Lesson 5.10 — Contactors, Relays, and Overload Relays

§00 — Risk Intercept

🔴 Red Risk Intercept — Read Before Proceeding

Contactors, relays, and overload relays are the components that directly control whether a motor runs or stops. Misunderstanding these devices causes:

Motors that won't start
Motors that won't stop
Machines that start unexpectedly after an OL reset — a fatal hazard

CRITICAL RULES — NON-NEGOTIABLE:

NEVER reset an overload relay without first finding and fixing the cause of the overload
NEVER bypass an overload relay — this removes the only protection between an overloaded motor and thermal destruction
LOTO required before working inside any MCC or motor starter enclosure
Assume all motor starters have 480V AC on the line side until verified de-energized

§01 — Overview

What This Lesson Covers

This lesson covers the three core switching components found in every industrial motor starter:

The contactor — the high-current power switch
The control relay — the low-current control signal switch
The overload relay (OL) — the thermal protection device

Together, these three components form the Full Voltage Non-Reversing (FVNR) motor starter — the most common motor control package in industrial facilities. Every Multi-Craft Technician at LEO will encounter these devices in MCC rooms, panel boards, and equipment enclosures daily.

Understanding how contactors, relays, and OL relays interact is the prerequisite foundation for troubleshooting motor control circuits (covered in Lesson 5.14). Get this right, and motor troubleshooting becomes systematic. Skip it, and you're guessing.

📌 Industry Context

The FVNR motor starter configuration in this lesson appears in: pump stations, conveyor drives, fan and blower controls, compressor motor controls, mixer drives, and virtually every fixed-speed motor application in industrial environments. If it runs a motor, it has these components.

§02 — Learning Objectives

Learning Objectives

Upon completing this lesson, you will be able to:

L5-10-01
Describe the construction and operation of a magnetic contactor
L5-10-02
Distinguish between NO (normally open) and NC (normally closed) contact designations
L5-10-03
Explain the difference between a contactor and a control relay — load rating and application
L5-10-04
Describe the operation of a bimetallic overload relay and an electronic overload relay
L5-10-05
Set an overload relay to the correct trip current for a given motor FLA
L5-10-06
Identify the difference between manual reset and automatic reset overload relays
L5-10-07
Trace the power and control circuit in a FVNR motor starter
L5-10-08
Perform a field inspection of a motor starter and identify signs of failure

§03 — Prerequisites

Prerequisites

The following lessons must be completed before beginning this lesson:

✓ Lesson 5.1 — LOTO Procedures ✓ Lesson 5.6 — Series Circuits ✓ Lesson 5.9 — Overcurrent Protection

⚠ If You Haven't Completed Prerequisites

This lesson assumes you understand LOTO, can trace a series circuit, and know the difference between a fuse and a circuit breaker. If those concepts are not solid, complete the prerequisite lessons first — this content builds directly on them.

§04 — The Magnetic Contactor

The Magnetic Contactor

A contactor is an electrically-operated switch designed for high-current loads. When a coil is energized, an electromagnet attracts the armature, mechanically closing the main contacts. When coil power is removed, a return spring opens the contacts.

Key Distinction

A contactor is controlled by a low-power control signal (the coil) but switches high-power loads (the main contacts). The control voltage and load voltage are almost always different. A 480V motor can be switched by a 120VAC or 24VDC control signal.

Parts of a Magnetic Contactor

⚡ Main Contacts (Power Contacts)

Close and open the power circuit to the motor. Rated in continuous amps and horsepower. Designed specifically for high-current make-and-break — they must withstand the inrush current surge when a motor starts (typically 6–8x FLA). Main contacts on an AC motor contactor are always Normally Open (NO).

🔗 Auxiliary Contacts

Smaller, lower-rated contacts on the same mechanical assembly. Move with the armature — when the main contacts close, the aux contacts actuate simultaneously. Can be configured as NO or NC. Used for: sealing the control circuit (the "seal-in" contact), interlocking with other circuits, pilot light status signaling, and feedback to PLCs.

