Q. part of an induction motor? Q.What is the example of operation of a synchronous generator.

Explain
Ans: i)Stator: The stationary part containing the core and windings where Ans: An example of the operation of a synchronous generator is in a power Capacitor Start Motor: A type of single-phase induction motor that
the magnetic field is generated. plant, where it converts mechanical energy from a turbine (driven by steam, uses a capacitor in series with the starting winding to create a phase
ii)Rotor: The rotating part, either a squirrel cage or wound type, which water, or gas) into electrical energy. The generator's rotor rotates at a speed shift, improving the starting torque and efficiency.
interacts with the magnetic field to produce torque. synchronized with the frequency of the alternating current (e.g., 50 Hz or 60 Synchronous Impedance Method: A method used to calculate the
iii)Shaft: Connects to the rotor and transmits mechanical power to the load. Hz) to maintain steady power output to the grid. synchronous reactance and impedance of an alternator by using the
iv)Bearings: Support the shaft and allow smooth rotation. open-circuit and short-circuit tests to determine the machine's voltage,
Q.Describe the principle of operation of an alternator. Q.Explain the concept of synchronizing a synchronous generator with the current, and power characteristics.
Ans: An alternator operates on the principle of electromagnetic induction. Gird. AC Servo Motor: A type of electric motor used for precise control of
When a conductor (typically coils of wire) rotates within a magnetic field, Ans: Synchronizing a synchronous generator with the grid involves matching angular position, velocity, and acceleration, typically used in
or a magnetic field rotates around a stationary conductor, it induces an its voltage, frequency, and phase angle with those of the grid before applications requiring high performance like robotics and CNC
alternating current (AC) in the conductor. connecting it. This ensures smooth integration, prevents power surges, and machinery.
This occurs because the relative motion between the magnetic field and allows stable operation without damaging the generator or grid. No Load Test on an Induction Motor: A test performed to determine
the conductor causes a change in magnetic flux, generating an the core losses, friction, and windage losses of an induction motor,
electromotive force (EMF) as per Faraday’s Law of Electromagnetic Q.What are the advantage and disadvantage of using a wound rotor where the motor is run at its rated voltage and frequency with no load
Induction. In an alternator, mechanical energy (usually from a turbine or induction motor. attached.
engine) drives the rotation of the rotor, creating the relative motion Ans: Advantages of Wound Rotor Induction Motor Synchronizing of Alternator: The process of matching the frequency,
i)Speed Control: Allows smooth and efficient speed control by varying voltage, and phase of an alternator to the grid or another alternator
necessary to produce electricity . external resistance in the rotor circuit. before connecting them in parallel to avoid damage and ensure stable
ii)High Starting Torque: Achieves high starting torque by inserting external operation.
Q.The methods of starting a synchronous motor are resistance.
i)Using an auxiliary motor - An external motor (e.g., an induction motor) iii)Reduced Starting Current: Lower starting current compared to squirrel Q.Difference between salient pole and cylindrical pole alternator.
brings the synchronous motor up to near-synchronous speed before cage motors. Ans: A salient pole alternator has a rotor with poles that are
connecting it to the supply. Disadvantages of Wound Rotor Induction Motor protruding (salient), typically used in low-speed applications like
ii)Using damper windings - The motor starts as an induction motor using i)Cost: More expensive due to the slip rings, brushes, and external hydropower plants. The rotor is usually made of laminated iron cores
damper bars in the rotor until it rea ches synchronous speed. resistances. with pole pieces attached to it.
iii)Using a variable-frequency drive (VFD) - Gradually increases the supply ii)Maintenance: Requires more maintenance due to brushes and slip rings. A cylindrical pole alternator (also called a non-salient or round rotor
frequency to bring the motor to synchronous speed. iii)Efficiency: Less efficient compared to squirrel cage motors due to alternator) has a smooth, cylindrical rotor, typically used in high-speed
additional losses in the rotor resistances. applications like thermal power plants. It provides a uniform magnetic
Q.Describe the performance of a synchronous motor with change in field and is made of a solid steel rotor without distinct pole projections.
excitation with the help of vector diagrams. Q. Explain the concept of a brushless DC motor and it is advantage.
Ans:Step 1: Understanding Synchronous Motor Operation Ans: A brushless DC (BLDC) motor is an electric motor that uses electronic Q.Explain the working principle of a linear induction motor. state the
A synchronous motor operates by converting electrical energy into commutation instead of mechanical brushes to switch the current in the types in their application.
mechanical energy through the interaction of magnetic fields. The motor windings. The rotor consists of permanent magnets, while the stator has Ans: A Linear Induction Motor (LIM) works on the same principle as a
has a stator and a rotor, with the rotor being excited by a DC current. windings. A controller regulates the current to the stator windings based on standard rotary induction motor, but instead of producing rotational
Step 2: Vector Diagram for Synchronous Motor the rotor position, ensuring efficient operation. motion, it generates linear (straight-line) motion. The stator produces a
The vector diagram for a synchronous motor shows the relationship Advantages of brushless DC Motors: traveling magnetic field, which induces a current in the rotor (or
between the stator flux (φs), rotor flux (φr), and the armature current (Ia). i)Higher Efficiency: Reduced energy losses due to the absence of brushes. secondary) of the motor, creating a force that moves the rotor along a
Step 3: Effect of Changing Excitation ii)Low Maintenance: No brushes to wear out, reducing maintenance needs. linear path.
