Level 1, Unit 4, Section 5 – Control Fundamentals

Unit 4 — Electrical Fundamentals

Section 5 — Control Fundamentals

Section 5 Overview

Sections 2 through 4 built the foundation: single-phase and three-phase motors, capacitors, overload protection, motor types, and variable frequency drives. Section 5 puts that equipment into context by examining the control systems that govern how it starts, modulates, and stops. These lessons develop the ability to read control drawings, identify system components and their roles, trace signal paths through control and safety circuits, and understand the feedback principles behind modern DDC and electronic control systems.

5.0.1 — General Learning Outcomes

Upon successful completion of this section, the apprentice will be able to:

Define the standard control system terminology found on HVAC/R drawings and specifications, including setpoint, controlled variable, manipulated variable, dead band, differential, and normally open/normally closed contact positions.
Distinguish between open loop and closed loop control, explain the role of feedback in closed loop systems, and identify which control mode (two-position, modulating, PI, PID, PWM) is appropriate for a given HVAC/R application.
Identify and compare the four types of control systems used in HVAC/R — electromechanical, electronic, DDC, and pneumatic — and describe the signal medium, precision, and typical application of each.
Identify the function and specifications of actuators, relays, and switches within a control loop, and read ladder diagrams to trace the signal path from sensor through controller to controlled device.
Distinguish between control circuits, safety circuits, and operator circuits; explain how safety devices are wired in series to provide fail-safe protection; and describe the function of Hand–Off–Auto selector switches.
Identify common HVAC/R sensing devices for temperature, pressure, and humidity; compare their operating principles, accuracy ranges, and output signal types; and apply correct sensor placement guidelines for each.

5.0.2 — Section 5 — Lessons at a Glance

Section 5 builds progressively from vocabulary to hardware to sensing. Lesson 5.01 establishes the language of control drawings — the terms and symbols that must be understood before any wiring diagram can be traced. Lesson 5.02 applies that vocabulary to real equipment — identifying system types, physical components, circuit categories, and the sensors that feed data back to controllers.

5.01

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Terminology of Drawings & Specifications

The standardized vocabulary of HVAC/R control drawings: setpoint, controlled variable, manipulated variable, dead band, and differential. Open loop vs. closed loop control. The seven control modes from two-position through PID and PWM. Ladder diagram conventions, symbol abbreviations, and reading legends.

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5.02

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Control Systems, Components, & Functions

The four control system types (electromechanical, electronic, DDC, pneumatic) compared by signal medium and precision. Actuators, relays, and switches: types, specifications, and how to verify them. Control, safety, and operator circuits. Temperature, pressure, and humidity sensing devices with accuracy and output comparisons.

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5.0.3 — Key Terms — Section 5 Preview

These terms appear throughout both lessons and are essential for reading any HVAC/R control drawing or sequence-of-operation document encountered in the field.

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Closed Loop Control

A control system in which a sensor continuously measures the controlled variable and feeds the result back to the controller, which adjusts its output to minimize the difference between the measured value and the setpoint. Self-correcting under varying loads — the standard approach in modern DDC, EEV, and inverter applications.

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Dead Band / Differential

The dead band is the range around setpoint within which the controller holds its output steady to prevent short-cycling. The differential is the numerical gap between the cut-in and cut-out values of a switch or pressure control. Both prevent rapid cycling of compressors, contactors, and valves.

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DDC (Direct Digital Control)

Microprocessor-based controllers that accept analogue and digital sensor inputs, execute programmable control logic (including PI and PID algorithms), and produce analogue and digital outputs to HVAC/R equipment. DDC is the backbone of commercial building automation systems; it communicates over BACnet, Modbus, or LonWorks networks.

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Safety Circuit

A series-wired chain of protective devices (high-pressure cutout, low-pressure cutout, overload relay, freeze-stat, flow switch) that de-energizes equipment when any unsafe condition is detected. Fail-safe by design: opening any one device in the chain interrupts the protected load. Many devices require manual reset before restart.

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Actuator

Converts a control signal into physical motion to regulate a valve, damper, or other component. Key specifications: signal type (two-position, 0–10 VDC, 4–20 mA, PWM), torque rating (N·m), stroke time (seconds), and fail-safe position (fail-open or fail-closed on loss of power or signal).

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NO / NC Contacts

The contact positions of a switch or relay in their de-energized state as drawn on a ladder diagram. Normally Open (NO): open circuit at rest, closes when actuated. Normally Closed (NC): closed circuit at rest, opens when actuated. This convention is universal on all ladder diagrams regardless of manufacturer or system type.

