Study guide: Electrical Machines and Electronic Devices

📋 Course Outline

1. Basic Circuit Laws and Quantities
2. Network Theorems and Analysis
3. AC Circuits and Wiring Systems
4. Electrical Machines
5. DC Motor Working Principles
6. Single-Phase Induction Motors
7. DC Generator Excitation Types
8. DC Shunt Motor Characteristics
9. Transformer Tests and Construction
10. DC Machines and Induction Motors
11. Alternators and Semiconductor Diodes
12. PN Junction Temperature Effects
13. Operational Amplifiers and Converters
14. BJT Construction and Operation
15. BJT Configurations and Amplifiers
16. Transducers and Their Classification
17. Measurement Transducers and Oscilloscopes
18. Instrument Transformers

📖 1. Basic Circuit Laws and Quantities

🔑 Key Concepts & Definitions

• Electric current: The time rate of change of charge, measured in amperes: $i=\frac{dq}{dt}$.

★ Must-know

• Ohm’s law states that, at constant temperature, the voltage across a resistor is directly proportional to the current through it: $V=IR$.

• Kirchhoff’s current law states that the algebraic sum of currents entering a node or closed boundary is zero, so currents entering equal currents leaving.

• Kirchhoff’s voltage law states that the algebraic sum of all voltages around a closed path is zero, so voltage drops equal voltage rises.

Further detail

📌 A network is the interconnection of two or more simple circuit elements, whereas a circuit is a network containing at least one closed path.

📐 Formula - For an electric iron rated 1000 W and 240 V, the heating-element resistance is $R=\frac{V^2}{P}=57.6\,\Omega$ and the current is $I=\frac{V}{R}=4.166\,\mathrm{A}$.

💡 Memory Hook

KCL for currents at nodes, KVL for voltages around loops

📖 2. Network Theorems and Analysis

★ Must-know

• 🔄 Nodal analysis proceeds by:
  1. selecting a reference node
  2. assigning voltages to the remaining nodes
  3. applying KCL to each unknown-voltage node
  4. solving the simultaneous equations

• Mesh analysis assigns mesh currents, applies KVL to each mesh, and solves the resulting simultaneous equations for the mesh currents.

• Thevenin’s theorem states that any linear bilateral network can be replaced by a single voltage source $V_{TH}$ in series with a single impedance $Z_{TH}$.

• Norton’s theorem states that any linear bilateral network can be replaced by a single current source in parallel with a single impedance $Z_{TH}$.

• Maximum power is transferred to a load impedance when the load impedance is the complex conjugate of the source impedance.

Further detail

📌 A loop is a closed path in a network, whereas a mesh is an elementary closed path that contains no other loop within it.

💡 Memory Hook

Thevenin uses a voltage source in series, whereas Norton uses a current source in parallel

📖 3. AC Circuits and Wiring Systems

🔑 Key Concepts & Definitions

• Impedance: The opposition offered by a circuit to the flow of alternating current, denoted by Z and measured in ohms.
• Resonance: The condition in which the applied voltage and resulting current are in phase and the power factor is unity.

★ Must-know

📌 In series resonance, inductive and capacitive reactances are equal and opposite, so $X_L=X_C$; in parallel resonance, the imaginary part of total admittance is zero and the power factor is unity.

📐 Formula - AC circuits use real power $P=EI\cos\theta$, reactive power $Q=EI\sin\theta$, and apparent power $S=EI$.

📌 For a balanced three-phase star connection, $V_{ph}=V_L/\sqrt{3}$ and $I_{ph}=I_L$, whereas for a delta connection, $V_{ph}=V_L$ and $I_{ph}=I_L/\sqrt{3}$.

• Safe installation requires:
  • using protective equipment
  • identifying exposed live parts
  • disconnecting the supply before wiring
  • controlling the supply with a circuit breaker
  • keeping conductive tools away from live parts
  • using correctly rated non-conductive tools

Further detail

📐 Formula - Complex power is $S=P+jQ$, with real power as its real part and reactive power as its imaginary part.

