DC Generators Quiz

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

• This topic covers DC motor and generator theory, components, construction, operation, and factors affecting output, direction of current flow, torque, speed, and the different types of DC motors: series wound, shunt wound, and compound. Also included is the construction and operation of starter generators.

Introduction

• On completion of this topic, students should be able to:
  • Describe basic motor and generator theory.
  • Identify components of a DC generator and describe their construction and purpose.
  • Describe the operation of DC generators and factors affecting output and direction of current flow.
  • Describe the operation of DC motors and factors affecting output power, torque, and speed; and direction of rotation.
  • Describe the operation and features of series wound, shunt wound, and compound DC motors.
  • Describe the construction and operation of starter generators.

Left Hand Rule

• The direction of the magnetic field is dependent on the direction of the current flowing in the conductor.
• Left-hand rule: Grasp the conductor in your left hand with your thumb pointing in the direction of the current. Your fingers will curl in the direction of the magnetic field.
• This rule is for electron flow only.

Magnetic Field Patterns

• When looking at a conductor end-on:
  • A cross indicates the tail of an arrow/feather/conductor (heading away from you).
  • A dot indicates the front of an arrow/feather/conductor (coming towards you).
• Using the left-hand rule, the direction of the magnetic field can be determined.
• The magnetic field around a current-carrying conductor is perpendicular to the conductor.

Magnetic Field Around Parallel Conductors

• Two parallel conductors with currents in the same direction:
  • Increase the strength of the field around the conductors.
  • Aid each other.
• Two parallel conductors with currents in opposite directions:
  • The lines of force oppose each other.
  • Deform the field around each conductor.
  • Repel each other.

Basic AC Generation

• One way to produce an electromotive force (emf) is through magnetism and movement.
• A changing magnetic field produces an emf in a conductor.
• An emf is induced in a conductor if placed in a magnetic field, and either:
  • The field is changing around the conductor.
  • The conductor moves through the magnetic field.
• This effect is called electromagnetic induction.

Voltage Produced by Magnetism and Motion

• Three conditions must exist before an emf is produced by magnetism:
  • A conductor in which voltage will be produced.
  • A magnetic field in the vicinity of the conductor.
  • Relative motion between the field and the conductor.
• The conductor must move to cut the magnetic lines of force.
• When the conductor cuts the lines of force, electrons are propelled.
• This creates an electric motor force (emf) or voltage.

Left-Hand Rule for Generators

• Position your left hand with your fingers as shown.
• Your thumb will point in the direction of rotation (relative movement of the wire to the field).
• Your index finger will point in the direction of the magnetic flux (north to south).
• Your middle finger will point in the direction of the electron current flow.

Basic Alternating-Current Generator

• Describes how a conductor rotating in a magnetic field produces an alternating current (AC) voltage. Covers various positions of the conductor during rotation and how that affects the voltage produced. Explains that no emf is induced when the conductor is parallel to the magnetic field lines, while the maximum induced emf is evident when the conductor is perpendicular to the field.

DC Generator Theory

• A sine curve shows the value of induced voltage at each instant of time during rotation.
• Two alternations represent one complete cycle of rotation.
• DC generators induce AC in the output windings.
• Generated AC is rectified to DC mechanically by a commutator and brushes.

Elementary DC Generator

• Each conductor end is connected to a segment of a 2-segment metal ring.
• Two segments insulated from each other form a simple commutator.
• The commutator in a DC generator replaces the slip rings of an AC generator.

Commutator

• Mechanically reverses armature loop connections to the external circuit.
• This occurs at the same instant that the polarity of voltage in the armature loop reverses.
• The commutator changes generated AC voltage to a pulsating DC voltage.
• This action is known as commutation.

Voltage developed across brushes in pulsating and unidirectional.

• Varies twice during each revolution between zero and maximum.
• This variation is called ripple.

Typical Generator Construction

• Lists major parts of a DC generator.
  • Field frame or yoke
  • Armature
  • Commutators
  • Brushes
• Notes laminated field poles to reduce eddy currents.

Typical 24-Volt DC Generator

• Provides an image of one particular 24-volt DC Generator
• Shows the labels of various components of the generator

Generator Field Frame

• The field frame (or yoke) completes the magnetic circuit between poles.
• It acts as a mechanical support for other generator parts.

Field Coil And Pole Shoe

• Magnetising force is produced by electromagnets (field coils).
• The core of the electromagnet is called a field pole or pole shoe.
• Pole shoes are usually laminated to reduce eddy current losses.
• Concentrates lines of force produced by field coils.
• Always one north pole for each south pole.

