Core Functions of Motor Protection Relays with Corresponding ANSI Codes
1. Overload Protection (ANSI 49)
Description: The most fundamental and critical protection. It monitors motor current to simulate the thermal accumulation process, preventing insulation damage and burnout caused by prolonged operation above the rated current.
Sub-functions:
Thermal Capacity Calculation / Thermal Overload
Inverse-Time Characteristic (shorter trip time with higher current)
2. Locked Rotor / Stall Protection (ANSI 48 / 14)
Description: Protects the motor if it becomes jammed during starting or running, drawing 4-8 times the rated current. It ensures rapid disconnection to prevent burnout within seconds.
ANSI 48: Starting Stall
ANSI 14: Under-speed / Over-speed Protection (can also cover running stalls)
3. Short-Circuit Protection (Instantaneous Overcurrent) (ANSI 50)
Description: Detects phase-to-phase short-circuit faults in the motor circuit, providing instantaneous or very short-time delayed tripping.
4. Time Overcurrent Protection (ANSI 51)
Description: Acts as a backup to short-circuit protection or for protecting the motor feeder cable. Its operating time is fixed (or fixed within a range), independent of the current magnitude.
5. Earth Fault / Ground Fault Protection (ANSI 50G / 51G or 50N / 51N)
Description: Detects single-phase-to-ground faults caused by insulation failure in the motor or cable.
ANSI 50G/50N: Instantaneous Earth Fault
ANSI 51G/51N: Time-Delayed Earth Fault
6. Phase Reversal / Loss of Phase / Unbalance Protection (ANSI 46)
Description:
Loss of Phase: Prevents motor damage from single-phasing.
Phase Reversal: Ensures the motor starts in the correct rotation direction.
Current Unbalance: Detects and protects against negative-sequence current caused by voltage unbalance or single-phasing, which causes severe rotor heating.
7. Underload / Loss of Load Protection (ANSI 37)
Description: Detects an unexpected loss of mechanical load (e.g., pump run dry, fan damper failure, broken conveyor belt), helping to prevent equipment damage and process interruption.
8. Overvoltage Protection (ANSI 59)
Description: Excessive voltage can cause magnetic saturation of the motor core, leading to a sharp increase in magnetizing current and excessive heating.
9. Undervoltage Protection (ANSI 27)
Description: Prevents the motor from drawing excessive current to maintain torque during low voltage conditions, which causes overload and overheating. Also prevents line current surges from simultaneous re-acceleration of multiple motors after voltage recovery.
10. Restricted Earth Fault / Zero-Voltage Protection (ANSI 59N)
Description: Used to detect internal ground faults within the motor windings, typically applied in impedance-grounded or ungrounded systems.
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Advanced Protection Functions (Common in Motor Differential and Comprehensive Protection Relays)
11. Differential Protection (ANSI 87M)
Description: The most sensitive and rapid protection for internal motor faults (e.g., phase-to-phase faults, turn-to-turn faults). It operates by comparing the current at the motor line terminals with the current at the neutral end. It is a core function for protecting large and medium-voltage motors.
12. Start Supervision / Number of Starts (ANSI 66)
Description: Limits the number of successive motor starts within a short period or enforces a cooling time between starts, preventing cumulative thermal damage.
13. Temperature Protection (ANSI 38 / 49T)
Description: Provides direct temperature monitoring of critical points like motor bearings and windings (via PT100/PT1000 RTDs), offering the most direct thermal protection.
ANSI 38: Bearing Temperature / Winding Temperature
ANSI 49T: Temperature Detection
14. Reverse Power / Underpower Protection (ANSI 32)
Description: Similar to underload protection but monitors active power. For pump and fan loads, a drop in power often indicates a process abnormality.
15. Power Factor Protection (ANSI 55)
Description: Monitors the motor’s power factor, which can be used to determine if the motor is unloaded or lightly loaded for energy savings or process monitoring.
16. Out-of-Step Protection / Incomplete Sequence Protection (ANSI 78)
Description: Primarily for synchronous motors, it rapidly disconnects the motor from the supply if it falls out of synchronism.
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