Motor Protection Faults, Protection Schemes and ANSI Functions

Overview

This paper mainly illustrates motor fault categories, protection schemes as well as relay setting calculation for motor protection, assisting overseas electrical engineers in mastering motor protection devices and their practical application specifications.

10kV high-voltage motors act as critical power equipment across industrial enterprises. Their safe and stable operation directly affects power grid reliability and power quality. These motors entail high procurement costs; prolonged abnormal operation will easily lead to winding burnout and permanent equipment damage.

For this reason, complete protective relays shall be installed to identify all internal faults and abnormal operating conditions at an early stage, eliminate hidden risks, and prevent severe damage such as motor burnout caused by extended faulty operation.

Common Motor Protection Faults

Internal Permanent Motor Faults

1. Single-phase grounding fault or phase-to-phase short circuit on stator windings
2. Inter-turn short circuit within one phase of stator winding
3. Loss of excitation current in rotor excitation circuit; single-point or two-point grounding faults on rotor windings

Abnormal Operating Conditions

Long-term operation under the following conditions will gradually deteriorate motor components:

1. Stator overcurrent induced by external short-circuit faults
2. Negative-sequence overcurrent resulting from asymmetric external short circuits or unbalanced three-phase loads
3. Symmetric three-phase overload when actual load exceeds motor rated capacity
4. Transient stator overcurrent caused by sudden load surges
5. Reverse power operation triggered by the closure of turbine main steam valves
6. Rotor winding overload due to excitation circuit faults or prolonged forced excitation duration

Motor Protection Functions and ANSI Codes

The ANSI (American National Standards Institute) device number system provides a standardized way to identify protection functions in electrical equipment. Below are the key protection functions for motor protection relays, organized by application category.

Motor Protection Functions

ANSI	Protection Function	Application
49	Thermal Overload	Motor overload protection using thermal image model
50	Instantaneous Overcurrent	Short-circuit protection, instantaneous trip
51	Time Overcurrent	Stalled rotor protection with time delay
51LR	Locked Rotor / Excessive Start Time	Protection against prolonged starting
46	Phase Unbalance / Negative Sequence	Current unbalance and phase loss detection
47	Phase Sequence Voltage	Phase reversal detection
37	Undercurrent / Underpower	Loss of load protection (dry running, belt breakage)
27	Undervoltage	Low voltage protection
59	Overvoltage	High voltage protection
50N/51N	Ground Fault	Earth fault / ground fault protection
48	Incomplete Sequence	Motor starting supervision
66	Starts Per Hour	Frequent starting limitation
86	Lockout Relay	Trip logic and lockout
87M	Motor Differential	Differential protection for motor

Motor Protection Schemes by Motor Size and Voltage

Motor protection requirements vary significantly with motor size and voltage level. The following outlines protection schemes by motor category.

Low-Voltage Motor Protection

Low-voltage motors (<1000V) are the most common in industrial facilities, powering pumps, fans, compressors, and conveyors.

Motor Size	Recommended Protection	Key Functions
< 50kW	Thermal overload relay + fuses/MCCB	Overload (49), short-circuit (50/51)
50kW ~ 200kW	Digital motor protection relay	Overload, phase loss/unbalance (46), ground fault (50N/51N)
> 200kW	Full-featured intelligent relay	All above + thermal modeling, communication, event logging

Setting Guidelines: Overload pickup 115-125% of FLA; instantaneous 8-12× FLA; Trip Class 10 or 20 typical.

Medium-Voltage Motor Protection

Medium-voltage motors (1000V ~ 15kV) power critical equipment with higher replacement costs, requiring more comprehensive protection.

Motor Size	Scheme Type	Key Protection Functions
200kW ~ 2000kW	Comprehensive relay	Overload (49), locked rotor (51LR), unbalance (46), earth fault (50N/51N), start supervision (48), under/over voltage (27/59)
2000kW ~ 5000kW	Comprehensive + Differential	Above + differential protection (87M)
> 5000kW	Dual-relay redundancy	Two independent relays with separate CTs

Special Considerations: RTD inputs for winding/bearing temperature monitoring; CTs sized to handle 6-8× FLA starting current without saturation.

