Level 1, Unit 4, Section 3 – Working With Motors

Unit 4 — Electrical Fundamentals

Section 3 — Working With Motors

Section 3 Overview

Section 2 built the theory — motor types, starting devices, operating characteristics. Section 3 puts it to work. These six lessons develop the practical skills needed to select and test capacitors, apply and size overload protection, diagnose electrical and mechanical failures, install and commission motors to code, and set up and maintain V-belt drive systems.

3.0.1 — General Learning Outcomes

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

Select, test, and replace start and run capacitors — including dual-round capacitors — applying correct capacitance tolerances, voltage ratings, discharge procedures, and safe handling practices.
Distinguish between start capacitors (electrolytic, intermittent duty) and run capacitors (oil-filled, continuous duty) based on construction, ratings, and failure modes, and correctly match replacements within allowable tolerances.
Select and apply the appropriate type of motor overload protection, size heater elements to motor full-load amperage, choose the correct trip class for the application, and identify and correct common causes of overload tripping.
Diagnose electrical motor failures — open windings, shorted windings, ground faults, capacitor faults, and starting device faults — using correct instrument procedures and interpret test results to recommend repair or replacement.
Diagnose mechanical motor failures — sleeve and ball bearing wear, shaft and rotor damage, misalignment, and loose mounting — identify probable causes, and recommend appropriate corrective action.
Install and commission a motor in accordance with NEC requirements, including conductor sizing, overcurrent and overload protection, grounding, terminal connections, and rotation verification, and document baseline operating data for future reference.
Select, install, align, and tension V-belt drives; calculate speed ratios from pulley diameters; and identify the correct replacement belt by number, measurement, or calculation.

3.0.2 — Section 3 — Lessons at a Glance

Section 3 moves from component knowledge through fault diagnosis to field practice. Lessons 3.01 and 3.02 address capacitors in depth; 3.03 and 3.04 cover protection and failure analysis; 3.05 and 3.06 develop installation, commissioning, and mechanical drive skills. Each lesson is self-contained, but the diagnostic lessons (3.03, 3.04) assume familiarity with capacitor types from 3.01 and 3.02.

3.01

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Capacitor Fundamentals

What capacitors are and how they create phase shift in single-phase motors. Capacitance ratings, voltage ratings, selection and sizing rules, the consequences of wrong values, visual and electrical testing procedures, and safe discharge and installation practices.

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3.02

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Types of Capacitors

Start capacitors (electrolytic, intermittent duty, 50–1200 μF) vs. run capacitors (oil-filled, continuous duty, 2–80 μF) — construction differences, ratings, failure modes, and replacement rules. Dual round capacitors: HERM, FAN, and COM terminals explained.

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3.03

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Overload Protection

Bimetallic, melting-alloy, and electronic overload relays. Internal thermal protectors and thermistor sensors. Fuses and motor circuit protectors. Heater element sizing, trip class selection (Class 10 / 20 / 30), and the eight common causes of motor overload.

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3.04

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Common Motor Failures

Electrical failures: open windings, shorted windings, ground faults, capacitor failures, current relay, potential relay, and centrifugal switch faults — causes, symptoms, test methods, and remedies. Mechanical failures: bearing wear, bent shafts, rotor bar damage, misalignment, and loose mounting.

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3.05

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Installation & Commissioning

Pre-installation checks and NEC conductor sizing. Overcurrent protection, overload protection, grounding, and terminal connections. Four-stage commissioning procedure: pre-start checks, initial start, running tests, and documentation. Baseline data recording and maintenance scheduling.

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3.06

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Pulleys and Belts

Speed ratio formula and pulley selection. Pulley installation and two-plane alignment (straightedge, string, and laser methods). Belt tension by deflection and strand tension methods. V-belt types (classical, narrow, cogged), synchronous belts, and flat belts. Belt sizing by number, measurement, and calculation.

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3.0.3 — Key Terms — Section 3 Preview

These terms appear throughout all six lessons. A working understanding of each is required before undertaking any capacitor, overload, or drive-system task in the field.

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Microfarad (μF)

The unit of electrical capacitance. One microfarad equals one millionth of a farad. Motor capacitors are rated in microfarads — start capacitors typically 50–1200 μF, run capacitors typically 2–80 μF. Capacitance values are measured with a digital capacitance meter or a multimeter with capacitance function.

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Start Capacitor

An electrolytic capacitor rated for intermittent duty only — typically 1–3 seconds per start cycle. High capacitance provides maximum starting torque but the construction cannot sustain continuous current. Always disconnected by a relay or switch once the motor reaches approximately 75% of full speed. Fails open or shorted.

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Run Capacitor

An oil-filled capacitor rated for continuous duty. Connected permanently in series with the auxiliary winding to maintain phase shift and improve running efficiency, power factor, and torque. Lower capacitance than start capacitors. Typically fails gradually by losing capacitance, causing increased current and overheating before complete failure.

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Dual Run Capacitor

Two capacitor sections housed in one cylindrical case with three terminals: C (common, to power supply), HERM (compressor), and FAN (condenser fan motor). Common in residential split-system condensing units. Test each section independently between its terminal and common. Replace the entire unit if either section fails.

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Thermal Overload Relay

A current-monitoring protection device that simulates motor heating characteristics to distinguish acceptable starting overloads from damaging sustained overcurrent. Uses bimetallic elements, melting-alloy solder pots, or electronic current sensing. Selected by heater element (sized to motor FLA) and trip class. Wired in series with the motor starter coil.

