Key Concepts Study Guide

AC electricity is produced by rotating a conductor (loop of wire) inside a magnetic field. As the conductor spins, it cuts through magnetic field lines at varying angles, which produces a voltage that continuously changes magnitude and direction.

Think of it like a Ferris wheel — your height above the ground changes smoothly as you go around. At the top, you're at peak positive. At the bottom, peak negative. And at the sides, you pass through zero. That's exactly what a sine wave looks like!

One complete cycle of AC = 360 degrees of rotation. The wave traces this path:

Zero → Peak (+) → Zero → Peak (−) → Zero

The positive half-cycle covers 0° to 180°, and the negative half-cycle covers 180° to 360°. The waveform is perfectly symmetrical.

The number of poles on the generator and the rotational speed together determine frequency. In North America, we standardize at 60 Hz. A 2-pole generator needs to spin at 3600 RPM; a 4-pole at 1800 RPM.

Want to know the voltage at 30°? Just plug in: e = Em × sin(30°) = Em × 0.5 = half the peak voltage. Easy!

An inductor is simply wire wound around a core (often iron). When current flows through it, it creates a magnetic field that stores energy. The key property is inductance (L), measured in Henrys (H).

Think of an inductor like a heavy flywheel — it resists changes in current, just like a flywheel resists changes in rotational speed. Once spinning (current flowing), it wants to keep going.

Faraday's Law: A changing magnetic field induces a voltage (EMF) in a conductor. More turns of wire = more induced voltage. Faster change = more voltage.

Lenz's Law: The induced EMF always OPPOSES the change that created it. This is nature's "pushback" — it resists change!

Self-Induction: One coil inducing EMF in itself. When current changes in a coil, the changing magnetic field induces a voltage right back in the same coil (counter-EMF).

Mutual Induction: One coil inducing EMF in another nearby coil. This is exactly how transformers work! The primary coil's changing field induces voltage in the secondary coil.

Four factors determine how much inductance a coil has:

• More turns ↑ = MORE inductance
• Higher permeability (core material) ↑ = MORE inductance
• Larger cross-sectional area ↑ = MORE inductance
• Shorter length ↓ = MORE inductance

Capacitors store energy in an electric field between two conductive plates separated by an insulator (dielectric). Different dielectric materials create different types:

• Higher dielectric constant (K) ↑ = MORE capacitance
• Larger plate area ↑ = MORE capacitance
• Smaller distance between plates ↓ = MORE capacitance

Every time constant, the circuit gets 63.2% closer to its final value. After 5 time constants (5τ), you're at 99.3% — essentially fully charged or discharged.

Discharge Resistors: When current through an inductor is suddenly interrupted, the collapsing magnetic field can generate dangerously high voltage spikes (remember, Lenz's Law fights the change). A discharge resistor provides a safe path for the energy to dissipate.

Reactance is like resistance, but for AC — measured in Ohms. The key insight: inductors and capacitors behave oppositely with frequency.

ELI the ICE man

ELI: In an inductor (L), EMF (voltage) leads I (current) by 90°.

ICE: In a capacitor (C), I (current) leads EMF (voltage) by 90°.

Phasor diagrams show voltage and current as rotating vectors. In purely inductive circuits, the voltage phasor is 90° ahead of current. In purely capacitive circuits, current is 90° ahead of voltage.

Both use an electromagnetic coil to move contacts, but they serve different weight classes:

• Relays = smaller loads (control circuits, lighting, small equipment)
• Contactors = bigger loads (motors, heaters, high-current equipment)

The basic parts are the same: Coil (electromagnet), Core (iron), Armature (the moving piece), and Contacts (switching points).

AC coils have special needs that DC coils don't:

• Laminated cores — thin iron layers to reduce eddy currents (circulating currents that waste energy and generate heat)
• Shading coils — small copper rings on the pole face that prevent 120 Hz buzz/chatter (the magnetic field goes to zero 120 times per second on 60 Hz AC)

Contact Materials: Silver (most common, low resistance), Copper (heavy duty), Cadmium oxide (arc resistant).

• Seal-in Voltage: ~85% of rated voltage is needed to hold the armature in once it's been pulled in. (It takes less to hold than to pull in!)
• Drop-out Voltage: ~50% of rated voltage — below this threshold, the spring wins and the armature releases.

Bridge Contacts: Two contact points bridge across a gap to make the circuit. This double-contact design is more reliable and can handle higher currents than single-point contacts.

• Spring-wound timers — Simple clock mechanism. Wind it up, it counts down. Think: kitchen egg timer.
• Time switches — Scheduled on/off at specific times. Think: outdoor lighting timer.
• Timing relays — Circuit-controlled. Activated by other circuit events.

Control methods: Pneumatic (uses air), Dashpot (uses fluid), Electronic (uses RC circuits or microcontrollers). Electronic is most common today because it's the most precise and adjustable.

Time Delay On Energization = On-Delay. When the coil is energized, a timer starts. Contacts change state AFTER the delay. When de-energized, contacts revert instantly.

Time Delay On De-Energization = Off-Delay. Contacts change instantly when coil is energized. When de-energized, the timer starts and contacts stay changed for the set time BEFORE reverting.

• Interval Timing: Contacts change for a set time, then revert. Like a stairway light timer — press the button, lights come on for 3 minutes, then automatically turn off.
• One-Shot Timing: Like interval timing, but triggered by an external signal rather than the coil itself. A single pulse triggers a timed output.
• Repeat Cycle: Contacts cycle on/off continuously.
  • Symmetrical = same on/off time (e.g., 5s on, 5s off)
  • Non-symmetrical = different on/off time (e.g., 2s on, 8s off)

Smart relays are essentially mini PLCs (Programmable Logic Controllers). They can be programmed using function block diagrams or ladder logic and can perform ALL timing functions plus scheduling.

Common applications:

• Car washes (sequencing wash/rinse/dry cycles)
• Automatic door openers
• Lighting management systems
• Access control systems
• HVAC control (heating/cooling sequences)

The fundamental motor control circuit:

• Stop button = NC (normally closed), wired in SERIES
• Start button = NO (normally open), wired in PARALLEL with holding contacts

Holding contacts (also called seal-in or maintaining contacts) are wired in parallel with the start button. Once the start button is pressed and the coil energizes, the holding contacts close and keep the coil energized even after you release the start button.

Multiple stations: Additional stops → add in SERIES. Additional starts → add in PARALLEL.

These devices detect changes in the environment and send signals to control circuits:

Temperature Sensors:

• Short Circuits: Very high current, very fast. A low-impedance fault path lets current skyrocket. Think: two wires touching — BOOM.
• Overloads: Moderate excess current, builds over time. Like running too many things on one circuit. The wires slowly heat up beyond their rating.

• Non-time-delay / Single-element: Blows fast on any overcurrent. No tolerance for temporary surges. Good for circuits that don't have inrush current (like lighting).
• Time-delay / Dual-element: Has TWO protection mechanisms:
  • Thermal cutout for sustained overloads (slow, gives motors time to start)
  • Fuse link for short circuits (fast, clears in milliseconds)

• Instantaneous-trip (Magnetic only): Uses an electromagnet only. Trips instantly on short circuits. No overload protection — used with separate overload relays.
• Inverse-time / Thermal-magnetic: The standard breaker in your panel. Has BOTH:
  • Bimetallic strip for overloads (heats up and bends slowly)
  • Electromagnet for short circuits (trips instantly)

Trip-Free: A breaker WILL trip even if someone holds the handle in the ON position. This is a critical safety requirement per CEC Rule 14-300. You can't defeat the protection by holding the handle!