💡 Memory Hook

Inductive loads make current lag, whereas capacitive loads make current lead

📖 4. Electrical Machines

🔑 Key Concepts & Definitions

• Transformer: Operates by mutual induction between inductively coupled coils: an AC source produces flux in the core that links both coils and induces an EMF in the secondary coil.

★ Must-know

📐 Formula - The EMF equations of a transformer are $E_1=4.44N_1fB_mA$ and $E_2=4.44N_2fB_mA$.

📌 A synchronous motor runs at synchronous speed independently of load, whereas an induction motor runs below synchronous speed and its speed decreases when load increases.

📌 A single-phase induction motor is not self-starting because its stator winding produces an alternating flux along one space axis rather than a synchronously revolving magnetic field.

📐 Formula - The electromagnetic force on a current-carrying conductor in a magnetic field is $F=BIl$.

Further detail

📌 The commutator of a DC motor converts alternating EMF into unidirectional EMF, while the brushes collect current from the commutator.

• DC motor applications include:
  • constant-speed drives
  • machine tools
  • electric locomotives
  • cranes
  • elevators
  • rolling mills

💡 Memory Hook

A synchronous motor runs at synchronous speed, whereas an induction motor runs below it

📖 5. DC Motor Working Principles

🔑 Key Concepts & Definitions

• DC motor: An electrical machine that converts direct-current electrical energy into mechanical energy through the force on a current-carrying conductor in a magnetic field.

📝 Essential Points

📌 A current-carrying conductor placed in a magnetic field experiences a mechanical force whose direction is given by Fleming’s left-hand rule.

📐 Formula - The force on a current-carrying conductor is $F = BIl$ newtons, where B is magnetic flux density in weber per square metre, I is current in amperes, and l is conductor length in metres.

• In a basic DC motor, current flows from the source through the brushes and commutator into the loop, whose magnetic field interacts with the permanent-magnet field and makes the loop rotate.

💡 Memory Hook

Current in a magnetic field → mechanical force → rotation

📖 6. Single-Phase Induction Motors

🔑 Key Concepts & Definitions

• Single-phase induction motor: An AC motor whose stator is supplied by a single-phase source and whose squirrel-cage rotor receives induced current from the alternating stator flux.

★ Must-know

• 🔄 The induction process is:

  1. The stator is connected to a single-phase supply.
  2. Alternating stator flux is produced.
  3. Current is induced in the squirrel-cage rotor bars.
  4. The rotor current produces alternating rotor flux.

• A single-phase induction motor is not self-starting because the two alternating fluxes do not produce a starting torque from rest; an external initial rotation allows it to accelerate and continue running.

• Self-starting methods include:

  • split-phase operation
  • capacitor-start operation
  • capacitor-start capacitor-run operation
  • permanent-split-capacitor operation
  • shaded-pole operation

• In a shaded-pole motor, the alternating flux induces current in the short-circuited shaded coil, causing the shaded-pole flux to lag the unshaded-pole flux and producing a rotating resultant flux.

Further detail

• Single-phase motors are widely used in homes, offices, and workshops because the available supply is generally single phase and these motors are reliable, inexpensive, simple to construct, and easy to repair.

• In a split-phase motor, the auxiliary winding is disconnected by a centrifugal switch when the motor reaches approximately 75–80% of synchronous speed.

💡 Memory Hook

Single-phase flux alone fails to start; an auxiliary phase difference enables starting

📖 7. DC Generator Excitation Types

🔑 Key Concepts & Definitions

• Separately excited generator: A field winding energized by a separate external voltage source, with output voltage depending on armature speed and field current.

★ Must-know

• A self-excited DC generator obtains field-winding excitation from the generator’s own output and is classified as series, shunt, or compound.

• In a series generator, the low-resistance field winding is connected in series with the armature and load, whereas in a shunt generator, the high-resistance field winding is connected in parallel with the armature.

