During operation, motors shall be protected or alarmed against overload, phase loss, locked rotor, short circuit, overvoltage, undervoltage, earth leakage, three-phase unbalance, overheating, bearing wear as well as stator and rotor eccentricity. Therefore, dedicated protection is essential for electric motors.
Motor Protection Relay types
| English Name | ANSI Code | Function Description |
|---|---|---|
| Instantaneous Overcurrent | 50 | Trips immediately in case of severe short circuit faults |
| Time-delay Overcurrent | 51 | Acts with time delay against motor overload and minor locked rotor |
| Ground/Earth Leakage Overcurrent | 50G / 51G | Detects stator and line ground faults and earth leakage |
| Negative Sequence / Three-phase Unbalance / Phase Loss | 46 | Protects against open phase, current unbalance and abnormal phase sequence |
| Locked Rotor Protection | 51LR / 66 | Provides protection when rotor is stuck during startup or operation |
| Thermal Overload Protection | 49M | Prevents motor burnout caused by long-term overheating via thermal model |
| Winding Temperature Protection | 26 | Monitors motor winding temperature and triggers action at over temperature |
| Undervoltage Protection | 27 | Protects motor when grid voltage drops abnormally |
| Overvoltage Protection | 59 | Activates protection against excessively high grid voltage |
| Phase Reversal Protection | 46R | Prevents motor reverse rotation for pumps, fans and similar loads |
| Frequency Deviation Protection | 81 | Monitors abnormal grid frequency |
| Underload Protection | 37M | Detects no-load condition and loss of load |
| Reverse Power Protection | 32 | Avoids reverse power caused by motor driven by external equipment |
| Motor Differential Protection | 87M | Protects large high-voltage motors against internal inter-turn and inter-phase faults |
| Bearing Wear Monitoring | / | Monitors abnormal wear of motor bearings |
| Stator & Rotor Eccentricity Monitoring | / | Detects eccentricity faults of stator and rotor |
The above is a summary of electric motor relay protection functions. Below is a detailed introduction to the common electric motor relay protection features.
Introduction to Motor Protection Functions of Electric Motor Protection Relays
87M Protection Relay
For motors with a rated power of 2000 kW and above, or motors below 2000 kW where the instantaneous overcurrent protection relay fails to meet the sensitivity requirements, differential protection shall be adopted to protect against phase-to-phase faults inside the motor and on outgoing leads, with the protection operating for tripping.
Motor differential protection calculation
Taking a 2000 kW, 6 kV motor as an example (rated current approx. 220 A, CT ratio selected as 300/1 A, secondary rated current Ie2≈0.73A), the differential protection calculation is presented as follows:
| Item | Parameter Description | Setting Value |
|---|---|---|
| Operating current | Set to 0.4Ie2 to avoid the maximum unbalanced current | 0.29 A |
| Inflection point and slope | Inflection point current: 1.0Ie2; Ratio restraint slope Kb1=0.4 | 0.73 A (inflection point current) |
| Differential high-speed tripping | For rapid clearance of severe faults, set to 6Ie2 | 4.38 A |
| Auxiliary functions | CT broken circuit blocking threshold: approx. 1.2Ie2; 15% second harmonic restraint enabled to prevent inrush current mal-operation | 0.88 A (CT broken circuit blocking threshold) |
This setting can reliably distinguish internal faults from external abnormalities and guarantee protection selectivity. Specific values shall be verified according to on-site short-circuit current.
Motor protection relay diagram
50/51 protection relay
The protection for phase-to-phase faults of motor stator windings is used for motors under 2000 kW and trips on fault. This overcurrent relay protection shares essentially the same principle as transformer and line overcurrent protection.
49 Protection Relay
This overload relay motor protection prevents overcurrent caused by overload of the mechanical equipment driven by the motor. It operates to issue an alarm signal, or trips after a preset time delay
Motor earth fault protection relay
In small earth current systems, single-phase earth fault protection is mandatory for motors with earth current over 5 A. It gives an alarm at 5~10 A and trips above 10 A.
A single-phase earth fault on high-voltage motors does not require immediate shutdown, as it generally does not interfere with normal operation and only disables leakage protection.
For personal safety and to avoid permanent damage, electronic motor protection relays prohibit energizing motors with internal electric leakage.