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Hand – Off – Auto (HOA)

A three-position selector switch that determines how a device is controlled. Auto: responds to automatic signals from sensors or BAS. Hand: runs continuously, overriding automatic commands (safety circuits remain active). Off: de-energized regardless of any automatic or safety signal.

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Thermistor (NTC)

A temperature-sensing resistor whose resistance decreases as temperature rises. The most common temperature sensor in refrigeration and electronic HVAC controllers: accuracy ±0.2–0.5 °C, simple two-wire connection, low cost. Used for superheat sensing in EEV controllers, space temperature sensing, and suction/discharge line temperature monitoring.

5.0.4 — How Section 5 Connects to the Rest of Unit 4

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Building on Section 4 — Motor Types and Variable Speed Drives

Section 4 introduced ECM motors with integrated electronic controllers, VFDs that accept 0–10 VDC or 4–20 mA speed reference signals, and DDC communication interfaces such as BACnet and Modbus. Section 5 provides the control-system context for all of those features: the speed reference signal to a VFD is an analogue output from a DDC controller; the VFD’s run-enable input is wired through a control circuit interlock; the fault relay output feeds back into a safety circuit.

The motor overload relay covered in Section 3, the high-pressure and low-pressure cutouts introduced in Section 2, and the VFD fault contacts from Section 4 are all safety circuit devices — the series-wired chain that Section 5 explains in detail.

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Foundation for Level 2 — System Controls and Commissioning

Level 2 units on air handling, hydronic systems, and refrigeration systems build directly on Section 5 knowledge. Reading a sequence of operation for a rooftop unit, commissioning a chiller safety circuit, or troubleshooting a BAS point requires exactly the vocabulary, circuit tracing skills, and sensor knowledge developed here. DDC programming, BACnet point mapping, and analogue I/O verification — all Level 2 competencies — assume that the Section 5 fundamentals are in place.

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313A / 313D Regulatory Context

The control drawing terminology, ladder diagram conventions, and safety circuit requirements in Section 5 align with the drawing and specification standards referenced in Ontario’s 313A (Refrigeration and Air Conditioning) and 313D (Domestic and Commercial Refrigeration) apprenticeship standards. Control system knowledge is tested in both the in-school curriculum and on-the-job training logs — the ability to read a wiring diagram and explain a safety interlock is a core competency in both trades.

5.0.5 — Key Principles for This Section

Keep the following principles in mind across all lessons in Section 5:

Always locate and read the legend before tracing any control drawing. Symbol conventions are not fully standardized across manufacturers — a circle with a diagonal line may mean different things on different drawings. The legend on that drawing is the authority.
A safety circuit is only as reliable as its weakest connection. A corroded terminal, a loose wire nut, or a bypassed device is as dangerous as the condition the circuit is designed to prevent. Never bypass or jumper a safety device to keep equipment running — identify and correct the underlying fault first.
In a closed loop system, the sensor is the foundation of accuracy. A perfectly tuned PID controller produces poor results if the sensor is located in a stratified pocket, poorly insulated, or out of calibration. Sensor placement and maintenance are not secondary tasks.
The dead band and differential settings directly affect equipment life. Too narrow a dead band causes short-cycling that wears contactors, compressors, and actuators. Too wide a dead band allows excessive deviation from setpoint. Set both to the manufacturer’s recommended range for the specific application.
Control circuit voltage (typically 24 VAC) and power circuit voltage (120–600 V) are often present in the same enclosure. Always identify which class of voltage you are measuring before connecting instruments. Use a CAT III or CAT IV rated multimeter for HVAC/R control work.
Hand–Off–Auto selector switches in Hand mode override automatic control but must not override safety circuits. Before placing any device in Hand mode, confirm that all series-wired safety devices remain active and that the equipment will shut down correctly on a safety trip.
4–20 mA current loop signals are more immune to electrical noise than 0–10 VDC voltage signals over long cable runs. In noisy environments (near VFDs, large motors, or fluorescent lighting), prefer 4–20 mA for sensor and actuator wiring and route signal cables in separate conduit from power wiring.
Document every control system setpoint and parameter at commissioning. Deadbands, differential settings, time delays, and safety cutout values must be recorded in the equipment log. Without a commissioning baseline, trending deterioration and verifying correct operation after service are impossible.