Electrical Theory, Controls, and Canadian Electrical Code

Electrical Theory, Controls, and the Canadian Electrical Code

Chapter Introduction

This chapter covers all the essential electrical concepts for the Red Seal exam in Refrigeration and Air Conditioning. You must master the fundamental principles, control circuits, electromechanical and electronic components, as well as the requirements of the Canadian Electrical Code (CE Code) . Exam questions cover both pure theory and the practical application of safety rules.

Fundamental Principles of Electricity

Basic Electrical Quantities

Electricity is the movement of electrons through a conductor. Five fundamental quantities must be fully understood:

Ohm's Law: Voltage equals the product of current multiplied by resistance.

E = I × R

Power Law: Power equals the product of voltage multiplied by current.

P = E × I

For single-phase circuits: P = E × I × power factor (PF)

For three-phase circuits: P = √3 × E × I × PF

Series and Parallel Circuits

Series Circuit: Current is identical through all elements. Total resistance is the sum of individual resistances.

Rₜ = R₁ + R₂ + R₃ + ...

Parallel Circuit: Voltage is identical across each branch. Total conductance is the sum of individual conductances.

1/Rₜ = 1/R₁ + 1/R₂ + 1/R₃ + ...

For two resistors in parallel: Rₜ = (R₁ × R₂) / (R₁ + R₂)

Alternating Current and Direct Current

Direct current (DC) flows in one direction only. It is used in low-voltage control circuits (24 V DC), variable-speed motors, and electronic circuit boards.

Alternating current (AC) changes direction periodically. In Canada, the standard frequency is 60 Hz. RMS (root mean square) values are used for calculations: the nominal voltage of 120 V AC corresponds to a peak voltage of √2 × 120 ≈ 170 V.

Power Factor

Power factor (PF) is the ratio of active power (W) to apparent power (VA). It equals the cosine of the phase angle between voltage and current.

PF = P / S where S is the apparent power in volt-amperes (VA)

Induction motors typically have a PF of 0.7 to 0.9. A low PF increases the current flowing in conductors without producing useful work. Power factor correction is achieved by adding capacitors.

Electrical Components of Refrigeration Systems

Contactors and Relays

A contactor is an electrically operated switch. It consists of a coil, main (power) contacts, and auxiliary (control) contacts.

The contactor coil can be powered at 24 V, 120 V, or 240 V. The coil voltage must match the control circuit voltage exactly.

Overload Relays and Motor Protection

Overload relays protect motors against prolonged overcurrents. Three main types:

The overload relay must be set to the motor's rated current, as indicated on the nameplate. A setting too high does not protect the motor; a setting too low causes nuisance trips.

Capacitors

Capacitors are used for starting and running single-phase motors.

The start capacitor is connected in series with the start winding and is removed from the circuit by a relay or centrifugal switch once the motor is running. The run capacitor remains in the circuit at all times.

Transformers

A transformer changes alternating voltage. It consists of a magnetic core and two windings: primary and secondary.

Transformation ratio: N₁/N₂ = E₁/E₂ = I₂/I₁

In refrigeration systems, transformers step down voltage from 120 V or 240 V to 24 V for control circuits. The secondary is often equipped with overcurrent protection.

Control Circuits

Wiring and Control Diagrams

Two types of diagrams are used:

On a control diagram, vertical lines represent power conductors. Components are numbered for easy identification. Contacts are identified by their normal (at-rest) position.

Typical Compressor Control Circuit

A standard control circuit includes:

The circuit is in series: all contacts must be closed for the coil to be energized. Opening any contact stops the compressor.

Interlocks and Sequences

Interlocks prevent the simultaneous operation of incompatible components. For example:

Sequences impose an operating order. For example, the condenser fan must start before the compressor, or crankcase heaters must be energized before compressor start-up.

Timers and Time Delays

Time delays are essential for protecting compressors:

Time delays can be implemented with time-delay relays, electronic timers, or programmable control boards.

Electric Motors

Single-Phase Motors

Four types of single-phase motors are commonly encountered:

The direction of rotation of a single-phase motor is reversed by reversing the connections of the start winding.

Three-Phase Motors

Three-phase motors are used for large-capacity compressors. They offer better efficiency and higher starting torque than single-phase motors.

Rotation reversal: Swap any two of the three supply phases.

Protection: Three-phase motors require protection against:

Variable Speed Drives

Variable frequency drives (VFDs) control motor speed by varying the supply frequency and voltage. They are used for:

The VFD converts alternating current to direct current, then back to alternating current at a variable frequency. The relationship between frequency and speed is: N = (120 × f) / P where N is speed in revolutions per minute, f is frequency in Hz, and P is the number of poles.

Canadian Electrical Code

Structure and Application

The Canadian Electrical Code, Part I (CE Code) is a national standard published by the Canadian Standards Association (CSA). It applies to all electrical installations in Canada.

The code is organized into numbered sections:

Essential Rules for Refrigeration

Rule 8-200: Calculation of branch circuit loads. The maximum load on a circuit must not exceed 80% of the rated capacity of the circuit breaker for continuous loads.

Rule 14-100: Overcurrent protection. Each circuit must be protected against overcurrent by a fuse or circuit breaker.

Rule 26-256: Protection of motors against overloads. The overload protection device must be set to a value not exceeding 125% of the motor's rated current.

Rule 28-602: Motor supply conductors. The conductor ampacity must be at least 125% of the motor's rated current.

Grounding and Bonding

Grounding is mandatory for all electrical equipment. It protects against electric shock by providing a low-impedance path to earth.

Rule 10-200: Grounding of equipment. All electrical equipment must be grounded.

Rule 10-400: Bonding. All non-current-carrying metal parts must be bonded together and connected to ground.

In refrigeration systems, compressors, condensers, and evaporators must be grounded. Metallic refrigerant lines must be bonded if they are likely to be touched.

Circuit Protection

Fuses are classified by their response time: fast-acting, time-delay (RK5, RK1), and slow-blow. Time-delay fuses are used for motors because they tolerate high starting currents.

Conductors and Sizes

Conductors are identified by their AWG size (American Wire Gauge). Common sizes in refrigeration:

Ampacity depends on the insulation temperature rating (60 °C, 75 °C, 90 °C) and installation conditions (ambient temperature, number of conductors in the conduit).

Troubleshooting and Diagnostics

Verification Procedure

Safety Measures

Before any intervention:

Component Testing

Testing a capacitor: Measure resistance with an ohmmeter. A charged capacitor will deflect the needle then return to infinity. A short-circuited capacitor indicates zero resistance. An open capacitor indicates infinite resistance.

Testing a coil: Measure resistance. An open coil indicates infinite resistance. A short-circuited coil indicates very low resistance.

Testing a motor: Measure resistance between each winding and between the windings and the frame. Infinite resistance between a winding and the frame indicates good insulation.

Common Pitfalls to Avoid

Summary

Self-Assessment Questions

Normative References

These standards are cited for reference. The Red Seal exam evaluates your knowledge of principles and practical application, not the memorization of rule numbers. However, familiarity with the most common rules (8-200, 14-100, 26-256, 28-602) is expected.