Motors & Controls – Electrician Journeyman Exam Study Guide
Overview
This study guide covers the essential motor theory, NEC code requirements, protection strategies, control circuits, and calculations needed for the Electrician Journeyman Exam. Motors and controls is one of the most heavily tested topics, requiring both conceptual understanding and the ability to apply NEC tables and formulas accurately. Mastery of this material requires knowing when and why rules apply, not just memorizing numbers.
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Motor Theory & Fundamentals
Synchronous Speed & Slip
Synchronous speed is the theoretical speed of the rotating magnetic field in an AC motor — the rotor never actually reaches this speed in an induction motor.
Formula:
$$N_s = \frac{120 \times f}{P}$$
| Poles | 60 Hz Speed |
|-------|-------------|
| 2 | 3600 RPM |
| 4 | 1800 RPM |
| 6 | 1200 RPM |
| 8 | 900 RPM |
Slip is the difference between synchronous speed and actual rotor speed, expressed as a percentage:
$$\text{Slip \%} = \frac{N_s - N_r}{N_s} \times 100$$
Key Motor Types
| Motor Type | Rotor Construction | Speed/Torque Control |
|---|---|---|
| Squirrel-cage induction | Cast aluminum/copper bars with end rings | Limited (fixed speed) |
| Wound-rotor induction | Actual windings with slip rings | External resistance can adjust speed/torque |
| DC motor | Armature windings on commutator | Excellent variable speed control |
Counter-Electromotive Force (CEMF)
Service Factor (SF)
Direction Reversal
| Motor Type | How to Reverse |
|---|---|
| Three-phase | Swap any two of the three supply leads |
| Single-phase | Reverse leads of either the start winding or the run winding (not both) |
Key Terms – Motor Theory
> Watch Out For: Synchronous speed is NOT the same as nameplate RPM. The nameplate shows actual running speed (synchronous speed minus slip). Don't confuse the two on exam questions.
> Watch Out For: NEMA Code Letters go from A through V — higher letters mean higher inrush currents, requiring larger protective devices.
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NEC Code Requirements
Conductor Sizing – NEC 430.22 & 430.6
Critical Rule: Always use NEC Table values (Tables 430.247–430.250) for FLC — NOT the motor nameplate current — when sizing conductors and protection devices.
$$\text{Minimum Conductor Ampacity} = \text{FLC (from NEC table)} \times 1.25$$
Branch-Circuit Protection – NEC 430.52 & Table 430.52
Maximum ratings for branch-circuit short-circuit and ground-fault protection:
| Device Type | Single-Phase | Three-Phase |
|---|---|---|
| Non-time-delay fuse | 300% | 300% |
| Dual-element (time-delay) fuse | 175% | 175% |
| Inverse time circuit breaker | 250% | 250% |
| Instantaneous trip breaker | 800% | 800% |
> If the calculated value doesn't match a standard size, round UP to the next standard size — but only up to the maximums allowed. If that exceeds the maximum percentage, round down.
Standard fuse/breaker sizes to know: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A
Overload Protection – NEC 430.32
| Motor Condition | Maximum Overload Setting |
|---|---|
| SF ≥ 1.15 OR temperature rise ≤ 40°C | 125% of FLC |
| All other motors | 115% of FLC |
> If the overload trips at the above settings, NEC 430.32 permits increasing to 140% (SF ≥ 1.15 motors) or 130% (all others).
Disconnecting Means – NEC 430.102
Controller Requirements – NEC 430.83
Key NEC Sections Quick Reference
| NEC Section | Topic |
|---|---|
| 430.6 | Use table FLC values, not nameplate |
| 430.22 | Branch-circuit conductor sizing (125% FLC) |
| 430.32 | Overload protection sizing |
| 430.52 | Branch-circuit protection maximums |
| 430.83 | Controller ratings |
| 430.102 | Disconnecting means location |
| Tables 430.247–430.250 | FLC values by voltage and HP |
> Watch Out For: NEC 430.6 is the most commonly missed concept. Exams frequently ask questions where the nameplate current differs from the table value. Always use the table.
> Watch Out For: Overload protection and branch-circuit protection are two separate systems with different purposes and sizing rules. Don't mix them up.