🔌 Coil (Electromagnet)

The wire-wound coil that, when energized, creates an electromagnetic field pulling the armature. Coil voltage is the CONTROL voltage — not the motor voltage. Most common: 120VAC or 24VDC. The coil voltage is stamped on the contactor body or nameplate. Applying wrong voltage destroys the coil immediately.

🔩 Armature (Moving Core)

The movable iron core that is pulled into the fixed core when the coil energizes. Its mechanical motion closes all main and auxiliary contacts simultaneously. A return spring opens the armature (and all contacts) when the coil de-energizes.

⚡ Arc Suppression

When contacts open while carrying current, an arc forms between them. Contactors use arc chutes (ceramic barriers that cool and split the arc) and sometimes magnetic blowout coils to extinguish this arc quickly. This is a major reason contactors are different from ordinary switches — they are designed to safely interrupt load current. This is also why burned contact pitting is a failure mode.

NEMA Size Chart — 480V 3-Phase Motors

NEMA Size	HP Rating (480V 3φ)	Continuous Amps	Typical Application
Size 0	3 HP	18A	Small pumps, conveyors
Size 1	10 HP	27A	Fans, small compressors
Size 2	25 HP	45A	Medium pumps, mixers
Size 3	50 HP	90A	Large fans, conveyors
Size 4	100 HP	135A	Compressors, crushers
Size 5	200 HP	270A	Large industrial drives
Size 6	400 HP	540A	Heavy industrial
Size 7	900 HP	810A	Very large motors

⚠ NEMA vs. IEC Sizing

IEC (European) contactors are rated by utilization category — AC-3 is the standard for squirrel-cage motors (starting and stopping). IEC contactors are generally smaller and lighter than NEMA equivalents for the same HP, but must be sized more precisely to the load. You'll find IEC contactors in most newer OEM equipment (Siemens 3RT, ABB A-series, Schneider LC1D). They are not always interchangeable with NEMA contactors — always check the replacement spec sheet.

§05 — Contact Types

NO vs. NC Contact Designations

"Normal" state always means coil de-energized. When you read NO or NC on a schematic, ask yourself: "Is the coil energized right now?" If no — that's the normal state.

NO — Normally Open NO

Open when coil is de-energized.
Closed when coil is energized.

— —

Used for: seal-in contacts, motor run indication, interlocks that activate with the coil

NC — Normally Closed NC

Closed when coil is de-energized.
Open when coil is energized.

—/—

Used for: STOP buttons, OL relay contact in control circuit, interlocks that break when coil energizes

Why the Distinction Matters — Fail-Safe Analysis

Scenario	NO Contact Behavior	NC Contact Behavior	Application
Coil energized (normal run)	CLOSED — allows current	OPEN — blocks current	—
Coil de-energized (power loss)	OPEN — blocks current	CLOSED — allows current	Safety relays: use NC → open on power loss = safe state
Control wire breaks	OPEN — load de-energizes	CLOSED — load stays energized	Motor START: use NO → wire break stops motor, not starts it

⛔ Critical Rule

The STOP pushbutton in a motor control circuit is always wired NC (normally closed — current flows in normal state). This ensures that a broken control wire in the STOP circuit causes the motor to stop, not to keep running. A NO stop button would be a safety violation. Know your schematics.

IEC 60947 Contact Notation

Contact	IEC Number Code	Schematic Symbol	Note
Main contacts (power)	1/2, 3/4, 5/6	Heavy lines, always NO	Motor power circuit
Auxiliary NO	13/14, 23/24, 33/34	— — (gap)	Seal-in, interlock, indication
Auxiliary NC	11/12, 21/22, 31/32	—/— (diagonal through gap)	Interlock, OL contact

§06 — Control Relays vs. Contactors

Control Relays vs. Contactors

Both contactors and control relays are electromagnetically-operated switches. The difference is their size, current rating, and intended application.

