Ohm’s Law

Ohm’s Law states that the current flowing through a conductor is directly proportional to the potential difference (voltage) applied across the conductor and inversely proportional to the resistance of the conductor.

This fundamental relationship can also be expressed as:

• V = I × R (to find voltage)
• R = V/I (to find resistance)

Applications of Ohm’s Law

• Calculating current, voltage, or resistance in circuits
• Designing electrical circuits with specific requirements
• Troubleshooting electrical problems
• Determining power consumption (P = VI = I²R = V²/R)

Kirchhoff’s Laws

Kirchhoff’s Current Law (KCL)

The algebraic sum of all currents entering and leaving a node (or junction) in an electrical circuit is zero.

Kirchhoff’s Voltage Law (KVL)

The algebraic sum of all voltages around any closed loop in a circuit is zero.

Applications of Kirchhoff’s Laws

• Analyzing complex circuits with multiple loops and branches
• Solving for unknown voltages and currents in circuits
• Verifying circuit designs and simulation models

Series & Parallel Combinations

Series Combination of Resistors

When resistors are connected in series, the same current flows through each resistor.

Parallel Combination of Resistors

When resistors are connected in parallel, the same voltage appears across each resistor.

Series Combination of Inductors

When inductors are connected in series, the total inductance is the sum of individual inductances.

Parallel Combination of Inductors

When inductors are connected in parallel (assuming no mutual inductance):

Series Combination of Capacitors

When capacitors are connected in series:

Parallel Combination of Capacitors

When capacitors are connected in parallel:

Wheatstone Bridge

A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component.

Applications of Wheatstone Bridge

• Precise measurement of resistance
• Strain gauge measurements
• Temperature measurement using RTDs
• Sensor interfaces where small changes in resistance need to be detected

PVC Wires, Conductors & Cables

PVC Wires

PVC (Polyvinyl Chloride) insulated wires are commonly used in electrical installations due to their durability, flexibility, and insulating properties.

Types of Conductors

Conductors are materials that allow the flow of electric current with minimal resistance. Common conductor materials include:

• Copper: Excellent conductivity, widely used
• Aluminum: Lighter and cheaper than copper, but less conductive
• Silver: Best conductivity but expensive
• Gold: Used in specialized applications due to corrosion resistance

Types of Cables

Cables consist of one or more conductors with insulation and protective coverings:

• Single Core Cables: Contains one conductor
• Twin Core Cables: Contains two insulated conductors
• Multi-core Cables: Contains three or more insulated conductors
• Armored Cables: Additional metal protection layer for mechanical strength
• Underground Cables: Specially designed for underground installations
• Coaxial Cables: Used for signal transmission

Cable Ratings

Cables are rated based on:

• Current carrying capacity (ampacity)
• Voltage rating
• Temperature rating
• Environmental conditions (moisture, heat, chemical exposure)

Wire Joints and Soldering

Types of Wire Joints

• Straight Joint (Married Joint): Connecting two wires running in the same straight line
• T-Joint: Connecting a wire at a right angle to another wire
• Western Union Joint: Strong joint for overhead lines
• Britannia Joint: For joining larger diameter wires
• Tap Joint: Connecting a branch wire to a continuous main wire

Soldering

Soldering is the process of joining electrical connections using a filler metal (solder) with a low melting point.

Soldering Process

1. Clean the surfaces to be joined
2. Apply flux to prevent oxidation
3. Heat the joint with a soldering iron
4. Apply solder to the heated joint
5. Allow to cool without movement

Types of Solder

• Lead-based: Traditional tin-lead alloys (60/40, 63/37)
• Lead-free: Modern environmental-friendly alternatives (SAC305, SN100C)

Effects of Electric Current

Heating Effect

When current flows through a conductor, it generates heat due to the resistance of the conductor. This is known as Joule heating.

Applications: Electric heaters, kettles, irons, incandescent lamps, fuses

Lighting Effect

Electric current can produce light through different mechanisms:

• Incandescence: Material gets hot enough to emit light (incandescent bulbs)
• Luminescence: Material emits light without significant heat (fluorescent lamps, LEDs)
• Arc Discharge: Electric arc between electrodes produces light (arc lamps)

Magnetic Effect

Current flowing through a conductor creates a magnetic field around it. This is the principle behind electromagnets.

Applications: Electric motors, generators, relays, solenoids, transformers, speakers

Chemical Effect

Electric current can cause chemical reactions when passed through certain solutions. This process is called electrolysis.

Applications: Electroplating, electrorefining of metals, production of chemicals, battery charging

Joule’s Law

Joule’s Law states that the heat produced in a conductor is:

• Directly proportional to the square of the current (I²)
• Directly proportional to the resistance of the conductor (R)
• Directly proportional to the time for which current flows (t)

Power Calculation

The rate of heat generation (power) is given by:

Electrolysis & its Laws

Electrolysis

Electrolysis is the process of using electric current to drive an otherwise non-spontaneous chemical reaction. It involves the decomposition of an electrolyte by the passage of electricity through it.

