Motor protection relays are designed to detect abnormal electrical, thermal, and mechanical conditions and initiate alarms or trips to prevent motor damage, fire risk, and production loss.
🔹 Main Motor Protection Functions
49 – Thermal Overload Protection
Protects stator winding from overheating using thermal model / I²t calculation.
50 – Instantaneous Phase Overcurrent
Trips motor instantly during short-circuits or severe faults.
51 – Time-Delay Phase Overcurrent
Backup protection for sustained overload or downstream faults.
50N / 50G – Instantaneous Earth Fault
Detects high-magnitude ground faults with fast tripping.
51N / 51G – Time-Delay Earth Fault
Selective earth fault protection with time coordination.
46 – Negative Sequence / Phase Unbalance
Prevents rotor overheating due to phase unbalance or single phasing.
47 – Phase Failure / Phase Reversal
Protects against wrong phase sequence and missing phase.
48 – Incomplete Sequence / Locked Rotor (Start)
Trips if motor fails to accelerate during starting.
51LR / 14 – Locked Rotor (Running)
Detects mechanical jamming during normal operation.
66 – Excessive Starts / Start Inhibit
Limits number of starts to protect motor thermal life.
37 – Undercurrent Protection
Detects loss of load, dry run (pumps), or broken shaft.
38 – Bearing Temperature Protection
Uses RTDs to monitor bearing overheating.
49R – Rotor Thermal Protection
Special protection for slip-ring motor rotor heating.
26 – Apparatus Temperature
RTD-based protection for winding or bearing temperature.
63 – Pressure / Level Switch Protection
Auxiliary protection linked with pumps, compressors, driven loads.
⚡ Voltage & Frequency Protection
27 – Undervoltage
Prevents stalling and overheating during low voltage.
59 – Overvoltage
Protects insulation and connected equipment.
81U – Underfrequency
Avoids overheating due to reduced cooling.
81O – Overfrequency
Protects against overspeed and mechanical stress.
🔧 Starting & Control Related
64 – Stator Earth Fault (Sensitive)
High-sensitivity protection for LV & HV motors.
86 – Lockout Relay
Master trip relay requiring manual reset after major fault.
94 – Trip Relay
Interfaces protection relay with breaker trip circuit.
52 – Circuit Breaker
Motor feeder switching and fault isolation device.
53 – Exciter / Field Relay
Used in synchronous motor field control.
🏭 Special Motors (HV / Critical Drives)
40 – Loss of Field
Protects synchronous motors from excitation failure.
78 – Pole Slipping
Detects instability in synchronous motor operation.
87M – Motor Differential Protection
High-speed internal fault protection for large HV motors.