When the excitation (DC current) is changed, the rotor flux (φr) changes. iii)Longer Lifespan: Durable due to minimal mechanical wear. Types and Applications:
This, in turn, affects the armature current (Ia) and the power factor (PF) of iv)Quiet Operation: Reduced noise compared to brushed motors. i)Single-sided LIM: The stator is mounted on one side of the track, and
the motor. the secondary is on the other. Used in applications like conveyors and
Step 4: Vector Diagrams for Different Excitation Levels Q.What is the principal of operation of a switched reluctance motor? transportation systems.
Here are three vector diagrams illustrating the effect of changing excitation Ans: The principal of operation of a switched reluctance motor (SRM) is ii)Double-sided LIM: The stator is mounted on both sides of the track.
on the synchronous motor: based on the tendency of its rotor to align with the stator poles that produce It's used in high-speed trains (e.g., maglev trains) and other
the minimum reluctance path for the magnetic flux. By sequentially transportation systems requiring high efficiency.
Q.Differentiate between silent pole and cylindrical Rotor machine. energizing the stator windings in a specific order, the magnetic attraction iii) Flat LIM: Used for flat, horizontal motion in applications like
Ans: Salient Pole Rotor Machine between the rotor and stator creates torque, causing the rotor to rotate. material handling or in certain transport systems.
1. Rotor construction: The rotor has projecting poles, which are typically
made of steel laminations. Q. why is the single phase induction motor not self- starting?Mention the
2. Magnetic field: The magnetic field is non-uniform, with a higher flux method used to make it self starting?
density at the poles. Ans: A single-phase induction motor is not self-starting because:
3. Torque production: Torque is produced due to the interaction between - It has only one phase winding, which produces a stationary magnetic field.
the stator field and the rotor poles. - This stationary field does not produce any rotating torque, making it
4. Speed: Salient pole machines typically operate at lower speeds. impossible for the motor to start on its own.
Cylindrical Rotor Machine External means, such as a capacitor or an auxiliary winding, are needed to
1. Rotor construction: The rotor is cylindrical in shape, with a smooth create a rotating magnetic field to start the motor.
surface. To make an engine self-starting, an electric starter motor is commonly
2. Magnetic field: The magnetic field is uniform, with a constant flux used. It converts electrical energy from the battery into mechanical energy to
density along the rotor surface. crank the engine until it begins operating on its own.
3. Torque production: Torque is produced due to the interaction between
the stator field and the rotor's magnetic field. Q.Compare between squirrel cage and slip-ring rotors?
Ans:The main difference between squirrel cage and slip-ring rotors is their
Q.Define slip and slip speed. construction, with slip-ring rotors being more complex and having more
Ans:Slip (s): Slip is the difference between the synchronous speed (Ns) of control options:
the motor and the actual rotor speed (Nr), expressed as a percentage of Construction:Slip-ring rotors are wound with more turns than squirrel cage
the synchronous speed. rotors, and include slip rings and brushes. Squirrel cage rotors are simpler
Mathematically, slip is represented as: s = (Ns - Nr) / Ns and more robust, with shortened rotor conductors and no brushes or slip
Where, s = slip (in decimal or percentage) rings.
Ns = synchronous speed (in rpm) Control:Slip-ring rotors have more control over speed and resistance, and
Nr = rotor speed (in rpm) can be controlled externally. Squirrel cage rotors have less control over
Slip Speed (Ns - Nr) speed and resistance.
Slip speed, also known as slip velocity, is the actual difference between the Starting:Slip-ring rotors have high starting torque and low starting current,
synchronous speed and the rotor speed. and don't generate abnormal heat during startup. Squirrel cage rotors have
Slip speed is represented as: Ns - Nr = slip speed (in rpm) low starting torque and high starting current, and generate heat during
In other words, slip speed is the speed at which the rotor is slipping behind startup.
the rotating magnetic field. Maintenance:Slip-ring rotors require more frequent maintenance than
squirrel cage rotors.
Q.Explain the working principle of a capacitor start motor with diagram. Efficiency:Slip-ring rotors have lower efficiency and higher copper loss than
Ans: Working Principle of a Capacitor Start Motor squirrel cage rotors.
1. Starting: When the motor is started, the capacitor and auxiliary winding
are connected in series with the main winding.
2. Current Flow: The capacitor allows a high current to flow through the
auxiliary winding, which creates a high starting torque.
3. Rotation: The rotor starts to rotate, and as it gains speed, the centrifugal
switch opens.
4. Capacitor Disconnection: The capacitor and auxiliary winding are
disconnected from the circuit, and the motor continues to run on the main
winding.
5. Running: The motor runs at a constant speed, and the capacitor is not
used during normal running conditions.
Q.Give the expression of distribution factor in alternator.
Ans: The distribution factor (Kd) in an alternator is a measure of how the
armature winding is distributed in the slots. It's an important factor in
determining the alternator's performance.
Here's the expression for the distribution factor:
Kd = sin(mα/2) / (m * sin(α/2))
where: - Kd = distribution factor
- m = number of slots per pole per phase
- α = angle between slots (in electrical degrees)
In simpler terms, the distribution factor depends on the number of slots
per pole per phase (m) and the angle between slots (α).
Q.Deduce the EMF equation of an alternator.
Ans: E.M.F. per turn = 2 × (e.m.f. per conductor) = 2 × (2f ϕ) = 4f ϕ volts.
Let Tph be the total number of turns per phase connected in series.
Assuming concentrated winding, we can say that all are placed in single slot
per pole per phase.
Q. Define synchronous reactanes
Ans: It is the imaginary reactance employed to account for the voltage effects in the
armature circuit produced by the actual armature leakage reactance and by the
change in the air gap flux caused by the armature reaction. The combination of
leakage reactance along with armature reaction is called as synchronous reactance.