Faraday’s Laws of Electrolysis

First Law: The mass of a substance deposited or liberated at an electrode during electrolysis is directly proportional to the quantity of electricity passed through the electrolyte.

Second Law: When the same quantity of electricity is passed through different electrolytes, the masses of substances deposited or liberated at the electrodes are directly proportional to their chemical equivalents (equivalent weights).

Applications of Electrolysis

• Electroplating of metals
• Extraction and purification of metals
• Production of chemicals
• Electroforming
• Anodizing of aluminum

Cells and Batteries

Primary Cells

Primary cells are non-rechargeable batteries that convert chemical energy to electrical energy. Once discharged, they cannot be efficiently recharged.

Types of Primary Cells:

• Zinc-Carbon (Leclanché) Cell: Voltage: 1.5V, uses zinc as anode, carbon rod as cathode, and ammonium chloride as electrolyte
• Alkaline Cell: Voltage: 1.5V, uses zinc as anode, manganese dioxide as cathode, and potassium hydroxide as electrolyte
• Silver Oxide Cell: Voltage: 1.55V, used in watches and calculators
• Lithium Cell: Voltage: 3.0V, high energy density, long shelf life

Secondary Cells

Secondary cells are rechargeable batteries that can be recharged by passing a current through them in the opposite direction to that of discharge.

Types of Secondary Cells:

• Lead-Acid Battery: Voltage: 2.0V per cell, uses lead and lead dioxide electrodes with sulfuric acid electrolyte
• Nickel-Cadmium (Ni-Cd) Battery: Voltage: 1.2V per cell, uses nickel oxide hydroxide and metallic cadmium electrodes
• Nickel-Metal Hydride (Ni-MH) Battery: Voltage: 1.2V per cell, environmentally friendlier alternative to Ni-Cd
• Lithium-Ion (Li-ion) Battery: Voltage: 3.7V per cell, high energy density, no memory effect

Lead-Acid Battery

The lead-acid battery is the most common type of secondary battery used in automotive applications.

Construction:

• Positive plate: Lead dioxide (PbO₂)
• Negative plate: Spongy lead (Pb)
• Electrolyte: Dilute sulfuric acid (H₂SO₄)

Chemical Reactions:

During discharge:

Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O

During charging (reverse reaction):

2PbSO₄ + 2H₂O → Pb + PbO₂ + 2H₂SO₄

Hybrid Cells

Hybrid cells combine features of both primary and secondary cells, allowing limited recharging of primarily single-use batteries.

Alkaline Cells

Alkaline batteries use potassium hydroxide (KOH) as the electrolyte instead of acidic substances. They provide longer life and better performance than zinc-carbon batteries.

Battery Charging & Maintenance

Charging Methods

• Constant Current Charging: Maintains a constant current throughout the charging process
• Constant Voltage Charging: Maintains a constant voltage while current decreases over time
• Trickle Charging: Low-rate, continuous charging to maintain full charge
• Float Charging: Maintains battery at full charge to compensate for self-discharge
• Pulse Charging: Uses current pulses to charge the battery

Care & Maintenance

• Regular Inspection: Check for cracks, leaks, and terminal corrosion
• Electrolyte Level: Maintain proper level in flooded batteries by adding distilled water
• Clean Terminals: Remove corrosion with a solution of baking soda and water
• Proper Storage: Store in cool, dry place; avoid extreme temperatures
• State of Charge: Maintain charge level, especially for lead-acid batteries
• Testing: Periodically check voltage, capacity, and internal resistance

Battery Testing

• Voltage Test: Measures battery terminal voltage
• Load Test: Measures battery performance under load
• Specific Gravity Test: Measures electrolyte density in flooded lead-acid batteries
• Impedance Test: Measures internal resistance of the battery

Terminology in Magnetic Circuits

Key Relationships

Principle of Electromagnet

An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. It consists of a coil of wire wound around a core made of magnetic material (typically iron).

Working Principle

When electric current flows through the coil:

1. A magnetic field is generated around the conductor according to Ampere’s Right-Hand Grip Rule
2. The magnetic field is concentrated in the core material
3. The core becomes magnetized, enhancing the overall magnetic field
4. When current stops, the magnetic field collapses (in soft magnetic materials)

Factors Affecting Electromagnet Strength

• Number of turns in the coil
• Amount of current flowing through the coil
• Core material (permeability)
• Core geometry and dimensions
• Air gaps in the magnetic circuit

Applications of Electromagnets

• Electric motors and generators
• Relays and contactors
• Solenoids and actuators
• Transformers
• Lifting magnets
• Magnetic recording
• MRI machines

Capacitor & its Types

A capacitor is a passive electronic component that stores electrical energy in an electric field. It consists of two conductive plates separated by a dielectric material.