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Trip Class

The maximum time an overload relay takes to trip from a cold start at 600% of full-load amperage. Class 10: 10 seconds (submersible pumps, low-inertia loads). Class 20: 20 seconds (general HVAC/R). Class 30: 30 seconds (compressors and high-inertia loads). Select based on motor starting characteristics, not just motor size.

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Insulation Resistance

The resistance between motor windings and the motor frame or ground, measured in megohms (MΩ) using a megohmmeter (megger). A minimum of 1 MΩ is required for fractional-HP motors; larger motors require higher values. Readings below threshold indicate a ground fault — the motor must be taken out of service until repaired or replaced.

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Ground Fault

A failure of winding insulation that allows current to flow from the motor windings to the motor frame or ground. Causes immediate GFCI tripping, blown fuses, or a shock hazard at the motor case. Detected with a megger. May result from moisture, overheating, contamination, or voltage surges. Minor moisture faults can sometimes be cleared by carefully baking the motor; winding failures require replacement.

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Belt Deflection

A field method for measuring and setting V-belt tension. Apply perpendicular force at the midpoint of the longest free span; target deflection is approximately 1/64 inch per inch of span length (1.5 mm per 100 mm). Too little deflection means the belt is over-tensioned; too much means it is loose. After initial tightening, recheck after the first run — new belts seat and loosen slightly.

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Speed Ratio

The relationship between motor shaft speed and driven equipment speed, determined entirely by pulley diameters: RPM_driven = RPM_motor × (D_motor ÷ D_driven). A 1750 RPM motor with a 3-inch drive pulley and a 12-inch driven pulley produces 437.5 RPM at the fan shaft. Changing the motor pulley is the standard field method for adjusting fan speed.

3.0.4 — How Section 3 Connects to the Rest of Unit 4

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Building on Section 2 — From Theory to Field Practice

Section 2 (Introduction to Motors) established the foundation: single-phase motor types, starting devices, capacitor roles in PSC and CS motors, operating theory, and the effects of load and voltage on motor performance. Section 3 assumes that knowledge and applies it to practical field tasks — selecting and testing the right capacitor for a specific motor, sizing the overload relay, diagnosing a motor that is failing, and commissioning a replacement.

The starting devices covered in Section 2 (current relay, potential relay, PTC thermistor, centrifugal switch) are the same devices that appear in the Common Motor Failures lesson (3.04). Understanding how they are supposed to operate is a prerequisite for diagnosing how they have failed.

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Leading Into Section 4 — Control Fundamentals

Section 4 (Control Fundamentals) covers the control circuits that start and stop motors as part of HVAC/R system sequencing: magnetic motor starters, contactors, reversing contactors, and reduced-voltage starting. Every magnetic starter contains an overload relay — the overload protection knowledge developed in Lesson 3.03 is assumed when reading starter wiring diagrams and selecting starter assemblies in Section 4.

The installation and commissioning procedures from Lesson 3.05 — conductor sizing, grounding, rotation verification, and baseline current documentation — apply identically when commissioning motor starters and control panels in Section 4.

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Belt Drives and Level 2 Air Systems

The pulley and belt knowledge from Lesson 3.06 extends into Level 2 air handling and ventilation units, where fan speed adjustment, drive efficiency, and airflow balancing are core competencies. The speed ratio formula, belt sizing methods, and alignment tolerances introduced here are used directly whenever a technician adjusts fan speed to achieve target CFM or static pressure.

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Safety runs through every lesson in Section 3

Capacitor discharge, lockout/tagout before motor work, megger testing protocols, correct overload settings, and belt drive guarding requirements are woven throughout. These are not optional steps — they are the standard of care for every motor task in the field.

3.0.5 — Key Principles for This Section

Keep the following principles in mind across all six lessons:

Always discharge capacitors before testing, handling, or replacing them — even with power disconnected, capacitors can retain lethal charge for extended periods. Use a 20,000 Ω 5 W resistor across the terminals for 30–60 seconds, then verify with a voltmeter.
A start capacitor is designed for 1–3 seconds of use per start cycle. If the starting relay fails to disconnect it, the capacitor overheats and fails within minutes — always verify that the starting device is operating correctly when replacing a start capacitor.
Never substitute a run capacitor for a start capacitor, or vice versa — they are constructed differently and rated for different duty cycles. Substitution will damage the replacement capacitor and may damage the motor.
When an overload relay trips, it tripped for a reason. Identify and correct the underlying cause before resetting — never bypass or jumper an overload relay to keep a motor running. Doing so risks motor destruction, fire, and injury.
Insulation resistance below 1 megohm means the motor has a ground fault. Do not re-energise a grounded motor; remove it from service, identify the cause, and repair or replace before returning to operation.
Current is the most reliable real-time indicator of motor health in the field. Measure and record it at commissioning, then trend it over time — rising current at constant load almost always indicates a developing problem.
Proper belt tension matters as much as alignment. An over-tight belt overloads motor and driven bearings, shortening both their service lives. An under-tight belt slips, generates heat, and wears rapidly. Check and re-tension after the first run on new belts.
Document every motor installation: nameplate data, date, installer, baseline voltage and current on all phases, and overload relay setting. You cannot trend deterioration without a baseline, and you cannot troubleshoot efficiently without installation records.