Induction Motors and DC Machines Quiz

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Questions and Answers

What is the principle upon which DC machines operate?

Ampere's Law

Faraday's Law of Electromagnetic Induction (correct)

Kirchhoff's Voltage Law

Lenz's Law

Which type of DC motor has its field winding connected in parallel with the armature winding?

Series Motor

Shunt Motor (correct)

Universal Motor

Compound Motor

What is the synchronous speed of a 6 pole induction motor supplied with a 50 Hz frequency?

1000 rpm (correct)

1500 rpm

500 rpm

750 rpm

What is the rotor speed of the 6 pole induction motor mentioned above, given a slip of 4%?

960 rpm (D) Signup and view all the answers

What is the formula for calculating synchronous speed (Ns) in a three-phase induction motor?

Ns = 120f/P (D) Signup and view all the answers

What is the percentage slip of a three-phase induction motor when the rotor is running at synchronous speed?

0% (C) Signup and view all the answers

A 3-phase, 4-pole induction motor running at 1440 rpm is supplied with a 50 Hz frequency. What is the full load slip of the motor?

4% (A) Signup and view all the answers

What is the value of slip at the starting of a three-phase induction motor?

100% (C) Signup and view all the answers

If a load needs to be driven at 700 rpm, what is the number of poles required for a 3-phase induction motor operating at 60 Hz?

10 (C) Signup and view all the answers

What is the actual speed of the 3-phase induction motor from the previous question, assuming a slip of 4%?

691.2 rpm (A) Signup and view all the answers

What is the purpose of the field winding in a DC generator?

To provide a magnetic field for the armature winding (A) Signup and view all the answers

What is the main difference between a DC motor and a DC generator?

DC motors convert electrical energy to mechanical energy, while DC generators convert mechanical energy to electrical energy (C) Signup and view all the answers

A 20 kW, 200V shunt generator has an armature resistance of 0.05 ohms and a shunt field resistance of 200 ohms. What is the power developed in the armature when it delivers a 3000 W output?

23 kW (C) Signup and view all the answers

Which of the following is NOT a type of AC motor?

DC Motor (D) Signup and view all the answers

What does the symbol "ia" represent in the DC shunt generator diagram?

Armature current (A) Signup and view all the answers

Which of the following equations is used to calculate the terminal voltage (Vt) in a DC shunt generator?

Vt = Ish x Rsh (C) Signup and view all the answers

A 6-pole wave-wound DC generator has 300 conductors and runs at 1000 rpm. The generated EMF is 400V. What is the flux per pole? (Round to three decimal places)

0.053 weber (C) Signup and view all the answers

A 4-pole wave-wound DC generator with 400 armature conductors has a flux per pole of 0.04 weber. What is the generated EMF if the machine runs at 1200 rpm?

640V (A) Signup and view all the answers

A shunt generator delivers 480A at 250V. The shunt field resistance is 50Ω and the armature resistance is 0.03Ω. What is the generated EMF?

243.64V (D) Signup and view all the answers

A long-shunt compound generator delivers 50A at 500V. The armature resistance is 0.05Ω, the series field resistance is 0.03Ω, and the shunt field resistance is 250Ω. What is the generated EMF if a 1V drop is allowed per brush?

506.16V (B) Signup and view all the answers

A short-shunt compound generator delivers 30A at 220V. The armature resistance is 0.05Ω, the series field resistance is 0.30Ω, and the shunt field resistance is 200Ω. What is the armature current if a 1V drop is allowed per brush?

31.1A (C) Signup and view all the answers

In a long-shunt compound generator, which of the following statements is TRUE?

The series field winding is connected in series with the armature winding and the shunt field winding is connected in parallel with the combination. (D) Signup and view all the answers

Which of the following statements is TRUE regarding the armature current in a compound generator?

The armature current is equal to the load current plus the shunt field current. (D) Signup and view all the answers

What is one of the advantages of a squirrel cage rotor?

Uniform torque production (C) Signup and view all the answers

What is the function of the slip rings in a phase wound rotor?

To provide a means for connecting external resistances (D) Signup and view all the answers

In the torque-slip characteristic, what happens as the load on the induction motor increases?

Torque increases to satisfy load demand (B) Signup and view all the answers

During which region of the torque-slip characteristic does the torque increase linearly?

Low Slip Region (D) Signup and view all the answers

What is the primary role of resistors in a phase wound rotor?

To increase starting torque and decrease the starting current (D) Signup and view all the answers

What type of rotor consists of a cylindrical laminated core with short-circuited rotor bar conductors?

Squirrel cage rotor (B) Signup and view all the answers

Which mathematical expression represents the torque in the low slip region for the rotor?