Basic Properties

• Capacitance (C): Measure of a capacitor’s ability to store charge, measured in farads (F)
• Voltage Rating: Maximum voltage that can be applied without breakdown
• Leakage Current: Small current that flows through the dielectric
• ESR (Equivalent Series Resistance): Resistance that causes power losses
• Tolerance: Allowable deviation from nominal capacitance value

Types of Capacitors

Based on Dielectric Material:

• Ceramic Capacitors: Use ceramic as dielectric, small size, low cost, suitable for high frequencies
• Film Capacitors: Use plastic film as dielectric, good stability and reliability
  • Polyester (Mylar)
  • Polypropylene
  • Polycarbonate
  • Teflon
• Electrolytic Capacitors: High capacitance values, polarized
  • Aluminum Electrolytic
  • Tantalum Electrolytic
• Paper Capacitors: Use paper as dielectric, older technology
• Mica Capacitors: Use mica as dielectric, high stability
• Glass Capacitors: Use glass as dielectric, high stability and reliability

Based on Construction:

• Fixed Capacitors: Have a single, non-adjustable value
• Variable Capacitors: Capacitance can be adjusted
  • Air Variable Capacitors
  • Vacuum Variable Capacitors
  • Trimmer Capacitors

Based on Polarization:

• Polarized Capacitors: Must be connected with correct polarity (most electrolytics)
• Non-polarized Capacitors: Can be connected in either direction (ceramic, film)

Capacitor Applications

• Energy storage
• Power factor correction
• Coupling and decoupling
• Filtering
• Timing circuits
• Tuning circuits
• Motor starting

Faraday’s Laws of Electromagnetic Induction

First Law

Whenever a conductor cuts magnetic flux or is cut by magnetic flux, an EMF (electromotive force) is induced in the conductor.

Second Law

The magnitude of induced EMF is directly proportional to the rate of change of magnetic flux linkage with the circuit.

Methods of Inducing EMF

• Moving a conductor in a static magnetic field
• Moving a magnetic field near a stationary conductor
• Changing the magnetic field strength around a conductor
• Changing the area of a coil in a magnetic field
• Rotating a coil in a magnetic field

Applications of Electromagnetic Induction

• Generators and alternators
• Transformers
• Induction motors
• Induction heating
• Wireless charging
• Metal detectors
• RFID technology

Fleming’s Rules

Fleming’s Left-Hand Rule (Motor Rule)

Used to determine the direction of force on a current-carrying conductor in a magnetic field (applied in motors).

When the thumb, forefinger, and middle finger of the left hand are held mutually perpendicular:

• Thumb: Direction of Force/Motion
• Forefinger: Direction of Magnetic Field (North to South)
• Middle finger: Direction of Current

Fleming’s Right-Hand Rule (Generator Rule)

Used to determine the direction of induced current in a conductor moving in a magnetic field (applied in generators).

When the thumb, forefinger, and middle finger of the right hand are held mutually perpendicular:

• Thumb: Direction of Motion of conductor
• Forefinger: Direction of Magnetic Field (North to South)
• Middle finger: Direction of Induced Current

B-H Curve

The B-H curve (magnetization curve) shows the relationship between magnetic flux density (B) and magnetic field intensity (H) for a particular material.

Key Points on the B-H Curve

• Origin (0,0): Unmagnetized state
• Linear Region: B increases linearly with H (initial permeability)
• Knee Region: Transition between linear and saturation regions
• Saturation Region: Further increase in H produces minimal increase in B

Hysteresis Loop

When a ferromagnetic material is subjected to a complete cycle of magnetization, the B-H relationship forms a closed loop called a hysteresis loop.

Key Points on the Hysteresis Loop:

• Saturation Points: Maximum magnetization in either direction
• Remanence (Br): Remaining flux density when H is reduced to zero
• Coercivity (Hc): Reverse field needed to reduce B to zero
• Hysteresis Loss: Energy lost as heat during each magnetization cycle (proportional to the area of the loop)

Classification of Magnetic Materials Based on B-H Curves

• Soft Magnetic Materials: Narrow hysteresis loop, easily magnetized and demagnetized, low hysteresis loss
  • Applications: Transformers, electromagnets, relays
  • Examples: Silicon steel, soft iron, permalloy
• Hard Magnetic Materials: Wide hysteresis loop, difficult to magnetize and demagnetize, high hysteresis loss
  • Applications: Permanent magnets
  • Examples: Alnico, ferrites, neodymium-iron-boron

RLC Circuit

An RLC circuit contains a resistor (R), inductor (L), and capacitor (C) connected in series or parallel.

Series RLC Circuit

In a series RLC circuit, the same current flows through each component.

Impedance:

Resonant Frequency:

Quality Factor (Q):

Bandwidth:

Parallel RLC Circuit

In a parallel RLC circuit, the same voltage appears across each component.

