is an ______________ magnet.
Ans: Artificial.
5. AC generator is also known as ______________.
Ans: Alternator.
6. DOL is used to safely start the ______________ motor.
Ans: Induction.
C. State whether the following statements are True or False:
1. Electromotive force is an electrical force, which is exerted on the electrons. This force is responsible for the movement of electrons.
Ans: True.
2. In an AC generator, current is collected by using a slip ring. While in a DC generator, current is collected by a split ring.
Ans: True.
3. Armature is the rotating part of a motor or generator.
Ans: True.
4. Brush is the stationary or fixed part of a generator.
Ans: False.
5. An electric motor is a device that converts electrical energy into mechanical energy.
Ans: True.
6. This rotating force is called torque.
Ans: True.
7. In synchronous motor the rotating speed of the rotor is the same as the rotating speed of the magnetic field.
Ans: True.
8. As per Lorentz law, if a current carrying conductor is placed in a magnetic field it will experience a force.
Ans: False.
9. In slip ring, motor inductor coils are connected in series with the inductive effect of the rotor winding.
Ans: True.
D. Short answer questions:
1. What type of energy is input to an electric generator?
Ans: A generator transforms mechanical energy into electrical energy. Mechanical force is applied on the axle of generator. This mechanical force generated by falling of water, steam or wind, is used to rotate the shaft or axle on which the armature or rotor is formed.
2. What type of energy is output from an electric generator?
Ans: A generator transforms mechanical energy into electrical energy.
3. Give the functions of an alternator.
Ans: AC generator is also known as alternator. It is used to generate alternating current (AC) power by converting mechanical energy into electrical energy.
4. What is brush?
Ans: A brush is a stationary or fixed part of a generator used for collecting current, which is produced due to induced electromagnetic force. Brushes in an AC generator are the sliding contacts that ride against slip rings and are used to connect the armature to the external AC circuit.
5. Discuss the use of a slip ring.
Ans: Slip ring (associated with AC machines) is like a finger ring that maintains contact with carbon brushes. A slip ring can be compared with a vehicle tyre, and how it maintains contact with the road while moving. A slip ring too maintains contact with carbon brushes in the same way. The purpose of a slip ring is to collect power from an AC generator or to give power to an AC motor.
6. Discuss the principle behind the alternator.
Ans: According to Faraday’s Law of Electromagnetic Induction, whenever a conductor moves in a magnetic field, an electromotive force is induced across the conductor. If a closed path is provided to the conductor, induced electromotive force causes current to flow in the circuit. Place the conductor coil, ABCD, in a magnetic field. The direction of the magnetic flux will be from N pole to S pole. The coil is connected to slip rings, and the load is connected through brushes resting on the slip rings.
7. What is the principle behind the motor?
Ans: An electric motor is a device that converts electrical energy into mechanical energy. This conversion is usually obtained through the generation of a magnetic field by means of current flowing into one or more coils.
8. Explain the principle of direct online starter.
Ans: Direct online starter (DOL) is the simplest form of motor starter. DOL is used to safely start the induction motor. DOL consists of a circuit breaker, contactor and an overload relay for protection. In DOL starter, electromagnetic contactors are used. This contactor will open, when thermal relay is overloaded due to fault conditions. Typically, the contactor will be controlled by separate start and stop buttons.
Working Principle of DOL: The internal parts of the DOL starter are shown in Fig. 8.30. DOL starter connects the three-phase main with the motor as shown in Fig. 8.31. The control circuit is connected to any two phases to energise them. When we press the start button, the current flows through the contactor coil (magnetising coil) and control circuit. The current energises the contactor coil and leads to close the contacts. The three-phase supply is then available for the three-phase motor. When we press the stop button, the current through the contact is discontinued, making supply to the motor unavailable and stopping the machine.
9. What are the steps for checking the motor?
Ans: Check an electric motor:
Material required: Electric motor, oil, grease, screw driver.
Procedure:
(i) Visually inspect the motor for the following problems:
• Broken mounting holes or feet.
• Discoloured paint in the middle of the motor indicating excessive heat.
• Evidence of dirt and other foreign matter pulled into the motor windings through openings in the motor body.
(ii) Read the specifications of the motor from the plate affixed on the motor body called the ‘stator’ or ‘frame’.
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