HVAC Training Academy - Volume 3 | 1 HVAC TRAINING ACADEMY Volume 3 - Electrical Fundamentals Voltage, current, resistance, Ohm's law, controls, motors, capacitors and schematic reasoning Prerequisites: Volumes 1 and 2. This volume teaches electrical concepts needed to understand HVAC controls and diagnostics before advanced troubleshooting. Critical safety boundary: HVAC electrical systems can contain lethal voltage and stored electrical energy. This material is educational. It does not instruct an unqualified person to open energized equipment, defeat interlocks, bypass safeties, or perform live electrical testing. Field work must follow manufacturer procedures, applicable codes, lockout/tagout practices, PPE requirements, and professional qualification requirements. HVAC Training Academy - Volume 3 | 2 1. Electricity in HVAC HVAC equipment uses electricity for controls, motors, compressors, heaters, valves, relays, contactors, sensors, and electronic boards. Understanding the electrical side means understanding both the power circuit and the control logic that commands it. A system can have correct mechanical components and still fail because the electrical command or power path is incomplete. 2. Voltage Voltage is electrical potential difference. It is the electrical 'push' that can drive current through a circuit when a complete path exists. Voltage may be present even when current is not flowing. Never equate 'voltage exists' with 'the load is operating.' 3. Current Current is the flow of electric charge through a circuit. In HVAC, current draw can help evaluate whether a load is operating and how heavily it is loaded, but readings must be interpreted against equipment specifications and operating conditions. A current value without knowing the load and expected rating is incomplete evidence. 4. Resistance Resistance opposes current flow. Conductors, motor windings, heaters, coils, sensors, and other components have electrical characteristics that can be evaluated in appropriate de-energized testing procedures. Resistance measurements are normally interpreted with the circuit safely de-energized and isolated according to proper procedures. 5. Ohm's Law The basic relationship is V = I x R, where V is voltage, I is current, and R is resistance. Rearranged: I = V/R and R = V/I. Ohm's law is a reasoning tool. Real HVAC loads may also involve inductance, capacitance, AC behavior, starting conditions, and electronic controls. 6. Electrical Power Electrical power is commonly expressed as P = V x I for simple cases. Power is measured in watts. HVAC equipment ratings and real AC circuits may require additional considerations. Do not infer equipment health from wattage alone. 7. AC and DC Alternating current changes direction periodically; utility-powered HVAC equipment commonly uses AC. Direct current flows in one direction and is common inside electronic control systems and sensor circuits. HVAC Training Academy - Volume 3 | 3 Do not assume every low-voltage signal is AC or every control board signal is DC; use documentation. 8. Line Voltage and Control Voltage Many HVAC systems separate higher-voltage power circuits from lower-voltage control circuits. A transformer is often used to supply control voltage, but exact voltages and architecture depend on the equipment. Charlie must not invent a control voltage when the equipment documentation is unknown. 9. Transformers A transformer transfers electrical energy between windings through electromagnetic induction and can change voltage levels in AC circuits. HVAC systems often use transformers to provide a lower control voltage from a higher supply voltage. A transformer has a primary side and a secondary side; exact terminals and ratings are equipment-specific. 10. Switches and Safeties A switch opens or closes an electrical path. HVAC safety devices may interrupt control or power when unsafe conditions are detected. Never recommend bypassing a safety device to make equipment run. A tripped safety is evidence that requires diagnosis. 11. Relays A relay uses an electrically operated coil or electronic control to change one or more contacts. It allows one circuit to control another. The coil/control side and the switched contact side are conceptually distinct. 12. Contactors A contactor is an electrically controlled switching device commonly used for higher-current loads such as compressors and motors. A control signal energizes its coil, causing power contacts to change state. A contactor being commanded does not prove that correct power reaches the load; both control and power paths matter. 13. Capacitors Capacitors store electrical energy in an electric field. In many HVAC motor applications, run or start capacitors support motor operation according to the equipment design. Capacitors can retain hazardous charge. Physical appearance alone does not establish electrical condition, and handling/testing requires proper safety procedures. 14. Motors HVAC systems use motors for blowers, condenser fans, pumps, and compressors. Different motor technologies have different control methods and diagnostic requirements. HVAC Training Academy - Volume 3 | 4 Do not apply capacitor-based assumptions to every motor; electronically commutated motors and other designs behave differently. 15. Compressor Electrical Concept A compressor motor is both an electrical load and part of the refrigeration system. Electrical symptoms can result from mechanical or refrigeration conditions, and vice versa. A high current reading is a symptom requiring context, not an automatic proof of a bad compressor. 16. Series Circuits In a simple series circuit, components share one current path. Opening any required point interrupts the path. Series safety switches can stop a control circuit even when other controls are calling. 17. Parallel Circuits Parallel branches provide multiple current paths across common electrical nodes. Loads can operate independently depending on the control design. Do not assume one failed parallel load necessarily prevents every other branch from operating. 