Admittance:

Resonant Frequency:

Series Resonance

Series resonance occurs when X_L = X_C, resulting in:

• Impedance is minimum (Z = R)
• Current is maximum
• Circuit is resistive (unity power factor)
• Voltages across L and C can be much higher than source voltage

Parallel Resonance

Parallel resonance occurs when B_L = B_C (or when inductive and capacitive currents are equal), resulting in:

• Impedance is maximum
• Line current is minimum
• Circuit is resistive (unity power factor)
• Circulating current between L and C can be much higher than line current

Applications of RLC Circuits

• Tuning circuits in radio receivers
• Filters (low-pass, high-pass, band-pass, band-stop)
• Oscillators
• Power factor correction
• Impedance matching networks

Working Principle of DC Generator

A DC generator operates on Faraday’s laws of electromagnetic induction, converting mechanical energy into electrical energy.

Basic Working Principle

1. The armature conductors rotate in a magnetic field produced by the field windings or permanent magnets
2. As the conductors cut through the magnetic flux, an EMF is induced in them according to Faraday’s law
3. The direction of the induced EMF is determined by Fleming’s right-hand rule
4. The commutator converts the alternating EMF in the armature into direct EMF at the brushes

Parts of a DC Generator

• Field System: Creates a magnetic field using field poles, pole shoes, and field windings
• Armature: Rotating part where EMF is induced, consists of core, windings, and commutator
• Commutator: Converts AC to DC, consists of copper segments insulated from each other and the shaft
• Brushes: Collect current from the commutator and transfer it to the external circuit
• Yoke: Provides mechanical support and forms part of the magnetic circuit
• Bearings: Support the rotating shaft
• End Shields: Protect the internal parts

Types of DC Generators

DC generators are classified based on the method of field excitation:

1. Separately Excited DC Generator

The field winding is excited by an independent external DC source.

• Field circuit is separate from the armature circuit
• Field current can be controlled independently
• Used in applications requiring precise voltage control

2. Self-Excited DC Generator

The field winding is connected to the generator’s own armature terminals.

Self-excited generators are further classified into three types:

a. Series Generator

• Field winding is connected in series with the armature winding
• The entire load current passes through the field winding
• Field winding has few turns of thick wire
• Output voltage varies significantly with load current
• Used in applications requiring increasing voltage with load (e.g., series arc lighting)

b. Shunt Generator

• Field winding is connected in parallel (shunt) with the armature winding
• Field winding has many turns of thin wire
• Field current is a small fraction of the total current
• Provides relatively constant voltage under varying load conditions
• Used in applications requiring fairly constant voltage (e.g., lighting, battery charging)

c. Compound Generator

• Has both series and shunt field windings
• Combines characteristics of both series and shunt generators
• Can be designed to maintain nearly constant voltage or to increase voltage with load

Compound generators can be further classified as:

• Cumulative Compound: Series field aids shunt field (more common)
• Differential Compound: Series field opposes shunt field
• Long Shunt: Shunt field is connected across both armature and series field
• Short Shunt: Shunt field is connected across armature only

EMF Equation of DC Generator

The EMF equation gives the theoretical value of the EMF generated in a DC generator.

This equation can also be written as:

Factors Affecting Generated EMF

• Flux per pole (Φ): Depends on field current and magnetic circuit design
• Speed of rotation (N): Directly proportional to generated EMF
• Number of conductors (Z): More conductors mean higher EMF
• Winding arrangement (A): Affects the number of parallel paths

Terminal Voltage

The terminal voltage (V) differs from the generated EMF (E) due to the armature voltage drop:

Characteristics of DC Generators

1. Series Generator Characteristics

Open Circuit Characteristic (OCC):

The OCC is identical for all types of DC generators. It shows the relationship between the generated EMF at no load and the field current.

Internal Characteristic:

Shows the relationship between the generated EMF (E) and the armature current (I_a). For a series generator, as load current increases:

• Initially, generated EMF increases rapidly due to increased field flux
• Eventually, the rate of increase reduces due to magnetic saturation

External Characteristic (Load Characteristic):

Shows the relationship between terminal voltage (V) and load current (I_L). Terminal voltage is lower than generated EMF due to:

• Armature resistance drop (I_aR_a)
• Armature reaction

2. Shunt Generator Characteristics

Internal Characteristic:

Shows the relationship between the generated EMF (E) and the armature current (I_a). As armature current increases:

• Generated EMF decreases slightly due to armature reaction

External Characteristic:

Shows the relationship between terminal voltage (V) and load current (I_L). Terminal voltage decreases with increasing load current due to:

• Armature resistance drop (I_aR_a)
• Armature reaction
• Reduced field current (as terminal voltage drops)

3. Compound Generator Characteristics

Cumulative Compound Generator:

External characteristic shows that terminal voltage:

• Can be made nearly constant at all loads (flat-compounded)
• Can increase with load (over-compounded)
• Can decrease slightly with load (under-compounded)

Differential Compound Generator:

Terminal voltage decreases rapidly with increasing load current as the series field opposes the shunt field.

Commutation in DC Generators

Commutation is the process of current reversal in armature conductors as they pass from one pole to another. It occurs when a specific armature coil is shorted by a brush as it moves from under one pole to another.

