Motor Relay Protection Formula
Motor relay protection settings are calculated using motor nameplate data, service factors, starting currents, and system parameters to ensure safe operation and prevent nuisance trips.
Key Steps for Calculation
1. Gather Motor Data: Start with accurate motor nameplate information: rated power (kW or HP), voltage, full-load amps (FLA), service factor (SF), locked rotor current (LRA), locked rotor time (hot and cold), NEMA design letter, and insulation class . 2. Overload (Thermal) Protection – ANSI Device 49:
- Set the overload relay pickup at 115–125% of FLA, adjusted for service factor.
- Example: Motor FLA = 78 A, SF = 1.15 → Overload pickup = 78 × 1.25 = 97.5 A (set at 98 A).
- Thermal time constant is typically 80% of cold stall time; hot/cold ratio is used to select the trip class (e.g., Class 20 for pumps), . 3. Short-Circuit and Overcurrent Protection – Devices 50/51:
- Device 50 (Instantaneous): Must exceed 1.5–2 times LRA to avoid nuisance trips during startup.
- Example: LRA = 468 A → Device 50 pickup = 1.7 × 468 = 795.6 A (set at 800 A).
- Device 51 (Time-Delayed): Provides inverse-time tripping for sustained overloads. Settings are coordinated with upstream breakers and contactors . 4. Consider System Parameters:
- Include CT ratios, grounding method, cable and transformer impedances, and fault levels.
- Calculate per-unit impedances for motors and sources to determine fault currents and ensure proper relay coordination . 5. Use Standards and Curves:
- IEC 60255 and IEEE C37.112 define inverse-time curves for overload and short-circuit protection.
- Adjust constants according to motor type, application, and environmental conditions (e.g., ambient temperature), . 6. Verification and Tools:
- Online calculators (e.g., EleCalculator, Mechavolt, IEC Overload Relay Calculator) can automate FLA, overload, and breaker sizing calculations.
- Always verify results against NEC or IEC standards and consider site-specific conditions .
Practical Tips
- Avoid using service factor current instead of true FLA for relay pickup; incorrect settings can cause frequent nuisance trips and production losses .
- For contactor-fed motors, Device 50 may be disabled if the contactor cannot interrupt short-circuit current; upstream breakers handle fault interruption.
- Include a short time delay (50–100 ms) for Device 50 to prevent trips from DC offset during startup .
- Review both overload and short-circuit protection separately, as they respond to different types of overcurrent events . By following these steps and using accurate motor and system data, you can calculate reliable motor relay protection settings that safeguard equipment while minimizing unnecessary downtime.
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