• In a short-shunt compound generator, the shunt field is connected across the armature while the series field is connected in series with the load, whereas in a long-shunt generator, the shunt field is connected across both the armature and series field.

Further detail

📌 A compound generator has both shunt and series field windings and may use either a short-shunt or long-shunt connection.

💡 Memory Hook

SSC: Series, Shunt, Compound

📖 8. DC Shunt Motor Characteristics

★ Must-know

• The important shunt-motor characteristics are:
  • torque versus armature current
  • speed versus armature current
  • speed versus torque

📐 Formula - For a DC motor, armature torque is proportional to the product of flux and armature current, $T_a \propto \Phi I_a$.

• Because the shunt-field flux is approximately constant, the torque-versus-armature-current characteristic of a DC shunt motor is a straight line through the origin.

• The speed of a DC shunt motor decreases only slightly as load increases because the back emf and flux decrease marginally due to armature resistance drop and armature reaction.

Further detail

📌 A DC shunt motor should not be started at full load because starting a heavy load requires a very large armature current.

💡 Memory Hook

Torque rises with armature current, whereas speed remains nearly constant

📖 9. Transformer Tests and Construction

🔑 Key Concepts & Definitions

• Transformer: A static device that transfers electrical energy between circuits by electromagnetic induction without changing frequency while changing voltage and current magnitudes.

★ Must-know

• In the transformer open-circuit test, rated-frequency voltage is gradually applied to the low-voltage side with the high-voltage side open, and voltmeter, ammeter, and wattmeter readings are recorded at rated low-voltage voltage.

• In the transformer short-circuit test, rated-frequency voltage is gradually applied to the high-voltage side with the low-voltage side short-circuited until rated high-voltage current flows, and all instrument readings are recorded.

• In the transformer short-circuit test, the wattmeter reading is approximately equal to full-load copper loss because the applied voltage and core loss are negligible.

Further detail

• The principal transformer parts are:
  • the core
  • the primary winding
  • the secondary winding
  • the enclosure

💡 Memory Hook

Open circuit reveals core loss; short circuit reveals copper loss

📖 10. DC Machines and Induction Motors

🔑 Key Concepts & Definitions

• DC machine: A rotating electrical machine that can operate as either a DC generator or a DC motor without fundamental constructional changes.

★ Must-know

📌 A DC generator converts mechanical energy into electrical energy, whereas a DC motor converts DC electrical energy into mechanical energy.

📌 A DC shunt motor has its field winding connected in parallel with the armature, a DC series motor has its field winding connected in series with the armature, and a compound motor has both parallel and series field windings.

• The main DC-machine parts are:
  • the yoke
  • the poles and pole shoes
  • the field winding
  • the armature core
  • the armature winding
  • the commutator
  • the brushes

📌 In a DC generator, the commutator reverses armature connections at the appropriate instant so that alternating current in the armature conductors becomes unidirectional current at the terminals.

Further detail

• The yoke provides mechanical strength and carries magnetic flux, while pole shoes support the field coils and spread flux uniformly across the air gap.

• The armature core is a laminated cylindrical rotor with slots for the armature winding, and its laminations reduce eddy-current losses.

💡 Memory Hook

Generator converts mechanical to electrical energy; motor converts electrical to mechanical energy

📖 11. Alternators and Semiconductor Diodes

🔑 Key Concepts & Definitions

• Induction motor: An asynchronous motor that does not run at synchronous speed and mainly consists of a stationary stator and a rotating rotor.
• Zener breakdown: Occurs in a reverse-biased junction and can provide a constant output voltage used as a reference voltage.
• Peak inverse voltage: The maximum reverse voltage that a diode can withstand before entering the Zener region or being damaged by reverse voltage.

★ Must-know

📐 Formula - For a synchronous generator with P poles, flux per pole Φ, speed N, and Zph conductors per phase, the induced RMS emf per phase is $E_{ph} = 4.44 K_c K_d f \Phi T_{ph}$, where Kc is the coil-span factor, Kd is the distribution factor, and Tph is the series turns per phase.