Process of Commutation

1. As a coil moves under a brush, it is short-circuited by the brush
2. The current in the coil must reverse direction during this period
3. Ideally, this reversal should be linear over the commutation period
4. After passing the brush, the coil becomes part of the next parallel path with current flowing in the opposite direction

Problems in Commutation

• Reactance Voltage: Self-induced EMF in the coil due to the change of current opposes the reversal
• Sparking: When commutation is not completed within the brushes’ contact period

Improving Commutation

• Brush Position Adjustment: Shifting the brushes to place the commutating coils in a suitable field
• Interpoles (Commutating Poles): Small auxiliary poles placed between main poles to create a field that neutralizes the reactance voltage
• Compensating Windings: Embedded in pole faces to neutralize armature reaction effects
• High-Resistance Brushes: Help to reduce circulating currents during commutation

Efficiency of DC Generator

The efficiency of a DC generator is the ratio of output power to input power, expressed as a percentage.

Types of Losses in DC Generator

1. Copper Losses (Electrical Losses):

• Armature Copper Loss: I_a²R_a (Power lost due to armature resistance)
• Field Copper Loss:
  • For shunt field: I_sh²R_sh or V×I_sh
  • For series field: I_se²R_se
• Brush Contact Loss: Due to the contact resistance between brushes and commutator

2. Iron Losses (Magnetic Losses):

• Hysteresis Loss: Due to the reversal of magnetization in the armature core
• Eddy Current Loss: Due to currents induced in the armature core

3. Mechanical Losses:

• Friction Losses: Bearing friction and brush friction
• Windage Loss: Due to air friction with rotating parts

4. Stray Load Loss: Additional losses that occur when the machine is loaded

Conditions for Maximum Efficiency

A DC generator operates at maximum efficiency when the variable losses equal the constant losses:

• Variable Losses: Mainly copper losses that vary with load
• Constant Losses: Iron losses and mechanical losses that remain approximately constant

Regulation of DC Generator

Voltage regulation is the measure of how well a generator maintains its output voltage as the load changes.

Regulation for Different Types of Generators

1. Separately Excited Generator:

• Moderate regulation
• Voltage drops with load due to armature resistance and armature reaction

2. Shunt Generator:

• Poor regulation (high percentage)
• Voltage drops significantly with load due to armature resistance, armature reaction, and reduced field current

3. Series Generator:

• Negative regulation
• Voltage increases with load (up to a point)

4. Compound Generator:

• Under-compounded: Voltage decreases slightly with load (positive regulation)
• Flat-compounded: Voltage remains nearly constant (almost zero regulation)
• Over-compounded: Voltage increases with load (negative regulation)

Applications of DC Generators

Applications of Separately Excited Generators

• Laboratory power supplies
• Applications requiring precise voltage control
• Excitation systems for large AC generators
• Industrial processes requiring adjustable DC voltage

Applications of Series Generators

• Series arc lighting (historical)
• Boosters in DC distribution systems
• Constant current applications

Applications of Shunt Generators

• Battery charging
• Power supply for constant voltage applications
• Electroplating
• Small lighting loads

Applications of Compound Generators

• Flat-compounded: General lighting and power supply
• Over-compounded: Power distribution with voltage drop compensation
• Under-compounded: Special applications requiring slight voltage drop with load

General Applications of DC Generators

• Welding generators
• Auxiliary power in automobiles and aircraft
• Emergency power supply
• Traction systems (historically)
• Excitation of large synchronous generators
• Field supply for DC motors

Principle of DC Motors

A DC motor converts electrical energy into mechanical energy using the principles of electromagnetism. It operates on the basic principle that when a current-carrying conductor is placed in a magnetic field, it experiences a force.

Basic Working Principle

1. The magnetic field is created by either permanent magnets or electromagnets (field windings)
2. Current is supplied to the armature conductors through brushes and commutator
3. When the current flows through the armature conductors, they experience a force according to Fleming’s Left-Hand Rule
4. This force produces a torque, causing the armature to rotate
5. The commutator ensures that the current direction in conductors under a pole is always such that the torque is produced in the same direction

Back EMF

As the armature rotates in the magnetic field, it also acts as a generator and produces an EMF (electromotive force) that opposes the applied voltage. This is called Back EMF or Counter EMF.

Net Voltage Equation

Torque Development

Types of DC Motors

DC motors are classified based on the connection of field winding with respect to the armature:

1. Separately Excited Motor

• Field winding is excited by a separate DC source
• Field circuit is independent of the armature circuit
• Provides excellent speed control
• Used in applications requiring precise speed control