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Motor Protection
Overload vs. Branch-Circuit Protection — Know the Difference
| Feature | Overload Protection | Branch-Circuit Protection |
|---|---|---|
| Device | Thermal overload relay | Fuses or circuit breakers |
| Protects against | Sustained low-level overcurrent (overheating) | High-magnitude short circuits & ground faults |
| Location | In the motor starter | At the panelboard |
| Trips at | ~115–125% FLC | Much higher (175–300% FLC) |
Thermal Overload Relay Operation
1. Excessive current flows through the heater element
2. Heat causes the bimetallic strip or eutectic alloy to trip
3. Normally closed contacts open → de-energizes the contactor coil
4. Motor is disconnected from power
5. Must be manually reset (standard) after fault is cleared
Locked-Rotor Current (LRC) Significance
NEMA Design Letters vs. Code Letters
> Watch Out For: "NEMA Code Letter" and "NEMA Design Letter" appear similar but mean completely different things. Code letters are for protection sizing; design letters describe starting torque characteristics.
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Motor Starters & Controllers
Across-the-Line (Full Voltage) Starter
Reduced Voltage Starting Methods
| Method | Starting Voltage | Starting Current Reduction |
|---|---|---|
| Wye-Delta | 57.7% of line voltage | Reduced to ~1/3 of across-the-line |
| Autotransformer | 50%, 65%, or 80% tap | Varies by tap selected |
| Soft Starter | Gradually increases | Smooth ramp-up |
| VFD | Variable frequency/voltage | Full control, most efficient |
Wye-Delta Detail:
Three-Wire vs. Two-Wire Control Circuits
| Feature | Two-Wire Control | Three-Wire Control |
|---|---|---|
| Pilot device | Maintained contact (float switch, thermostat) | Momentary push buttons |
| Holding contact | Not required | Required (seal-in contact) |
| After power loss | Restarts automatically | Does NOT restart automatically |
| Use case | Pumps, HVAC (auto restart acceptable) | Most motor applications (safety restart) |
The Holding (Seal-In) Contact
Reversing Starter Interlocks
Both mechanical and electrical interlocks should be used in reversing starters:
Variable Frequency Drive (VFD)
> Watch Out For: In a reversing starter, swapping two leads on one contactor and returning to original on the other creates the phase reversal needed for reverse rotation. Both contactors must NEVER close at the same time.
> Watch Out For: Two-wire control circuits provide low-voltage release not low-voltage protection. Three-wire circuits provide low-voltage protection (no automatic restart). Know which is which.
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Motor Calculations
Essential Formulas
Synchronous Speed:
$$N_s = \frac{120 \times f}{P}$$
Slip Percentage:
$$\text{Slip \%} = \frac{N_s - N_r}{N_s} \times 100$$
Motor Torque:
$$\text{Torque (lb·ft)} = \frac{\text{HP} \times 5{,}252}{\text{RPM}}$$
Horsepower to Watts:
$$1 \text{ HP} = 746 \text{ W}$$
Input Power with Efficiency and Power Factor:
$$\text{Input kW} = \frac{\text{Output W}}{\text{Efficiency}}$$
$$\text{Input kVA} = \frac{\text{Input kW}}{\text{Power Factor}}$$
Branch-Circuit Conductor Sizing:
$$\text{Ampacity} = \text{FLC (table)} \times 1.25$$
Branch-Circuit Protection (maximum):
$$\text{Max Protection} = \text{FLC (table)} \times \text{Percentage (from Table 430.52)}$$
Step-by-Step Calculation Examples
Example 1: Branch-Circuit Conductor Sizing
Example 2: Maximum Fuse Size (Dual-Element)
Example 3: Maximum Breaker Size (Inverse Time)
Example 4: Input kVA Calculation
Example 5: Torque Calculation
NEC Table FLC Quick Reference
Table 430.250 – Three-Phase AC Motors (Selected Values)
| HP | 230V | 460V | 575V |
|---|---|---|---|
| 1 | 3.6 A | 1.8 A | 1.4 A |
| 5 | 15.2 A | 7.6 A | 6.1 A |
| 10 | 28 A | 14 A | 11 A |
| 15 | 42 A | 21 A | 17 A |
| 20 | 54 A | 27 A | 22 A |
| 25 | 68 A | 34 A | 27 A |
> Watch Out For: When using the torque formula, always use actual running RPM (from nameplate), not synchronous speed. Using synchronous speed will give a wrong answer.
> Watch Out For: When the calculated protective device size falls between standard sizes, you round up to the next standard size only if it does not exceed the maximum percentage. If it does exceed the maximum, you must round down.
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Quick Review Checklist
Use this list to verify you can confidently answer exam questions on each topic: