AC Theory & Power Systems
AC fundamentals, power factor, true and apparent power, single-phase and three-phase system relationships, and Canadian voltage standards.
- State the standard frequency and residential voltage for Canadian AC power systems
- Explain the phase relationships in purely resistive, inductive, and capacitive circuits
- Calculate line voltage and line current in wye and delta three-phase systems
- Distinguish between true power, apparent power, and reactive power
- Describe the neutral current behavior in a 120/240V single-phase system
Lección 1
AC Fundamentals - Frequency, Phase & Impedance
Canadian AC Power Standards
The standard frequency of AC power in Canada is 60 Hz. This means the voltage and current waveforms complete 60 full cycles per second. This is the same frequency used in the United States, which allows equipment designed for one country to generally work in the other.
The standard voltage for residential single-phase service in Canada is 120/240V. This is delivered as a split-phase system with two 120V hot legs and a grounded neutral. The 240V is available between the two hot legs, while 120V is available between either hot leg and neutral.
Phase Relationships in AC Circuits
In AC circuits, the relationship between voltage and current depends on the type of load. This is a heavily tested concept on the Red Seal exam.
Purely resistive load - voltage and current are in phase. An incandescent lamp is the classic example of a resistive load.
Purely inductive load - current lags voltage by 90 degrees. Motors, transformers, and solenoids are inductive loads. Remember: "ELI" - voltage (E) leads current (I) in an inductor (L).
Purely capacitive load - current leads voltage by 90 degrees. Capacitor banks for power factor correction are capacitive loads. Remember: "ICE" - current (I) leads voltage (E) in a capacitor (C). This means a capacitor in an AC circuit causes current to lead voltage - a key exam fact.
Resistive (R)
Phase: V and I in phase
Example: Incandescent lamp, heater
Power Factor: 1.0
Inductive (L)
Phase: Current lags voltage by 90 degrees
Example: Motor, transformer
Memory Aid: ELI
Capacitive (C)
Phase: Current leads voltage by 90 degrees
Example: Capacitor bank
Memory Aid: ICE
Impedance in AC Circuits
Impedance is the total opposition to current flow in an AC circuit. While resistance opposes current in both DC and AC, impedance also includes the effects of inductance and capacitance. The impedance of an AC circuit is measured in ohms - the same unit as resistance. Impedance is represented by the symbol Z and combines resistance (R) with reactance (X) using the formula Z = square root of (R2 + X2). Understanding impedance is essential for AC circuit analysis and power calculations.
Canada uses 60 Hz AC power. Residential service is 120/240V single-phase. In a purely inductive circuit, current lags voltage by 90 degrees (ELI). In a purely capacitive circuit, current leads voltage by 90 degrees (ICE). An incandescent lamp is a resistive load. Impedance is measured in ohms.