Tα = SR2 / R2² (A) Signup and view all the answers

What does the stable region in the torque-slip characteristic indicate?

Stability of motor speed (D) Signup and view all the answers

What is the primary purpose of the auxiliary winding in a Split Phase Induction Motor?

To provide a high starting torque by increasing the phase difference between the main winding and the auxiliary winding. (C) Signup and view all the answers

What is the main reason for the poor starting torque of Split Phase Induction Motors?

The phase difference between the main winding and auxiliary winding is too small, resulting in a weak starting torque. (D) Signup and view all the answers

What is the typical speed range at which the centrifugal switch is opened in both the Split Phase and Capacitor Start Induction Motors?

75% to 80% of synchronous speed. (C) Signup and view all the answers

Which of the following statements accurately describes the nature of the windings in a Split Phase Induction Motor?

The main winding is inductive and the auxiliary winding is resistive. (A) Signup and view all the answers

How does the phase difference between the main winding and auxiliary winding affect the starting torque in an induction motor?

A larger phase difference results in a weaker starting torque. (C) Signup and view all the answers

Which of the following applications typically utilizes a Split Phase Induction Motor?

Small household appliances such as blowers, washing machines, and table fans. (B) Signup and view all the answers

What is the primary reason why the auxiliary winding in a Split Phase Induction Motor is typically disconnected once the motor reaches a certain speed?

To improve the efficiency of the motor by reducing the power consumption. (C) Signup and view all the answers

What is the fundamental difference between the shaded pole induction motor and the split phase induction motor?

The shaded pole induction motor uses a single winding with a shaded pole, while the split phase induction motor uses two separate windings. (C) Signup and view all the answers

What is the primary difference between a capacitor-start motor and a capacitor-start capacitor-run motor?

The capacitor-start motor uses a centrifugal switch to disconnect the capacitor during operation, while the capacitor-start capacitor-run motor keeps the capacitor connected. (C) Signup and view all the answers

What is the main disadvantage of a shaded pole induction motor compared to other types?

The shaded pole motor is less efficient due to copper losses from the shading bands. (D) Signup and view all the answers

Which of the following applications is most suitable for a capacitor-start capacitor-run motor?

A large air compressor demanding high starting torque and constant power factor. (D) Signup and view all the answers

Which type of induction motor has a rotor winding similar to the stator winding, requiring external resistance for starting?

Slipring (or wound) cage induction motor (C) Signup and view all the answers

What is the key feature of a shaded pole induction motor that distinguishes it from other types?

The presence of a copper band on each pole, partially shading the magnetic flux. (D) Signup and view all the answers

What is a significant drawback of the slipring (or wound) cage induction motor compared to the squirrel cage type?

Slipring motors require more complex maintenance due to the presence of brushes. (A) Signup and view all the answers

Why is the starting torque of a shaded pole induction motor considered very poor?

The shading band reduces the magnetic flux, lowering the torque developed. (D) Signup and view all the answers

Which of the following statements is TRUE about the capacitor start motor?

The capacitor is removed from the circuit using a centrifugal switch when the motor reaches a certain speed. (B) Signup and view all the answers

Study Notes

Electrical Machines

DC Machines: Concepts of principle, construction, types, equations of generators and motors, applications, and numerical problems.
Three-Phase DC Motors: Covers principle, construction, types, and characteristics, along with numerical problems and slip-related applications.
Single-Phase Induction Motors: Includes operational principles, introduction to starting methods, and applications.
Three-Phase Synchronous Machines: Focuses on operational principles, alternator, and synchronous motor applications.

DC Machines (Detailed)

Principle: DC machines operate based on Faraday's law of electromagnetic induction. Whenever a conductor cuts a magnetic field, an electromotive force (emf) is induced in it, causing current to flow if the conductor is closed.
Essential Parts: Field winding and armature winding (lap or wave).
Types: Series and shunt, with the field winding connected in either series or parallel with the armature winding.

Induction Motors (Detailed)

Slip Calculation: The percent slip is calculated using the formula: (NS - NR) / NS * 100, where NS is the synchronous speed and NR is the rotor speed.
Synchronous Speed: Calculated using the formula: NS = 120f/P, where f is the frequency of supply and P is the number of poles.
Slip at Starting: Slip at starting is 100%.
Slip at Synchronous Speed: Slip at synchronous speed is 0%.

Additional Topics

Questions and Problem Solving: Various solved examples and problems, illustrating the calculation of percentage slip, synchronous speed, and rotor speed for different induction motors.

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Description

Test your knowledge on the principles of DC machines and three-phase induction motors. This quiz covers topics such as synchronous speed, rotor speed, and slip calculations, providing a comprehensive review for students and enthusiasts alike. Dive into the essential concepts that govern motor operations and performance.