Feature	Control Relay	Contactor
Current rating	10–20A maximum	18A to thousands of amps
Construction	Lightweight, multiple aux contacts (4–8 contacts typical)	Heavy duty, main contacts + limited aux contacts
Primary use	Signal switching, interlocking, control logic	Power switching — motors, heaters, large loads
Coil life	1–10 million operations	1–5 million operations
Arc suppression	Minimal (low-current contacts)	Full arc chutes and/or blowout coils
Common examples	Ice cube relay, CR relay, plug-in socket relay	Allen-Bradley 500, Siemens 3RT, Eaton C25
Mounting	Plug-in socket, DIN rail	DIN rail, panel mount, MCC bucket

🧊 Ice Cube Relays (Plug-In Relays)

Named for their square translucent plastic body. Available in 8-pin and 11-pin versions. Mount in a matching socket base with DIN rail clip. Extremely common for control logic — easy to replace without rewiring (just unplug the relay body from its socket).

Coil voltages: 24VAC, 24VDC, 120VAC, 240VAC — must match your control supply
Contact rating: typically 10–15A at 120VAC
8-pin: 2 changeover (DPDT) contacts
11-pin: 3 changeover (3PDT) contacts

⛔ Coil Voltage Warning

Plugging a 120VAC coil relay into a 24VDC socket: relay won't pull in (under-voltage). Plugging a 24VDC coil into a 120VAC socket: immediate coil burnout. ALWAYS verify coil voltage before insertion. Check the label on the relay body and the socket wiring.

§07 — Overload Relays

Overload Relays — The Motor's Thermal Protector

The overload relay (OL relay) protects motor windings from sustained overcurrent. This is protection the branch circuit fuse or breaker cannot provide — the fuse is sized at 125–175% of FLA to allow motor inrush current through on starting. A motor running at 115% of FLA continuously will overheat and fail within hours. The OL relay catches this.

Fuse vs. OL Relay — Why Both Are Needed

Fuse/breaker: sized for inrush (typically 175% FLA for time-delay fuses). Protects against short circuits and gross overloads. Will NOT trip on a 120% overload running for hours.

OL relay: sized to 100–115% of FLA. Trips on sustained overcurrent that would damage motor windings. Uses inverse time-current characteristic — longer at 110%, very fast at 300%+.

Bimetallic Overload Relay — How It Works

Three bimetallic strips (one per phase) carry motor current through attached heater elements. On overload:

Heater elements warm from excess current
Heat transfers to bimetallic strips
Differential expansion bends the bimetal
At trip point, the bimetal actuates the trip mechanism
OL contact (NC in control circuit) opens
Contactor coil de-energizes → motor stops

⚠ Heater Element Selection

Bimetallic OL relays use interchangeable heater elements selected by motor FLA. Wrong heater = wrong trip point = wrong protection. Heater selection charts are on the OL relay body or in the manufacturer data sheet. Always match the heater to the motor nameplate FLA — not the wire size, not the breaker rating, not the horsepower.

Electronic Overload Relay — How It Works

Uses current transformers (CTs) to measure actual motor current in each phase. A microprocessor calculates thermal equivalent (accumulated heat model) and trips when the model indicates dangerous temperature. Advantages over bimetallic:

Fully adjustable trip current (dial or keypad) — no heater elements needed
Phase loss detection (won't trip fast enough on single-phase if one CT reads zero)
Phase imbalance detection
Ground fault detection (some models)
Thermistor input — reads actual motor winding temperature
Manual or auto reset, selectable
Trip history log — shows reason for last trip
4–20mA or network output to PLC/SCADA

Common examples: Allen-Bradley E300, Sprecher+Schuh CET4, Siemens 3RU.