2. Self-Excited Motors

Self-excited motors use their own armature voltage to energize the field circuit. They are further classified into:

a. Series Motor

• Field winding is connected in series with the armature
• Same current flows through field and armature
• Field winding has few turns of thick wire to carry the full armature current
• Produces high starting torque
• Speed varies widely with load (high at light loads, low at heavy loads)
• Applications: Electric traction, cranes, hoists, electric vehicles

b. Shunt Motor

• Field winding is connected in parallel (shunt) with the armature
• Field winding has many turns of thin wire
• Field current is much smaller than armature current
• Nearly constant speed with varying load
• Moderate starting torque
• Applications: Machine tools, fans, blowers, centrifugal pumps, conveyors

c. Compound Motor

• Has both series and shunt field windings
• Combines characteristics of both series and shunt motors
• Available in two configurations:
  • Cumulative Compound: Series field aids shunt field, providing higher starting torque than shunt motor and better speed regulation than series motor
  • Differential Compound: Series field opposes shunt field, resulting in more constant speed but poor starting torque
• Further classified as:
  • Long Shunt: Shunt field connected across both armature and series field
  • Short Shunt: Shunt field connected across armature only
• Applications: Elevators, rolling mills, punching machines, presses

3. Permanent Magnet DC Motor

• Uses permanent magnets instead of field windings
• No field winding power consumption
• Compact size and lighter weight
• Linear torque-speed characteristics
• Applications: Automotive accessories, toys, small appliances, computer peripherals

Characteristics of DC Motors

1. Torque-Armature Current Characteristics

Shows the relationship between torque and armature current.

a. Series Motor:

• Torque is approximately proportional to the square of the armature current at low currents (T ∝ I_a²)
• At higher currents, as magnetic saturation occurs, torque becomes nearly proportional to current (T ∝ I_a)

b. Shunt Motor:

• Torque is directly proportional to armature current (T ∝ I_a)
• Linear relationship as field flux remains nearly constant

c. Compound Motor:

• Cumulative Compound: Torque increases more rapidly with current than in a shunt motor
• Differential Compound: Torque increases less rapidly with current than in a shunt motor

2. Speed-Armature Current Characteristics

Shows the relationship between speed and armature current.

a. Series Motor:

• Speed is inversely proportional to the square root of the armature current at low currents
• At higher currents, speed is inversely proportional to current
• Speed decreases significantly as load (current) increases
• Dangerously high speeds at very light loads

b. Shunt Motor:

• Speed decreases slightly with increasing armature current
• Nearly constant speed operation (speed regulation typically 5-10%)
• Small speed drop due to armature reaction and armature voltage drop

c. Compound Motor:

• Cumulative Compound: Speed drops more with load than in a shunt motor
• Differential Compound: Speed increases with load (not commonly used)

3. Torque-Speed Characteristics

Shows the relationship between torque and speed.

a. Series Motor:

• High torque at low speeds
• Torque decreases rapidly as speed increases
• Hyperbolic relationship (T ∝ 1/N²)

b. Shunt Motor:

• Almost flat speed-torque curve
• Small decrease in speed as torque increases
• Good speed regulation

c. Compound Motor:

• Cumulative Compound: Intermediate between series and shunt characteristics
• Differential Compound: Speed increases as torque increases (unstable operation)

Commutation in DC Motors

Commutation is the process of current reversal in armature conductors as they pass from one pole to another. Proper commutation is essential for sparkless operation of DC motors.

Process of Commutation

1. As a coil moves under a brush, it is short-circuited by the brush
2. During this short-circuit period, the current in the coil must reverse direction
3. The current should ideally change linearly from one value to the opposite value
4. After passing the brush, the coil becomes part of the next parallel path with current flowing in the opposite direction

Factors Affecting Commutation

• Reactance Voltage: Self-induced EMF in the commutating coil due to rapid current change
• Armature Reaction: Distortion of main field by armature field
• Contact Resistance: Between brush and commutator
• Brush Width and Material: Affects the commutation period and resistance

Improving Commutation

• Interpoles (Commutating Poles): Small auxiliary poles placed between main poles to generate a voltage equal and opposite to the reactance voltage
• Compensating Windings: Embedded in pole faces to neutralize armature reaction
• Brush Shift: Adjusting brush position to place commutating coils in a suitable field (less common in motors than generators)
• High-Resistance Brushes: Help to reduce circulating currents during commutation
• Optimized Commutator Design: Proper segment width, insulation, surface finish

DC Motor Starters

When a DC motor starts, the armature has no back EMF (as it’s not rotating yet), resulting in a very high initial current that can damage the motor. Starters limit this starting current to safe values.

Need for Starters

Without a starter, the initial current can be:

Types of DC Motor Starters

1. Three-Point Starter

Used for shunt and compound motors.

• Components:
  • Starting resistance in series with armature
  • No-volt release (NVR) coil
  • Overload release (OLR) mechanism
  • Handle with spring return
• Three Points: Line (L), Armature (A), and Field (F)
• Operation:
  • Starting position: Full resistance in armature circuit
  • As handle is moved clockwise, resistance is gradually cut out
  • Final position: Zero resistance in armature circuit
  • NVR holds handle in run position as long as field current flows
  • If supply fails or field circuit opens, NVR releases handle to starting position
• Limitation: Not suitable for variable speed operation by field control, as weakening the field would cause the NVR to release the handle

2. Four-Point Starter

Modification of three-point starter to allow field weakening for speed control.