27 Relay Protection
This 27 undervoltage protection relay prevents motors from auto-restarting when voltage drops or fails, and operates to trip. It can be installed as required.
51LR Protection Relay
The locked rotor protection relay safeguards the motor during operation. When the rotor gets accidentally locked due to overload, it acts rapidly to shut down the motor, so as to prevent or mitigate damage to the drive unit, bearings and other motor components. This function is automatically enabled after motor start-up is completed.
This protection is equipped with two stages of definite-time overcurrent elements, each of which can be set independently. The two elements compare the positive-sequence current after motor start-up with the preset overcurrent threshold. A start signal will be triggered once the positive-sequence current exceeds the setting value. If no locked rotor condition is detected upon expiry of the user-defined time delay, a tripping signal will be issued.
For positive-sequence current greater than 0.2 In, the reset value is approximately 95% of the operating setting.
37 Undercurrent (or Underpower) Relay
The 37 undercurrent protection relay activates when all three-phase currents of the air-cooled motor drop below the preset current setting, and issues an alarm signal after a set time delay. It will also trigger an undercurrent alarm when the cooling fan stops running.
46 Relay Rrotection
The negative sequence relay is commonly used to protect motors against negative sequence current resulting from asymmetric faults or unbalanced loads in the motor system.
When negative sequence current flows through the stator windings of a three-phase induction motor, a reverse rotating electromagnetic field is generated, which produces double-frequency current in the rotor. This induces eddy currents on the rotor surface and causes overheating at rotor ends and slot edges.
If the negative sequence current exceeds the motor’s withstand capability, the rotor will overheat and endanger the safe operation of the motor.
This protection is configured with two stages of definite-time overcurrent and one stage of inverse-time overcurrent, all of which can be set independently. Analog channel selection is available for the operating quantity of negative sequence overcurrent protection, allowing users to choose either protection CT or measuring CT.
40 Protection Relay
The loss of excitation protection relay is specially designed for synchronous motors. When a synchronous motor loses excitation or operates with insufficient excitation, it will fall out of synchronism, along with changes in current, voltage, power factor and other electrical parameters. By monitoring these variations, the relay can detect and handle loss-of-excitation faults promptly to protect the motor from damage.
The Loss of Field Protection generally consists of out-of-step protection and under-excitation protection. The out-of-step protection detects the out-of-step condition of synchronous motors and trips the circuit once a fault is identified, preventing continuous operation under out-of-step status. The under-excitation protection monitors the excitation level.
It issues an alarm or initiates tripping when excitation is insufficient, so as to protect the motor from damage caused by under-excitation.
Thermal Overload Protection Relay
The thermal overload relay motor protection, one of the motor protective relays, protects the motor by monitoring its temperature.
The motor temperature will rise during prolonged operation or heavy loading, and may burn out without timely intervention.
Equipped with temperature sensors mounted on the motor, this protection tracks temperature in real time. When the temperature exceeds the preset threshold, it trips the circuit or triggers an alarm to prompt operators to lower the motor temperature.
Motor Dry Run Protection Relay
It is widely used for pump and fan motors. This protection functions under dry running and dry grinding conditions when the equipment operates without water or media, preventing component wear and motor overheating damage. It is commonly equipped in pump motor protection relays.
Summary of Motor Protection Functions
Refer to the motor protection relay setting guide for relay setting for motor protection, including the configuration of protection functions and parameters.
Motor Relay Protection Selection Method from China Motor Protection Relay Factory
Select protection relays according to motor capacity.
Power < 200 kW: Thermal relays or magnetic starters are sufficient.
Power range 200 kW to 2000 kW: Adopt smart motor protection relays (with protection functions against overload, locked rotor, phase unbalance, etc.).
Power > 2000 kW or for stringent process requirements: Differential protection shall be equipped, and protection devices shall be configured in dual redundancy.
The products are sorted by voltage level.
Motor protection falls into two categories: 400V low-voltage motor protectors and high-voltage motor protection for voltage levels of 6kV and higher.
Types of Motor Protection Devices Classified by Function and Product Type
Comprehensive Motor Protection Device
Motor Differential Protection Device
Motor Magnetic Balance Protection Device
Motor Protection Device for Variable Frequency Start-up
Conclusion
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