Motor protection — ANSI 49 / 50 / 51 / 46 / 27
Layered relay protection for industrial motors: thermal overload (49), instantaneous + time overcurrent (50/51), ground (50G/51G), negative-sequence (46), undervoltage (27), phase reversal (47), start-counter (66), motor differential (87M).
Step 1 — Motor protection: layered ANSI functions
Reference notes
Industrial motors are protected by a layered set of relay functions defined by ANSI device-number standards. Use Next → to walk through the major motor-protection ANSI functions and their typical settings, all of which are now implemented in a single microprocessor-based relay (SEL-710, GE Multilin 469, ABB REM615, Siemens SIPROTEC).
ANSI device numbers — motor protection set
| ANSI | Function | Typical pickup / setting |
|---|---|---|
| 49 | Thermal overload (I²t model) | ~105 % FLA, τ_th = 10–30 min |
| 50 | Instantaneous phase overcurrent (short-circuit) | 1.6–2.0× LRA, 30–100 ms delay |
| 51 | Time-delayed overcurrent (sustained overload) | 120–125 % FLA, definite or inverse-time |
| 50G / 51G | Ground overcurrent (zero-sequence CT) | 5–20 % FLA, 100–500 ms |
| 46 | Negative-sequence / unbalance | I_2²·t = 30 s capability |
| 27 | Undervoltage | 80 % nominal, 1–5 s |
| 47 | Phase reversal / reverse-sequence | Instantaneous |
| 66 | Start-frequency / start-count | 2 cold/hr, 1 hot/hr typ. |
| 37 | Undercurrent (load loss) | 50 % FLA, several seconds |
| 38 | Bearing-temperature RTD | Per manufacturer |
| 87M | Motor differential (large motors only) | 10–20 % FLA differential |
49 — Thermal overload (the cornerstone)
Maintains a thermal model that integrates motor heating (I²·t-style during run and start) and cooling (exponential decay with τ_th). Pickup typically 105 % FLA. Differentiates between:
- Running overload — current modestly above FLA → trip after extended duration (curve-shaped).
- Starting overload — current near LRA for several seconds during acceleration → ride through up to the manufacturer's locked-rotor-time rating (typically 10–30 s).
After a trip, the thermal model retains state, preventing restart until cooling drops capacity below ~60–70 % of trip threshold.
50 / 50G — Instantaneous overcurrent
50: phase overcurrent for short-circuit detection. Pickup set well above LRA (typically 1.6–2.0× LRA) to ride through starting inrush; 30–100 ms time delay. 50G: ground overcurrent using a zero-sequence (core-balance) CT whose output equals 3·I_0 (three times the zero-sequence component); detects high-impedance arcing ground faults at pickups of just 5–20 % FLA.
46 — Negative-sequence current (essential!)
Unbalanced supply or open phase produces I_2 (negative-sequence current) in the stator. I_2 generates a backward-rotating MMF at 2f relative to the rotor → severe rotor eddy heating. Damage scales as I_2² · t. Industrial motors typically rated I_2² · t = 30 s. Function 46 integrates I_2² · t and trips when integrated exposure exceeds the capability. Detects open-phase conditions within seconds, modest unbalance within minutes.
87M — Differential (large motors only)
For motors typically > 500 HP and > 1000 V (medium-voltage 4.16 / 6.6 / 13.8 kV). Requires 6 CTs: 3 line + 3 neutral. Compares I_line − I_neutral per phase; non-zero differential = internal fault. Same principle as transformer differential. Fast, sensitive, selective.
Implementation
- Microprocessor relay — single integrated device implementing all ANSI functions in software. Examples: SEL-710 (SEL), Multilin 469 (GE), REM615 (ABB), SIPROTEC 7SK (Siemens).
- Inputs — 3 phase CTs, 3 phase VTs, dedicated ground/zero-sequence CT, RTDs for bearings & windings, DC supply.
- Outputs — trip contact to contactor (low-voltage motors) or breaker (MV motors), status contacts for run/trip/alarm.
- Communications — Modbus, DNP3, or IEC 61850 to plant control / SCADA. Settings configured via PC, password-protected.
- Cost justification — microprocessor relay typically justified for motors > 100 HP; below that, simpler thermal-magnetic overload relays handle basic 49 + 50 + 51 at much lower cost. Mission-critical motors (fire pumps, large compressors, crushers) always get full microprocessor relay including 87M regardless of size.