• Components: Similar to three-point starter
• Four Points: Line (L), Armature (A), Field (F), and No-volt release (N)
• Key Difference: NVR coil is connected directly across the supply instead of in series with the field circuit
• Advantage: Field current can be varied without affecting the NVR, allowing speed control by field regulation

3. Face-Plate Starter

Used for series motors where high starting currents are expected.

• Similar principle to three-point starter but designed for higher currents
• Often used in traction applications

4. Automatic Starters

• Use time-delay relays or current-sensing relays
• Automatically control the acceleration process
• Provide better protection than manual starters
• Used in applications requiring remote control or frequent starts

Speed Control of DC Shunt Motor

The speed of a DC shunt motor can be controlled by changing either the armature circuit resistance or the field current.

Basic Speed Equation

1. Armature Control Method

Involves varying the voltage applied to the armature circuit while keeping field current constant.

a. Armature Resistance Control

• Method: Insert variable resistance in series with armature
• Effect: Increases (I_aR_a) drop, reducing effective armature voltage
• Speed Range: Only speeds below base speed are possible
• Advantages: Simple and inexpensive
• Disadvantages:
  • Poor speed regulation
  • Energy wastage in the resistance
  • Speed depends on load
• Applications: Small motors where efficiency is not critical

b. Armature Voltage Control

• Method: Vary the voltage supplied to the armature (using variable voltage source or power electronics)
• Effect: Changes the applied voltage (V)
• Speed Range: Only speeds below base speed are possible
• Advantages:
  • Good speed regulation
  • Higher efficiency than resistance control
  • Smooth control
• Disadvantages: More expensive, requires additional equipment
• Applications: Machine tools, conveyor drives, process control

2. Field Control Method

Involves varying the field current to change the field flux while keeping armature voltage constant.

a. Field Resistance Control

• Method: Insert variable resistance in series with field winding
• Effect: Decreases field current and flux (Φ)
• Speed Range: Only speeds above base speed are possible
• Advantages:
  • Simple and inexpensive
  • High efficiency as power loss in field circuit is small
  • Wide speed range
• Disadvantages:
  • Reduced torque capability at higher speeds
  • Cannot reduce speed below base speed
  • Field weakening limits (stability issues at very weak fields)
• Applications: Machine tools, winders, printing presses

b. Field Diverter Method

• Method: Connect variable resistance in parallel with field winding
• Effect: Diverts some current away from field, decreasing field flux
• Speed Range: Similar to field resistance control
• Applications: Alternative to field resistance control

Combined Methods

For wide-range speed control, both armature and field control methods can be combined:

• Armature voltage control for speeds below base speed
• Field weakening for speeds above base speed

Troubleshooting, Care and Maintenance

Common Problems and Troubleshooting

Care and Maintenance

Regular Inspection

• Check for unusual noise, vibration, or smell
• Monitor operating temperature
• Inspect mounting bolts and coupling alignment
• Check electrical connections for tightness

Brush Maintenance

• Check brush wear and replace when worn to about 50% of original length
• Ensure proper brush pressure (typically 1.5-2.0 psi)
• Verify proper seating on commutator
• Keep brush holders clean and free from carbon dust
• Maintain proper clearance between brush holder and commutator (3-5 mm)

Commutator Maintenance

• Clean regularly with fine sandpaper (never emery cloth)
• Check for roughness, grooving, or out-of-round condition
• Resurface if necessary
• Check for high mica (insulation between segments) and undercut if needed

Bearing Maintenance

• Lubricate according to manufacturer’s schedule
• Use recommended type and quantity of lubricant
• Check for bearing noise, overheating, or excessive play
• Replace bearings as needed

Cooling System

• Keep air passages clean and unobstructed
• Clean cooling fan
• Ensure proper functioning of any cooling systems

Insulation

• Perform periodic insulation resistance tests
• Keep windings clean and dry
• Check for signs of overheating or insulation breakdown

General Cleaning

• Remove dust and dirt, especially from ventilation openings
• Clean with dry, compressed air (low pressure)
• Avoid using liquid cleaners on windings unless specifically designed for electrical equipment

Applications of DC Motors

Applications of Series Motors

• Electric Traction: Trains, trolleys, and trams, due to high starting torque and speed variation with load
• Hoists and Cranes: High starting torque for lifting heavy loads
• Electric Vehicles: Historically used in electric cars and forklifts
• Conveyors: For heavy materials requiring high starting torque
• Elevators: Especially in older installations
• Heavy Duty Starters: For internal combustion engines

Applications of Shunt Motors

• Machine Tools: Lathes, milling machines, drill presses, where constant speed is required
• Centrifugal Pumps: Requiring constant speed operation
• Fans and Blowers: For continuous operation at constant speed
• Printing Presses: Where speed control and constant speed are important
• Woodworking Machinery: Requiring uniform cutting speed
• Textile Machinery: For consistent operation
• Conveyors: For lighter loads requiring constant speed