• The stator of a three-phase induction motor contains an outer frame, a laminated silicon-steel core, and three-phase windings whose pole number determines motor speed.

📌 A squirrel-cage rotor uses uninsulated copper or aluminium bars short-circuited by end rings, whereas a phase-wound rotor uses insulated three-phase windings connected in star and slip rings connected to external resistance.

📌 An ideal diode has zero resistance when forward biased and infinite resistance when reverse biased, whereas a practical diode has non-ideal forward and reverse characteristics.

• 🔄 The PN-junction bias states are:
  1. No bias: the net current is zero.
  2. Forward bias: the depletion region narrows and majority carriers cross the junction.
  3. Reverse bias: the depletion region widens and reverse saturation current remains.

📌 An inverting amplifier produces an output 180 degrees out of phase with its input, whereas a non-inverting amplifier produces an output in phase with its input.

Further detail

• Skewing squirrel-cage rotor conductors reduces humming, provides smoother torque, reduces magnetic locking, and increases rotor resistance because the bars are longer.

• The reverse saturation current of a semiconductor diode approximately doubles for every 10°C increase in temperature, and increasing temperature makes the forward characteristic more nearly ideal.

💡 Memory Hook

No bias → forward bias → reverse bias

📖 12. PN Junction Temperature Effects

★ Must-know

• In forward bias, the applied potential causes electrons and holes to recombine with ions near the junction, reducing the depletion-region width and allowing a heavy majority-carrier flow across the junction.

Further detail

📌 Silicon diodes have much lower reverse saturation currents than germanium diodes at comparable power and current levels.

• As temperature increases, the forward characteristics of a diode become more nearly ideal because the increasing reverse saturation current dominates the smaller percentage change in the thermal-voltage term.

💡 Memory Hook

Higher temperature → higher reverse saturation current → lower threshold voltage

📖 13. Operational Amplifiers and Converters

📝 Essential Points

• Operational amplifiers are used in: audio amplifiers, instrumentation amplifiers, power amplifiers, analog computers, combinations of integrators, differentiators, summing amplifiers, and multipliers

• Typical operational-amplifier parameters include an open-loop voltage gain of $10^5$ to $10^8$, an input resistance of $10^5$ to $10^{13}\ \Omega$, an output resistance of 10 to 100 Ω, and supply voltages from 5 to 30 V and from −30 V to 0 V.

📌 An analog-to-digital converter transforms an analog voltage or current into a binary number, whereas a digital-to-analog converter transforms a binary number into an analog voltage or current.

📐 Formula - The gain of an ideal inverting amplifier is $A_v=\frac{V_o}{V_i}=-\frac{R_f}{R_i}$.

💡 Memory Hook

ADC: analog to binary; DAC: binary to analog

📖 14. BJT Construction and Operation

🔑 Key Concepts & Definitions

• Bipolar junction transistor: a three-terminal device made of doped semiconductor regions that can amplify or switch signals using both electrons and holes

📝 Essential Points

• 🔄 NPN forward-active operation proceeds as follows: electrons are injected from the emitter into the base, electrons diffuse across the thin base, electrons are swept into the collector

• A BJT contains emitter, base, and collector regions, with the base lightly doped and physically located between the emitter and collector.

📌 An NPN transistor turns on when its base is at a higher potential than its emitter, whereas a PNP transistor turns on when its base is at a lower potential than its emitter.

📐 Formula - The common-emitter current gain is approximately $\beta_F=\frac{I_C}{I_B}$, while the common-base current gain is approximately $\alpha_F=\frac{I_C}{I_E}$.