Effects of Additional Coils

• Describes how adding more coils to the armature improves the output voltage, reducing the voltage ripple.

Effects of Additional Poles

• Adding more magnetic poles enhances the magnetic field strength.
• This increase in field strength results in more output voltage as the coils cut more flux lines per revolution.

Electromagnetic Poles

• Nearly all generators use electromagnetic poles instead of permanent magnets, consisting of coils of insulated copper wire wound around soft iron cores.
• Advantages of using electromagnetic poles include increased field strength and the ability to control field strength.

Brushes

• Ride on the surface of the commutator.
• Act as electrical contact between armature coils and the external circuit.
• Made of high-grade carbon (with molybdenum for lubrication when needed).
• Held in place by spring-loaded brush holders that are insulated from the frame.
• Free to slide to accommodate wear and irregularities. High altitude, self-lubricating carbon brushes have been found for certain aircraft applications, these take advantage of drier atmospheric conditions. Flexible braided copper conductor (pig-tail) connects each brush to the external circuit.

Commutator

• Located at one end of the armature.
• Consists of wedge-shaped segments of hard-drawn copper.
• Insulated from each other by thin mica sheets.
• Held in place by steel V-rings or clamping flanges.
• Also has raised portions called risers.
• Leads from armature coils are soldered into these risers in some cases. Some machines do not use risers and instead have those leads soldered into slots within the segments.

Commutation

• Successful reversal of current in an armature coil while under short-circuit by a brush.
• Mechanical conversion from AC to DC at the brushes of a DC machine.
• As a commutator segment moves under a brush, the armature coil is shorted.
• When a coil is under short circuit of a brush, the current in the coil reverses direction.

Commutation (Waveform)

• Current in coils being commutated should reverse linearly with time, as shown.
• If the reversal is not complete by the time the coil leaves the brush, severe sparking results.
• Good commutation requires the coil to be commutated when it is in neutral position, and brush assemblies are positioned to ensure correct commutation

Commutation in a DC Generator

• Commutation occurs simultaneously in 2 coils, briefly short-circuited by brushes.
• Brushes are positioned on the commutator such that each coil is short-circuited in the neutral plane.
• No voltage is generated in that coil at that time, so no sparking occurs.
• Sparking between brushes and the commutator indicates improper commutation.
• Improper brush placement is the main cause of improper commutation

Armature

• Mounted on a shaft that rotates in bearings located in the generator's end frames.
• Most commonly used winding is a two-layer winding.
• Number of coils equals the number of armature slots.
• Coil span is made equal to 180 electrical degrees.

Armature Two-Layer Winding

• Coils are placed in slots in the armature core.
• There is no electrical connection between coils and the core.
• The coils are held in place using wedges.
• Coil ends are brought out to individual segments of the commutator.
• The windings can be either lap windings or wave windings.

Armature Reaction

• Magnetic fields produced by the current in the armature affect the main field's flux pattern.
• Changing the neutral plane and the interaction of the magnetic field on the armature is called armature reaction.
• For proper commutation, the coil short-circuited by the brushes must be in the neutral plane

Armature Reaction (with Field Windings)

• If only field windings are excited and the armature is not connected, the neutral plane is unaffected.
• Excitation of both the field and armature windings shifts the neutral plane in the direction of rotation.
• This shift in the neutral plane causes a distortion of the main field.

Armature Reaction

• Discusses the impact of armature current on the main magnetic field and the resulting shift in the neutral plane, which affects commutation.
• Explains that shifting brushes does not fully resolve the issue, and that load current changes cause shifts in the neutral plane.

Armature Reaction in Generator with Field Windings

• Provides specific scenarios illustrating how armature reaction affects the neutral plane under various excitation states of the field windings and armature. (e.g., field only, armature only, and both field and armature windings excited

Compensating Windings and Interpoles

• Describes how compensating windings oppose armature reaction, keeping the neutral plane stationary and consistent for different loads. This means more consistent commutation across different loads. Interpoles serve a similar function and are a key component in larger generators.

DC Generator Interpoles

• Discusses how interpoles are designed to counteract armature reaction in a DC motor.
• Emphasizes that interpoles utilize a specific polarity to achieve compensation that is different in design compared to DC generators.
• Underscores that with corrected interpoles, brushes don't need to be shifted as load current varies.

Starter Generators

• Combination of a starter motor and a generator in a single unit.
• This design provides an electric starter and power generation.
• Eliminates the need for a clutch, as the unit continually spins and is connected to the engine via a quill shaft.

