Motor Winding Calculator
Diagnose motor health with our interactive checklist and winding resistance calculator for single-phase and three-phase HVAC motors.
Professional Diagnostic Tool — Electrical Safety Required
Motor winding tests involve working with electrical components that can carry lethal voltages and stored energy in capacitors. This tool is intended for qualified HVAC technicians as a diagnostic reference.
Always verify the system is fully de-energized before testing. Capacitors must be discharged. Do not attempt these tests unless you are trained in electrical safety and lockout/tagout procedures.
Motor Winding Health Checklist
Work through each step in order — from safety preparation through winding resistance, ground fault testing, capacitor checks, and three-phase diagnostics. Each check includes troubleshooting tips if the test fails.
Before testing motor windings, ensure the system is completely de-energized and locked out. Disconnect power at the breaker and verify with a non-contact voltage tester.
Winding Resistance Analyzer
Enter your multimeter readings to verify motor winding health. The tool will check for open circuits, shorts, ground faults, and winding balance.
Motor Looks Good
Insulation to Ground
OL — No ground fault detected
Understanding Motor Windings
HVAC motors use copper wire coils (windings) to create electromagnetic fields that spin the rotor. Single-phase motors have a start winding and a run winding connected to a common terminal. Three-phase motors have three identical windings.
Single-Phase Terminal Identification
- C (Common): Connected to both windings — one lead of each coil
- S (Start): Start winding — higher resistance (thinner wire, more turns)
- R (Run): Run winding — lower resistance (thicker wire)
Tip: The highest ohm reading between any two terminals is S→R. The two lower readings identify which is Start and which is Run by comparing to Common.
Common Reading Interpretations
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Our goal is to provide the HVAC community with helpful resources. If you have ideas for new calculators, improvements to this tool, or technical suggestions, we'd love to hear them.