Motor overload protection devices guard against overcurrent damage from mechanical
overload, low voltage, and phase loss. This lesson covers thermal overload relays,
internal protectors, fuses, relay selection, and common causes of motor overload.
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3.3.1 — Purpose and Principles of Motor Protection
Motor overload protection devices safeguard motors from damage caused by overcurrent
conditions resulting from mechanical overload, low voltage, phase loss, or other
abnormal operating conditions. Properly selected and applied overload protection
extends motor life, prevents fires, and reduces equipment downtime.
Motors are designed to carry their rated full-load current continuously, with
additional capacity for brief overloads up to their service factor. However,
sustained overcurrent causes excessive heating that rapidly degrades winding
insulation.
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Temperature and insulation life
Each 10 °C (18 °F) increase above rated temperature can
reduce insulation life by approximately 50%. Even moderate
sustained overloads dramatically shorten motor life.
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Acceptable vs. Harmful Overcurrent
Effective motor protection must distinguish between two fundamentally different
overcurrent situations:
✓ Acceptable â Allow to Pass
Starting current (inrush) — brief, high current lasting 1–3 seconds
Momentary load spikes from brief mechanical disturbances
Service factor loads within rated limits
✗ Harmful â Must Disconnect
Sustained overcurrent from mechanical overload
Continuous operation at locked rotor current
Phase loss causing remaining phases to carry excess current
Protection devices are calibrated with inverse time–current
characteristics: severe overloads trip quickly, moderate overloads
trip after longer periods, matching motor thermal characteristics.
3.3.2 — Types of Motor Overload Protection
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Thermal Overload Relays
External — Motor Starter Circuit
Bimetallic Overload Relays
Bimetallic elements warp when heated by motor current flowing through heater
elements. When current exceeds the setpoint for sufficient time, the strip bends
enough to trip a mechanism that opens contacts, de-energizing the starter coil
and disconnecting the motor.
Available in manual reset (requires operator intervention after
trip) and automatic reset (resets when bimetal cools) models.
Automatic reset is useful for remote applications but potentially hazardous if
the overload cause has not been corrected.
Melting Alloy Overload Relays
Use a eutectic alloy (solder pot) that melts at a specific temperature. When the
alloy melts, a ratchet wheel turns and opens the starter contacts. The relay must
cool completely and be manually reset before restart. Melting
alloy relays provide excellent repeatability and are less affected by ambient
temperature than bimetallic types.
Electronic Overload Relays
Microprocessor-based relays use current transformers to monitor motor current
continuously. Algorithms calculate thermal accumulation using precise motor
heating models, providing superior accuracy compared to electromechanical devices.
Wide adjustment range covering multiple motor sizes
Diagnostic displays and fault indication
Communication capabilities for remote monitoring
Insensitive to ambient temperature
Faster response to severe overloads
Used with:Magnetic Motor StartersCommercial & Industrial Applications
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Internal Thermal Protectors
Internal thermal protectors are temperature-sensitive switches embedded
directly in the motor windings during manufacturing. They provide the most
direct measurement of motor temperature, responding to actual winding temperature
rather than current.
Bimetallic Disc Protectors
A snap-action bimetallic disc opens contacts when winding temperature exceeds
the trip point — typically 90 °C to 150 °C
(194 °F to 302 °F) depending on motor insulation class.
When the motor cools, the disc snaps back and allows restart.
Most internal protectors are automatic reset designs that
cycle the motor if the overload persists. Repeated cycling indicates motor
problems or inadequate ventilation.
Thermistor Sensors
Some motors use PTC or NTC thermistor sensors embedded in the windings.
Connected to external relay modules that monitor resistance, they trip when
temperature limits are exceeded.
This approach provides more sophisticated temperature monitoring and can
interface with building management systems for data logging
and remote alarming.
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Fuses and Circuit Breakers
Fuses and circuit breakers provide short-circuit protection and backup
overload protection. They are sized according to electrical codes,
typically at 125% to 175% of motor full-load current depending on motor type
and starting characteristics.
Dual-Element (Time-Delay) Fuses
Allow motor starting current while providing relatively fast protection
against short circuits and sustained overloads. A thermal element
tolerates brief overloads; a magnetic element responds instantly
to short-circuit currents.
Motor Circuit Protectors (MCP)
Specialized circuit breakers with magnetic trip elements that allow starting
current while providing short-circuit protection. MCPs typically
do not provide overload protection and must be used in
combination with overload relays.
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Inverse Time Circuit Breakers
Standard thermal-magnetic breakers provide both overload and short-circuit
protection but may not adequately coordinate with motor starting currents. Sized
at 250% to 400% of motor FLA for high starting-current motors;
serve as backup protection when coordinated with overload relays.
3.3.3 — Overload Relay Selection and Settings
Sizing Heater Elements
Thermal overload relays use replaceable heater elements matched to the motor
full-load current. Select heater elements based on motor nameplate
FLA — not on conductor ampacity or circuit breaker size.
Manufacturers provide selection tables correlating heater part numbers to current
ratings.
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Service factor motors
For motors with service factors, heater elements may be sized for up to
125% of nameplate FLA, allowing continuous operation at
service factor load. However, this provides less protection margin —
motors operated continuously at service factor will have reduced life
expectancy.
Adjustable Overload Settings
Many overload relays provide adjustment ranges (typically ±15% to ±25%)
around the nominal heater element rating. Set the overload relay trip point at
motor nameplate FLA for maximum protection. Higher settings
(up to 125% of FLA) may be acceptable for service factor motors or motors subject
to brief, harmless overloads, but increase the risk of damage from sustained
overcurrent.
Class Designation
Overload relays are classified by maximum trip time from a cold start at a
specified overload current. Select the class based on motor starting
characteristics and load requirements.
Class
Max Trip Time at 600% FLA
Typical Applications
10
10 seconds
Motors with very short starting time; submersible pumps
20
20 seconds
Standard motors; general-purpose HVAC/R applications
30
30 seconds
High-inertia loads; compressors with long starting time
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Class 20 is most common for HVAC/R
Class 20 suits most general HVAC/R motor applications. Class 30 may be
necessary for compressors and other high-inertia loads to prevent nuisance
tripping during starting.
3.3.4 — Common Causes of Motor Overload
When overload protection devices trip, identifying and correcting the underlying
cause is essential before restarting the motor. Systematically diagnose by
measuring supply voltage, checking for mechanical problems, verifying proper
rotation, inspecting ventilation, and measuring actual operating current.
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Common Overload Causes — Investigate Before Restart
Low Voltage — inadequate supply voltage causes increased current draw for a given load
Single Phasing — loss of one phase in three-phase motors causes remaining phases to carry excessive current
Voltage Imbalance — unequal phase voltages cause current imbalance and overheating
Locked Rotor — motor stalled by mechanical blockage draws locked-rotor current continuously
Cycling Loads — rapid start-stop cycling does not allow adequate motor cooling between starts
Inadequate Ventilation — blocked ventilation or high ambient temperature causes overheating even at normal load
Insulation Failure — winding insulation breakdown causes internal short circuits and excessive current
Improper Motor Selection — motor undersized for the application
Incorrect Overload Setting — protection set too low for actual motor requirements
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Never bypass or defeat overload protection
Bypassing overload protection devices to keep a motor running risks motor
destruction, fire, and personal safety hazards. Always identify and correct
the underlying cause before restarting. If tripping recurs, the motor or
protection settings must be investigated — not bypassed.