Unit 3: Electrical Drawing, Generators, and Transformers

1. Electrical Drawing and Symbols

Electrical drawings are used to represent circuits in a standard way.

Main Electric Circuit Elements

These are the basic building blocks:

• Source: Provides energy (e.g., Battery, AC Voltage Source).
• Path: Conductors (wires) for the current to flow.
• Load: Consumes energy (e.g., Resistor, Lamp, Motor).
• Control: Regulates the flow (e.g., Switch).

Drawing Types

• Blueprints: A general term for any detailed technical drawing (often blue, historically).
• Schematics (Circuit Diagrams): The most important type. Uses standard symbols to show the *function* and *connections* of a circuit, not its physical appearance or layout.
• Ladder Diagrams: A specific type of schematic used for industrial control logic (like in PLCs). It looks like a ladder, with two vertical "rails" (power) and horizontal "rungs" representing control circuits (switches, relays, motors).

Common Drawing Symbols

It's essential to memorize the standard symbols for components.

[Image of common electrical schematic symbols: Resistor, Capacitor, Inductor, Switch, DC Source (Battery), AC Source, Lamp, Motor, Ground, Fuse]

2. DC Circuit Analysis

Rules to Analyze DC Circuits

To analyze a DC circuit, you find the voltage across and current through each component. The main rules are:

1. Ohm's Law: V = IR
2. Kirchhoff's Current Law (KCL): Σ I_in = Σ I_out at any node.
3. Kirchhoff's Voltage Law (KVL): Σ V_rises = Σ V_drops in any loop.

Current and Voltage Drop

When current (I) flows through a resistor (R), it creates a voltage drop (V = IR). This is a loss of electrical potential, which is converted to heat.

Example: In a simple series circuit with a 12V battery and two resistors (R₁=4Ω, R₂=2Ω):

1. Total Resistance: R_eq = R₁ + R₂ = 4 + 2 = 6Ω.
2. Total Current: I = V / R_eq = 12V / 6Ω = 2A.
3. Voltage Drop (R₁): V₁ = I × R₁ = 2A × 4Ω = 8V.
4. Voltage Drop (R₂): V₂ = I × R₂ = 2A × 2Ω = 4V.
5. Check (KVL): 8V + 4V = 12V (Total drop = Total rise).

3. Power Factor

Power Factor (PF) is a concept for AC circuits only.

In AC circuits, we have two types of power:

• True Power (P): The actual power used to do work (e.g., create heat or motion). Measured in Watts (W).
• Apparent Power (S): The "total" power drawn by the circuit (S = V_rms × I_rms). Measured in Volt-Amperes (VA).

In circuits with inductive or capacitive loads, some power is "borrowed" and "returned" to the circuit each cycle to build magnetic or electric fields. This "reactive power" doesn't do work, but it increases the Apparent Power (S).

Definition: Power Factor (PF) = True Power (P) / Apparent Power (S)

• PF is a number between 0 and 1 (or 0% to 100%).
• A purely resistive circuit (heater) has PF = 1 (or 100%). All power is used.
• An inductive circuit (motor) has a "lagging" PF < 1.
• A capacitive circuit has a "leading" PF < 1.

Low power factor is inefficient. Power companies charge penalties for it because they have to supply the high Apparent Power (S) even if the True Power (P) being used is low.

4. Generators and Transformers

DC Power Sources

Sources that provide Direct Current (constant voltage/current).

• Batteries: Chemical power sources.
• Solar Cells: Convert light to DC.
• DC Generators (Dynamo): Convert mechanical rotation to DC.
• Power Supplies: Convert AC from the wall into DC for electronics.

AC/DC Generators

A generator converts mechanical energy into electrical energy using Faraday's Law of Induction (rotating a coil of wire in a magnetic field).

• AC Generator (Alternator): The rotating coil is connected to slip rings. These continuous rings output the natural sinusoidal AC voltage. This is how all power plants work.
• DC Generator (Dynamo): The rotating coil is connected to a split-ring commutator. This special switch reverses the connection to the external circuit every half-rotation, "flipping" the negative half of the AC wave. This produces a pulsating DC.

[Image of AC Generator (slip rings) vs DC Generator (split-ring commutator)]

Inductance, Capacitance, and Impedance

In AC circuits, components resist current in different ways:

• Resistance (R): "Normal" resistance. Opposes AC and DC.
• Inductive Reactance (X_L): An inductor's opposition to AC. It *increases* with frequency (X_L = 2πfL).
• Capacitive Reactance (X_C): A capacitor's opposition to AC. It *decreases* with frequency (X_C = 1 / (2πfC)).

Impedance (Z) is the total opposition to current in an AC circuit. It's the vector sum of resistance and reactance.

Formula: Z = √(R² + (X_L - X_C)²)

Operation of Transformers

A transformer is a static device that transfers AC power from one circuit to another, usually changing the voltage and current levels. It does not work for DC.

Principle (Mutual Induction):

1. An AC voltage (V_p) is applied to the primary coil (N_p turns).
2. This AC creates a continuously changing magnetic flux in the iron core.
3. The shared, changing magnetic flux induces an AC voltage (V_s) in the secondary coil (N_s turns).

[Image of a step-up and step-down transformer schematic]

Ideal Transformer Equation:

Voltage Ratio: V_p / V_s = N_p / N_s

Current Ratio: I_p / I_s = N_s / N_p

• Step-Up Transformer: N_s > N_p. Voltage increases, current decreases. (Used at power plants to transmit at high voltage).
• Step-Down Transformer: N_s < N_p. Voltage decreases, current increases. (Used on utility poles and in phone chargers).