18. Open Circuit An open circuit is an interrupted path. Opens can be intentional, such as an open switch, or caused by a fault such as a broken conductor or failed component. An open circuit can have voltage present at one point while no load current flows. 19. Short Circuit A short circuit is an unintended low-impedance path that can cause excessive current and protective-device operation. Short circuits are hazardous. Diagnosis must use safe procedures rather than repeated resetting or bypassing protection. 20. Grounding and Bonding Grounding and bonding support electrical safety and fault-current paths. Exact requirements are governed by applicable electrical codes and equipment instructions. Charlie should not improvise grounding modifications. 21. Fuses and Circuit Breakers Overcurrent protective devices interrupt a circuit when current exceeds their designed limits or conditions. A blown fuse or tripped breaker is a symptom, not the root cause. Do not repeatedly replace/reset protection without investigating why it operated. HVAC Training Academy - Volume 3 | 5 22. Thermostat Calls A thermostat or controller issues requests based on conditions and logic. Those requests travel through a control circuit to relays, contactors, boards, valves, motors, or other devices. A thermostat display showing 'cooling' does not prove the outdoor unit or compressor actually received and executed the command. 23. Reading a Schematic A schematic represents electrical relationships rather than physical component placement. Good schematic reasoning follows the circuit from source through controls and loads back to the return path. Do not read a schematic as though left-to-right placement always matches physical wiring location. 24. Ladder Logic Foundation Many HVAC control diagrams can be understood as paths between supply rails, with switches/controls and loads arranged along rungs. A complete energized path is required for the controlled load to operate. Ask: Is the source available? Is every required control in the correct state? Does the path reach the load? Is there a return path? 25. Normally Open and Normally Closed Contact labels such as normally open and normally closed describe the contact state in its defined normal/de-energized condition, not necessarily the condition while the equipment is operating. Always determine what 'normal' means in the device documentation. 26. Measurement Discipline Electrical diagnosis depends on measurement location, reference point, operating state, meter function, equipment rating, and circuit design. A number without these details is weak evidence. Charlie should ask for exact measured values and where/how they were obtained, while maintaining safety boundaries. 27. Control vs. Power Diagnosis A useful diagnostic distinction is whether the load lacks a command, lacks power, has power but cannot operate, or is being intentionally held off by a safety/control condition. This prevents random parts replacement and keeps reasoning organized. 28. Evidence-Based Electrical Reasoning Start from the symptom, identify the expected sequence of operation, determine what should be energized, and compare that expectation with safe, qualified observations and manufacturer data. Never jump directly from 'not running' to 'bad motor' without checking the command and power chain. HVAC Training Academy - Volume 3 | 6 29. Charlie's Electrical Rule If exact voltage, wiring, terminal identity, component rating, or sequence is not documented for the equipment, Charlie must not invent it. It should request the model, schematic, service literature, or qualified measurements. Equipment-specific documentation overrides generic HVAC assumptions. HVAC Training Academy - Volume 3 | 7 30. Electrical Reference Table Concept / component Foundation-level meaning Voltage (V) Electrical potential difference. Current (I) Flow of electric charge. Resistance (R) Opposition to current flow. Ohm's law V = I x R. Power For simple cases, P = V x I. Transformer Transfers AC energy between windings; can change voltage. Relay Control-operated switching device. Contactor Control-operated switch commonly used for higher-current loads. Capacitor Stores electrical energy; used in some motor circuits. Fuse / breaker Overcurrent protection; operation indicates a condition to investigate. Open circuit Interrupted current path. Short circuit Unintended low-impedance path. HVAC Training Academy - Volume 3 | 8 31. Soft Training Questions # Question Expected answer 1 What is voltage? Electrical potential difference. 2 What is current? Flow of electric charge. 3 What is resistance? Opposition to current flow. 4 State Ohm's law. V = I x R. 5 If V=24 V and R=12 ohms in a simple resistive example, what is I? 2 A. 6 What does a transformer do conceptually? Transfers AC energy between windings and can change voltage levels. 7 What is the difference between a relay and its controlled load? The relay switches a circuit; the load is the device receiving power through that circuit. 8 Does an energized contactor coil prove the load has correct power? No. 9 Can a capacitor retain dangerous electrical energy? Yes. 10 Does a blown fuse prove the fuse itself was the root cause? No; investigate why protection operated. 11 What is an open circuit? An interrupted current path. 12 What is a short circuit? An unintended low-impedance path that can cause excessive current. 13 Does a thermostat calling for cooling prove the compressor is running? No. 14 What should Charlie do if the equipment voltage or terminal identity is unknown? Request equipment-specific documentation or qualified measurements; do not invent values. 15 Should a safety switch be bypassed to see whether the unit runs? No. 16 What comes next after Volume 3? Airflow and duct systems: CFM, static pressure, blower performance, filters, supply/return, ducts and airflow diagnostics. Volume 3 pass standard: Charlie should correctly distinguish voltage, current, resistance, power, control and load circuits; understand the conceptual role of transformers, relays, contactors, capacitors, motors and protective devices; reason through simple schematics without inventing equipment-specific values; and preserve electrical safety boundaries. Next: HVAC Training Volume 4 - Airflow & Duct Systems.