Electrical Fundamentals Module 3 Quiz

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Questions and Answers

What is the primary function that must be controlled in a DC motor?

The input power supplied to the motor (correct)

The speed of the motor

The torque generated by the motor

The output produced by the motor

Which type of DC motor is characterized by the field being connected in series with the armature?

Series motor (correct)

Shunt wound motor

Separately excited motor

Compound wound motor

What is a significant disadvantage of a series motor?

It cannot generate high torque at low speeds

It cannot run on alternating current

It must always have a load connected before operation (correct)

It operates at a constant speed under varying load conditions

What type of motor is predominantly used for consistent applications?

Compound wound motor Signup and view all the answers

How does the speed of a series motor behave under changing load conditions?

It varies significantly between no-load and full-load Signup and view all the answers

What method can be used for below normal speed control of a series motor?

Connecting a rheostat in parallel with the armature Signup and view all the answers

Which of the following applications is least suited for a series motor?

Electric trains Signup and view all the answers

What is the primary purpose of the series motor's field winding design?

To carry the full armature current Signup and view all the answers

At Position A, what is the state of induced emf in the conductor?

Induced emf is zero. Signup and view all the answers

What happens to the induced voltage as the conductor moves from Position A to Position B?

Induced voltage increases. Signup and view all the answers

What characteristic is observed at Position C regarding the induced voltage?

Induced voltage is decreasing. Signup and view all the answers

What is the polarity of the induced emf at Position D compared to Position B?

Changing polarity. Signup and view all the answers

Which position indicates when the conductor is cutting directly across the magnetic field?

Position B Signup and view all the answers

What occurs to the induced voltage at Position D after it has been at a maximum?

It begins to decrease. Signup and view all the answers

What is the shape of the induced voltage value over time during the rotation?

Sine wave. Signup and view all the answers

In which position does the conductor complete one-quarter of a revolution?

Position B Signup and view all the answers

What happens to the armature reaction in a DC motor compared to a DC generator?

The neutral plane shifts backward, opposite to the direction of rotation. Signup and view all the answers

What is the primary method for compensating for armature reaction in large DC motors?

Employing interpoles for correction. Signup and view all the answers

Which statement is true regarding the polarity of interpoles in DC motors?

Interpoles have the opposite polarity of the main pole behind in DOR. Signup and view all the answers

What is the effect of shifting the brushes in a DC motor?

It reduces sparking but lessens field effectiveness. Signup and view all the answers

Why are compensating windings and interpoles important in DC motors?

They correct armature reaction and enhance performance. Signup and view all the answers

What occurs as the load varies in a DC motor in relation to interpole flux?

Interpole flux increases and decreases accordingly. Signup and view all the answers

What is a significant disadvantage of using compensating windings in DC motors?

They are relatively expensive. Signup and view all the answers

What is the purpose of shifting the neutral plane in a motor?

To improve the positioning of brushes. Signup and view all the answers

What is the primary function of a starter generator?

To serve as both a starter and a generator Signup and view all the answers

Which of the following components does a typical starter generator consist of?

Two sets of field windings and one armature winding Signup and view all the answers

What happens in the generator mode of a starter generator?

Current only flows through the shunt winding Signup and view all the answers

What is a key advantage of using starter generators in small turbine engines?

Significant weight savings Signup and view all the answers

What feature distinguishes the armature winding in the starter mode of operation?

It is wound with larger conductors Signup and view all the answers

Which of the following factors affects the output of DC generators?

Field resistance and direction of voltage Signup and view all the answers

What is the role of the quill shaft in starter generators?

To couple the starter generator to the engine Signup and view all the answers

What type of motor can be described as 'shunt wound'?

A motor where the field winding is connected parallel to the armature Signup and view all the answers

What is the relationship between the resistance of a material and its cross-sectional area?

Resistance is inversely proportional to cross-sectional area. Signup and view all the answers

What technique is used to reduce eddy current losses in generators?

Employing laminated cores. Signup and view all the answers

How do hysteresis losses in an armature chiefly manifest?

As heat generated from molecular friction. Signup and view all the answers

Which of the following factors does not affect the output voltage of a DC generator?

Resistance of the armature windings. Signup and view all the answers

Why is the insulation value in laminated cores not required to be very high?

Because voltages induced are very small. Signup and view all the answers

What heat treatment process is applied to steel laminations to reduce hysteresis loss?

Annealing. Signup and view all the answers

Which statement is true regarding the factors that affect the output voltage of a DC generator?

Armature speed is usually impractical to change. Signup and view all the answers

What happens to the resistance of an armature as hysteresis losses increase?

Resistance increases due to molecular friction. Signup and view all the answers

What is the primary function of the field frame in a DC generator?

To support mechanical components and complete the magnetic circuit. Signup and view all the answers

How are the laminated field poles of a DC generator beneficial?

They reduce eddy current losses. Signup and view all the answers

What does an increase in the number of coils in the armature change?

It increases the number of commutator segments required. Signup and view all the answers

What role does the field coil play in the generator?

It generates the magnetic field required for operation. Signup and view all the answers

What is the significance of having an even number of poles in a DC generator?

It ensures balanced magnetic forces are achieved. Signup and view all the answers

What happens to the emf induced in the coils during a 90° rotation of the armature?

One coil's emf increases while the other's decreases. Signup and view all the answers

Which structure of a generator provides the foundation for others while completing the magnetic circuit?

Field Frame/Yoke Signup and view all the answers

What does the commutator do in a DC generator structure?

It converts AC to DC. Signup and view all the answers

Study Notes

Module 3: Electrical Fundamentals - Topic 3.12: DC Motor/Generator Theory

This topic covers DC motor and generator theory, components, and operation
Students should be able to describe basic motor/generator theory, identify DC generator components, describe their construction/purpose, and explain DC generator and motor operation including factors affecting output, current flow, output power, torque and speed
Students should also be able to describe the operation and features of DC motors (series wound, shunt wound, compound) and starter generators
A left-hand rule is used to understand direction of magnetic fields around conductors
Cross indicates tail of arrow/feather/conductor (heading away), and a point indicates front (coming towards)
Current flow in 2 parallel conductors in the same direction increases the strength of the field around the conductors
Current flow in 2 parallel conductors in opposite directions weakens the field around the conductors; they repel each other
An electromotive force (emf) or voltage is produced in a conductor when placed in a changing magnetic field or when the conductor is moved through a magnetic field. This is called electromagnetic induction
Three conditions for emf generation must be met: the presence of a conductor, a magnetic field in the vicinity of the conductor, and relative motion between the conductor and the field
A DC generator's output voltage depends on factors like number of conductor loops in series, armature speed, and magnetic field strength
Separately excited generators are supplied by an external current source
Self-excited generators - voltage is supplied directly from the output, only if the field pole pieces retain some residual magnetism
Major parts of a DC Generator include the field frame or yoke, armature, commutators, and brushes
The field frame or yoke completes the magnetic circuit between poles, and acts as a mechanical support for other generator parts; in small generators it is one solid piece of iron; in large generators two pieces bolted together
The magnetising force is produced by an electromagnet (field coil) with a core (field pole/pole shoe) bolted into the frame
The core is laminated to reduce eddy current losses, and concentrates lines of force produced by field coils
The number of north poles equals the number of south poles
Armature reaction causes a distortion in the main field, which shifts the neutral plane
Shifting brushes to compensate for the armature reaction doesn't eliminate the problem entirely, although it helps
Interpoles, and compensating windings help compensate the armature reaction

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Description

Test your understanding of DC motor and generator theory, including their components and operations. This quiz covers topics such as construction, current flow, torque, and the left-hand rule. Dive into the specifics of series, shunt, and compound motors to solidify your knowledge.