Conclusion (Summary)

• A conclusion restating the objectives of the module.

DC Motors

• Lists major types of DC motors: shunt, series, compound, and separately excited.

Inducing a Force on a Conductor

• Two conditions are necessary to induce a force on a conductor:
• The conductor is carrying a current.
• The conductor is within a magnetic field.
• When these exist, the force attempts to move the conductor perpendicular to the field.

Principles of Operation

• Every current-carrying conductor has a magnetic field around it.
• The direction of this magnetic field is determined by the left-hand rule.
• Describes how the conductor's magnetic field interacts with the main field, creating weak and strong points.

Basic Single Coil DC Motor

• Explaining how a current-carrying conductor in a magnetic field has a force acting on it causing movement, and how the commutator in the DC motor allows for consistent rotational movement.

Torque

• Defined as the force that tends to produce and maintain rotation.
• Crucial in DC motors to provide mechanical output.
• The force (in lbs) multiplied by the radius of the armature (in feet) is equivalent to the torque.
• Combined actions of magnetic fields create force on the armature conductors.
• This force is directly proportional to the strength of the main field and the field around the conductor, with the field strength dependent on the amount of current flow.

Torque in a DC Motor

• Describes how torque characteristics vary depending on the angle between the conductors and the magnetic field. States that the most torque is produced at 90 degree angles.

Torque Variation in DC Motor

• Explains how the commutator in a DC motor reverses the current periodically to maintain consistent torque and spinning.

Counter emf

• An emf is produced every time the conductor moves in a magnetic field.
• In a DC motor, this induced emf is called counter-emf.
• Counter-emf directly corresponds to the speed of the armature and the strength of the field.
• Greater counter-emf increases the resistance to current flow into the armature.

DC Motor Types

• Discusses shunt motors, series motors, compound motors, and separately excited (or permanent magnet) motors, comparing their wiring configurations/characteristics to understand how torque and speed are impacted.

Series Motor

• Series motors have a field winding that is in series with the armature.
• These motors tend to develop higher levels of torque but have more variable/irregular speeds across loads.

Series Motor Speed Control

• Describes different ways for adjusting series motor speed below normal, at normal, and above normal operating points.

Shunt Motor

• Describes the use and characteristics of Shunt motors (field and armature are wired in parallel).
• High starting torque, but tends to have more stable/constant speeds across loads.

Shunt Motor Speed Control

• Explains how the speed of a shunt motor can be adjusted above its normal operating speed and below its normal operating speed.

Compound Motor

• Discusses compound motors.
• Discusses the long shunt and short shunt connections of field windings to the armature.

Cumulative Compound Motor

• Describes the current flow in the windings of a cumulative compound motor and how the series field aids the shunt field. Explains that torque and speed characteristics fall between the series' motor and the shunt's motor characteristics.

Differential Compound Motor

• Explains the current flow in the windings of a differential compound motor and how the series field opposes the shunt field.
• Explains that torque and speed characteristics are poorer than the shunt motor.

Separately Excited Motor

• Describes the differences in the circuit of a separately excited motor compared to other DC motors.

DC Motor Characteristics

• Presents a table summarizing various characteristics for different types of DC motors.

Reversing Motor Direction

• Details different methods to reverse the direction of a DC motor.

Split Field DC Motor

• Explains how using the split field approach on a series motor allows for switching current flow, thereby reversing the motor's direction of rotation.

Reversing Motor Direction (Switch Method)

• Describes using a double pole, double throw switch to reverse the direction of DC motor rotation, either through the field or the armature

Armature Reaction in DC Motors

• Describes how the armature's magnetic field interacts with the main field in DC motors, creating a distorted magnetic field (armature reaction), and the implications for commutation.
• Explains how this is similar and different from how armature reaction works in DC generators.

Cancelling Armature Reaction

• Describes the use of compensating windings to counter the effects of armature reaction and how interpoles similarly assist in correcting commutation issues induced by load changes. Emphasizes that interpoles, like compensating windings, are pivotal to the smooth functioning of DC motors.

DC Motor Interpoles

• Discusses the distinct design of interpoles in DC motors compared to generators, emphasizing their function in automatically compensating for armature reaction and load variations, dispensing with the need for manual brush adjustments.

Test your knowledge on DC generators with this quiz. Explore key concepts such as the sine curve representation, the function of the commutator, and how output voltage varies. Challenge yourself with questions about single-loop generators and commutation processes.