💡 Memory Hook

Emitter injects → base transports → collector collects

📖 15. BJT Configurations and Amplifiers

🔑 Key Concepts & Definitions

• Common-base configuration: connects the base as the common terminal for input and output, applies the input between emitter and base, and takes the output between collector and base
• Active region: the base-emitter junction is forward biased and the base-collector junction is reverse biased, allowing amplification

★ Must-know

• The BJT configurations are: common-base with voltage gain but no current gain, common-emitter with both voltage and current gain, common-collector with current gain but no voltage gain

📌 In cutoff, both BJT junctions are reverse biased and the transistor acts as an open switch with approximately zero collector current, whereas in saturation both junctions are forward biased and the transistor acts as a closed switch.

Further detail

📐 Formula - The BJT current-gain relationships are $\beta=\frac{\alpha}{1-\alpha}$ and $\alpha=\frac{\beta}{\beta+1}$.

💡 Memory Hook

Common-base: voltage gain without current gain; common-emitter: both gains; common-collector: current gain without voltage gain

📖 16. Transducers and Their Classification

🔑 Key Concepts & Definitions

• Transducer: converts a physical quantity or another form of energy into a proportional electrical signal for measurement or control

★ Must-know

📌 Active transducers generate a proportional voltage or current without an external power source, whereas passive transducers require external power and operate through changes in resistance, inductance, or capacitance.

• The main classifications of transducers are:
  • active or passive
  • according to transduction principle
  • analog or digital
  • primary or secondary
  • direct or inverse

Further detail

📌 Analog transducers produce continuous-time outputs, whereas digital transducers produce pulses or coded outputs for discrete sensed values.

• In pressure measurement, a Bourdon tube acts as a primary transducer by converting pressure into displacement, and an LVDT acts as a secondary transducer by converting that displacement into an electrical signal.

💡 Memory Hook

Active transducers generate energy; passive transducers require external power

📖 17. Measurement Transducers and Oscilloscopes

★ Must-know

📐 Formula - The capacitance of a parallel-plate capacitive transducer is $C=\frac{\varepsilon_r\varepsilon_0A}{d}$, where A is plate overlap area and d is plate separation.

• A capacitive transducer measures linear displacement by changing either the separation between plates or their area of overlap.

• A piezoelectric transducer uses materials such as quartz to measure pressure, acceleration, and dynamic forces including oscillation, impact, and high-speed compression or tension.

• In a digital storage oscilloscope, the input is attenuated and amplified, converted into digital data by an analog-to-digital converter, and stored in memory for display or further processing.

Further detail

📌 Resistance thermometers have a positive temperature coefficient, whereas thermistors have a negative temperature coefficient.

💡 Memory Hook

A capacitor, crystal, and resistance coil turn physical changes into electrical signals

📖 18. Instrument Transformers

🔑 Key Concepts & Definitions

• Current transformer: a transformer that produces a low secondary current proportional to the high current flowing in its primary winding
• Potential transformer: a step-down transformer connected in parallel across a line to produce a reduced voltage suitable for measurement by a low-range voltmeter

★ Must-know

• A measuring-instrument current transformer is expected to remain accurate up to 125% of its normal full-load current, whereas a protective current transformer is expected to remain accurate up to about 20 times its normal full-load current.

Further detail

📌 The line voltage measured with a potential transformer is obtained by dividing the voltmeter deflection by the transformation ratio, where the transformation ratio is $V_2/V_1$.

• Potential transformers are constructed as shell type below 5000 V, dry type or oil type from 5000 to 13800 V, and oil type above 13800 V.

💡 Memory Hook

CT measures current in series; PT measures voltage in parallel

📊 Synthesis Tables

Three-Phase Star and Delta Connections

Connection	Phase voltage	Phase current
Star	$V_{ph}=V_L/\sqrt{3}$	$I_{ph}=I_L$
Delta	$V_{ph}=V_L$	$I_{ph}=I_L/\sqrt{3}$

Electrical Machine Speed

Machine	Speed behavior	Starting
Synchronous motor	Runs at synchronous speed	Not self-starting
Induction motor	Runs below synchronous speed	Self-starting
Single-phase induction motor	Does not start from a single-phase alternating field	Not self-starting