OL Relay Setting — Field Procedure

Setting Procedure (Electronic OL)

Read motor nameplate FLA (Full Load Amps)
Set OL trip current to 100% of motor nameplate FLA
If motor has Service Factor ≥ 1.15: may set to 115% of FLA
Verify reset mode is set to MANUAL for industrial applications
Document the setting and the motor FLA on the starter door card

⛔ Never Exceed

Do NOT set OL trip current above motor nameplate FLA + 10% without written engineering authorization. "Just bump it up so it stops tripping" is a capital maintenance failure — find the overload cause, fix the cause.

Setting Calculator

Enter motor FLA from nameplate to calculate OL relay settings:

Motor Nameplate FLA: Service Factor:

Manual vs. Automatic Reset

Reset Type	Operation	When to Use	Safety Note
Manual Reset	Person must physically press RESET button on OL body	Default for all industrial motor applications	Requires site visit before motor restarts — forces investigation
Automatic Reset	OL resets automatically after cooling period (5–10 min)	Remote/unmanned locations where auto-restart is safe and acceptable	⚠ NEVER where unexpected restart endangers people or equipment. Motor can restart with no warning.

⛔ Auto-Reset OL Relay — Unexpected Restart Hazard

An auto-reset OL relay will restart the motor after the OL cools — even if no one is present
A technician working near a "stopped" motor that uses auto-reset OL is at risk if they don't LOTO
If you find an auto-reset OL in an application where people work near moving equipment: flag it, document it, escalate to engineering
Rule: if a motor restart could hurt someone → manual reset OL required

§08 — Interactive Simulator

FVNR Motor Starter Simulator

This simulator models a Full Voltage Non-Reversing (FVNR) motor starter control circuit. Use START/STOP/SIMULATE OVERLOAD to explore how the components interact. Watch how the M auxiliary seal-in contact keeps the motor running after you release START.

FVNR Motor Starter — Control Circuit Simulation

Control Power

ON

M Coil

DE-ENERGIZED

Motor

STOPPED

OL Relay

NORMAL

Contact States

STOP PB (NC)

━━━━━━

CLOSED (normal)

OL Contact (NC)

━━━━━━

CLOSED (normal)

M Seal-In (NO)

— · —

OPEN (motor off)

START PB (NO)

— · —

OPEN (not pressed)

Event Log

[INIT] Simulator ready. Control power ON. Motor stopped.

FVNR Control Circuit Schematic

How the Control Circuit Reads

For the M coil to energize, current must flow from L1 through the entire series path: STOP (NC closed) → OL (NC closed) → one of two parallel paths (seal-in OR START) → M coil → L2.

The seal-in auxiliary contact creates a "latch" — once the motor is running, it bypasses the START button. Pressing STOP breaks the main series path — coil drops out, seal-in opens, and you must press START again to restart.

§09 — FVNR Motor Starter

FVNR Motor Starter — Component Identification

A Full Voltage Non-Reversing (FVNR) motor starter is the complete assembly that provides power switching, protection, and control for a single-speed motor that only runs in one direction. In an MCC (Motor Control Center), each motor gets its own "bucket" — a plug-in or drawout unit containing all starter components.

MCC Bucket Component Stack (Top to Bottom)

1

⚡

Branch Circuit Fuse or Breaker

Disconnect means + overcurrent protection. Sized per NEC 430.52 (motor branch circuit protection). This is where LOTO lockout occurs. Time-delay fuses: typically 175% FLA for squirrel cage motors.

2

🔌

Contactor (M)

Three-pole power switch. Closes on START command, opens on STOP or OL trip. NEMA or IEC sized to motor HP. Main contacts rated for motor inrush. Auxiliary contacts used in control circuit for seal-in and status.

3

🌡

Overload Relay (OL)

Directly below the contactor — current flows through its heater elements or CTs. Bimetallic or electronic type. Set to motor FLA. NC contact in control circuit — opens on trip, de-energizing coil. Must be manual reset in most applications.