Applications of Compound Motors

• Presses: Requiring both high starting torque and reasonable speed regulation
• Rolling Mills: For metal processing
• Shears and Punches: For metal working
• Elevators: Modern installations requiring controlled starting and running
• Reciprocating Pumps: To handle load fluctuations
• Conveyors: For variable load applications

Applications of Permanent Magnet DC Motors

• Automotive Applications: Windshield wipers, power windows, seat adjusters
• Computer Peripherals: Disk drives, printers, scanners
• Small Appliances: Mixers, blenders, hand tools
• Toys: Remote-controlled cars, robots
• Robotics: Positioning and movement systems
• Medical Equipment: Precision instruments

Applications Based on Control Method

• Armature Voltage Control: Applications requiring speed control below base speed
  • Machine tools
  • Process control
  • Precision positioning
• Field Control: Applications requiring speed control above base speed
  • Winders
  • Printing presses
  • Paper machines
• Combined Control: Applications requiring wide speed range
  • Machine tool drives
  • Steel mill drives
  • Test equipment

Calculation for Work, Power & Energy

Work

Work is done when a force causes displacement of an object.

In electrical terms, work is done when charge moves through a potential difference:

Power

Power is the rate of doing work or the rate of energy transfer.

In electrical circuits, power can be calculated by several equivalent formulas:

In AC circuits, power involves the phase relationship between voltage and current:

Energy

Energy is the capacity to do work. It is power integrated over time.

Common units of electrical energy:

• Joule (J): Base unit of energy
• Watt-hour (Wh): Energy consumed by a 1-watt device in 1 hour (3600 J)
• Kilowatt-hour (kWh): 1000 watt-hours, commonly used for billing
• Megawatt-hour (MWh): 1000 kilowatt-hours, used for larger systems

Electrical Energy Calculation Examples

1. Daily energy consumption of a 100W bulb operating for 5 hours:

   E = P × t = 100W × 5h = 500Wh = 0.5kWh

2. Monthly energy consumption of a 1500W heater used 2 hours daily:

   E = 1.5kW × 2h × 30 days = 90kWh

3. Energy consumed by a 230V device drawing 10A for 45 minutes:

   P = V × I = 230V × 10A = 2300W

   E = 2300W × 0.75h = 1725Wh = 1.725kWh

Power Factor

Power factor is the ratio of active (real) power to apparent power in an AC circuit. It indicates how efficiently electrical power is being used.

Types of Power in AC Circuits

• Active Power (P): The actual power consumed to do useful work, measured in watts (W)
• Reactive Power (Q): Power that oscillates between source and load without doing useful work, measured in volt-amperes reactive (VAR)
• Apparent Power (S): The product of RMS voltage and RMS current, measured in volt-amperes (VA)

Nature of Power Factor

• Unity Power Factor (PF = 1): Current and voltage are in phase, all power is active
• Lagging Power Factor (0 < PF < 1): Current lags voltage, typical of inductive loads
• Leading Power Factor (0 < PF < 1): Current leads voltage, typical of capacitive loads

Typical Power Factor Values

Causes & Effects of Low Power Factor

Causes of Low Power Factor

1. Inductive Loads: Motors, transformers, and induction furnaces create lagging power factor
2. Motor Operation at Light Load: Motors draw magnetizing current regardless of load
3. Transformer Operation at Light Load: Similar to motors, they draw magnetizing current
4. Arc Lamps and Discharge Lighting: Inherently have low power factor due to their operating characteristics
5. Induction Heating Equipment: Creates large inductive component
6. Welding Equipment: Arc welders and resistance welders
7. Harmonic Currents: Non-linear loads like variable frequency drives and electronic equipment

Effects of Low Power Factor

For Consumers:

1. Increased Electricity Bills: Many utilities impose penalties for low power factor
2. Reduced Distribution System Capacity: More current is needed for the same active power
3. Increased Voltage Drop: Can cause poor performance of equipment
4. Need for Larger Equipment: Cables, transformers, and switchgear must be sized for apparent power

For Utilities:

1. Reduced Generation Capacity: Generators must be sized for apparent power
2. Increased Transmission Losses: Higher current causes more I²R losses
3. Reduced Transformer Capacity: Transformers must handle larger apparent power
4. Stability Issues: Low power factor can affect system stability
5. Poor Voltage Regulation: System voltage becomes more difficult to maintain

Methods of Improving Power Factor

Improving power factor reduces reactive power demand, increases system capacity, and reduces energy costs.

Static Capacitors

• Principle: Capacitors provide leading reactive power to counteract lagging reactive power of inductive loads
• Advantages:
  • Low maintenance (no moving parts)
  • Easy installation
  • Flexible capacity (can be added in steps)
  • Low losses
  • Can be installed at any voltage level
• Installation Methods:
  • Fixed Capacitors: Permanently connected
  • Switched Capacitors: Connected through contactors based on load requirements
  • Automatically Controlled Banks: Switched based on power factor measurements
• Installation Locations:
  • Individual Compensation: Connected directly to inductive equipment
  • Group Compensation: For a group of small motors or loads
  • Centralized Compensation: At main distribution board or substation