4

🔧

Control Terminal Strip

Connection points for field wiring — START/STOP pushbuttons, pilot lights, remote control signals, PLC I/O. All external control wiring terminates here. Each terminal is numbered per the control schematic.

5

🔄

Control Power Transformer (CPT) — if applicable

Steps down 480V power to 120V control voltage for the coil and pilot lights. Not always present — some starters run 480V coils or use 24V control from an external supply. Covered in Lesson 5.11.

Power Circuit vs. Control Circuit

⚡ Power Circuit (3-Phase)

L1, L2, L3 → fuse/breaker → contactor main contacts → OL heaters → motor T1, T2, T3
Voltage: 208V, 240V, 480V, or 600V 3-phase
Current: motor FLA (running) to 6–8x FLA (starting)
LOTO this circuit before any work

🔁 Control Circuit (1-Phase)

L1(or CPT secondary) → STOP NC → OL NC → [seal || START] → M coil → L2(N)
Voltage: 24VDC, 120VAC, or 240VAC typically
Current: milliamps to a few amps (coil current only)
Trace this circuit when troubleshooting motor won't start

💡 Troubleshooting Rule of Thumb

When a motor won't start: trace the CONTROL circuit first. When a motor won't stop (or has welded contacts): suspect the POWER circuit. When the motor trips the OL: you have a load problem (or wrong OL setting), not a starter problem.

§10 — Failures & Field Inspection

Common Failures and Field Inspection

Failure	Symptom	Likely Cause	Action
Welded main contacts	Motor runs even when coil is de-energized; won't stop on STOP command	Contacts welded by overcurrent arc — usually from a fault while running or incorrect contactor sizing	LOTO immediately. Replace contactor. Investigate why fault current passed through.
Burned coil	Contactor won't pull in; coil smells burnt; coil measures open on ohmmeter	Wrong coil voltage applied, coil overheated from excessive cycling, or short in coil winding	Replace coil or entire contactor. Verify control voltage matches coil rating before re-energizing.
Worn/pitted main contacts	Excessive arcing on start/stop; reduced contact life; motor may run rough	Normal wear; excessive inrush from oversized motor; incorrect contactor category for load	Inspect contact faces. Replace if pitting depth exceeds 50% of contact thickness. Do not file contacts.
OL trips frequently	Motor overheating; OL trips during run, not starting	Wrong heater element, motor mechanically overloaded, high ambient temperature in MCC, motor winding issue	Check heater vs. motor FLA. Check motor load (amp meter). Check MCC ventilation. Do NOT just increase OL setting.
Motor won't start — OL not tripped	START pressed, nothing happens; contactor won't pull in	Control circuit open: STOP button stuck open, seal contact failure, OL contact failed open, loss of control power, coil open	Trace control circuit from L1 to L2. Use voltmeter step-by-step — find where voltage is lost. (See Lesson 5.14)
Chattering contactor	Rapid buzzing/clicking from contactor; overheating coil; premature contact wear	Low control voltage (coil can't hold in), intermittent NC contact in series circuit, loose connection vibrating open	Measure control voltage at coil terminals while contactor chatters. Check all NC contacts in series for intermittent opens.
Motor trips OL on start only	OL trips within seconds of starting; motor doesn't reach full speed	OL class too fast (use Class 20 or 30 for high-inertia loads), mechanical jam at start, OL set too low	Verify OL class is appropriate for the load. Check for mechanical issues. Verify OL setting is at motor FLA (not below).

Field Inspection Checklist

⛔ LOTO Before Any Inspection Inside Starter Enclosure

Pilot light and indicator observations can be done with power on. Any physical inspection inside the enclosure requires LOTO. Verify absence of voltage before touching any component inside a starter panel.

Inspection Progress 0 / 8 items

Coil voltage verified: Nameplate coil voltage matches actual control supply voltage

Main contact inspection: No visible pitting, burning, or material transfer on contact faces. Contact gap appropriate (replace if gap >50% consumed)

Auxiliary contacts: Aux contacts seat properly when contactor is manually depressed (de-energized). No mechanical binding.

OL heater size: Heater element part number matches the selection table for the motor's actual FLA (not HP, not wire size)

OL current setting (electronic OL): Trip current set to motor nameplate FLA ±5%. Reset mode is MANUAL.

Terminal torque check: All control and power wire terminations are tight. No loose strands, no signs of arcing at terminals.

Enclosure temperature: Enclosure exterior is not excessively hot. MCC door closes fully. Ventilation unobstructed.

Operating current vs. FLA: Motor running amps (measured with clamp meter on all three phases) are at or below nameplate FLA and balanced within 5% across phases

§11 — Assessment

Knowledge Check — 5 Questions

Select the best answer for each question. Immediate feedback is provided after submission.

Q1. A contactor's AUXILIARY normally-open (NO) contact is wired in parallel with the START pushbutton in the control circuit. What is the specific function of this contact?

A) Provides overcurrent protection for the motor windings B) Seals (latches) the control circuit so the motor keeps running after the START button is released C) Provides a second independent start command path from a remote location D) Disconnects the motor power circuit in the event of an overload

Q2. An overload relay has tripped and the motor is stopped. What is the correct first response?

A) Press the RESET button immediately to get the motor running again B) Investigate and correct the cause of the overload before resetting the OL relay C) Increase the OL trip setting by 10% and then reset to prevent future trips D) Replace the OL relay heater element with the next larger size

Q3. A NEMA Size 2 contactor is rated for which motor application at 480V three-phase?

A) 10 HP, 27A continuous B) 25 HP, 45A continuous C) 100 HP, 135A continuous D) 5 HP, 15A continuous

Q4. A safety relay in a machine guard circuit must open and stop the motor if control power is lost. Which contact type should be used for this safety relay's contact in the motor control circuit?

A) A normally open (NO) contact — so the motor can only run when the safety relay coil is powered B) The START pushbutton contact — it's already in the circuit C) A normally closed (NC) contact on the safety relay — it opens when the coil de-energizes, stopping the motor D) Either type — it doesn't matter for safety functions

Q5. An "ice cube" plug-in relay has a 120VAC coil rating stamped on its body. A technician installs it into a socket that is wired to a 24VDC control supply. What is the most likely outcome?

A) Nothing — plug-in relays automatically adjust to the supply voltage B) The relay energizes but at reduced pull force, causing intermittent operation C) The relay coil will not energize; it may emit a slight hum from the AC harmonic in the DC supply D) The coil will be immediately damaged — 24VDC is the wrong voltage type AND magnitude for a 120VAC coil

§12 — Summary

Summary

Key Takeaways — Lesson 5.10

Contactors are high-current power switches controlled by a low-current coil. The coil voltage is the control voltage — separate from the load voltage.
NEMA sizing is by HP/current class (Size 0–7). IEC sizing is by utilization category (AC-3). Know which type you are working with.
NO contacts open on coil loss. NC contacts close on coil loss. This distinction determines fail-safe behavior — know your application.
Control relays switch signals, not power. Contactors switch power loads. Mixing them up damages equipment.
Overload relays protect motor windings from sustained overcurrent. Set to 100% of motor nameplate FLA. Never bypass.
Manual reset OL is required for any application where unexpected restart is a safety hazard — which is most of them.
The M auxiliary seal-in contact (NO) creates latch logic — once the motor is running, it holds itself in without the operator's hand on START.
Troubleshoot motor control circuits by tracing the control circuit systematically — never by guessing or bypassing protection.

📚 Next Lesson

Lesson 5.11 — Transformers and Power Supplies: How the Control Power Transformer (CPT) inside the MCC bucket steps down 480V to 120V control voltage, transformer sizing, polarity, and troubleshooting transformer-related control power loss.