Large Motor Protection

Large motors (> 5000kW or > 10kV) are critical assets requiring comprehensive protection with redundancy and advanced diagnostics.

Protection Type	Functions	Purpose
Primary Protection	Differential (87M)	Fast protection for winding internal faults
Backup Protection	Overcurrent (50/51), Overload (49)	Backup if differential fails
Thermal Protection	RTD monitoring + Thermal model	Real-time winding/bearing temperature
Ground Protection	Sensitive earth fault (51N)	High-impedance grounding systems
System Protection	Under/over voltage, frequency, reverse power	Abnormal system condition protection
Communication	IEC 61850, Modbus, DNP 3.0	DCS/SCADA integration

10kV Motor Protection Scheme

10kV motors are common in power plants, petrochemical, and mining applications.

System Characteristics: Starting current 6-8× FLA for 5-15 seconds; resistance grounding or ungrounded.

Recommended Protection Functions:

Protection	ANSI	Setting Range	Notes
Differential	87M	20-40% of rated current	Primary protection
Locked Rotor / Start Supervision	51LR/48	6-8× FLA, 5-15s	Must exceed normal start time
Thermal Overload	49	Class 10-30	Thermal model + RTD input
Negative Sequence	46	10-20% of FLA	Prevents rotor heating
Earth Fault	51N	5-15% of FLA	Sensitive detection
Under/Over Voltage	27/59	70-90% / 110-120%	Time-delayed trip

Typical Configuration: Differential relay (87M) as primary + multifunction comprehensive relay as backup + 6 RTD inputs (3 windings + 3 bearings) + IEC 61850/Modbus communication.

For detailed protection scheme design and setting calculations, please contact our engineering team.

Motor Protection for 10kV Motors

10kV motors power critical equipment in power plants, petrochemical, mining, and steel mills, requiring comprehensive protection to prevent costly failures.

System Characteristics

Parameter	Characteristic
Starting Current	6-8 × FLA
Locked Rotor Duration	10-30 seconds
Grounding Type	Resistance-grounded or ungrounded

Core Protection Functions

ANSI	Function	Typical Setting
87M	Motor Differential	20-40% of rated current
49	Thermal Overload	Class 10-30 + RTD inputs
51	Time Overcurrent	IEC inverse curve
50	Instantaneous Overcurrent	8× FLA
51LR/48	Locked Rotor / Start Supervision	6-8× FLA; exceed start time
46	Negative Sequence	10-20% of FLA
50N/51N	Earth Fault	5-15% of FLA
27/59	Under/Over Voltage	70-90% / 110-120%
66	Starts Per Hour	2-3 cold starts/hour

Differential Protection (87M)

Recommended for motors >2000kW or 10kV. Requires 6 leads and 5P20/PX class CTs.

Typical Settings (1.4MW, 11kV):

Parameter	Setting
Pickup	20% of rated current
Slope 1 / Slope 2	30% / 70%
Start Blocking	12s or current < 2× FLA

Thermal Overload Protection

Thermal model must include both positive and negative sequence currents.

Input	Description
FLA / LRA	Full-load / locked rotor current
LRHOT / LRCOLD	Hot/cold stall time (~20s / ~30s)
RTD Bias	6 RTDs (3 windings + 3 bearings)

Protection Coordination

Criteria	Setting Rule
Overcurrent Pickup (51)	115-125% of FLA
Time Dial	2-10s above starting curve
Instantaneous (50)	~8× FLA
Starts Limitation	2-3/hour; 15-20min between starts

Typical Configuration

Layer	Implementation
Primary	Differential (87M) with start blocking
Thermal	Thermal model + 6 RTD inputs
Backup	Multifunction relay: overcurrent, unbalance, earth fault, voltage, start supervision
Communication	IEC 61850 or Modbus TCP

Special Considerations

• Earth Fault: In resistance-grounded systems, currents limited to 5-15% of FLA — careful CT selection required
• Starting Mode: Settings differ for DOL, soft-starter, and VFD
• Steel Plants: Account for back-feed current and self-starting during voltage sags

Motor Protection Relay Testing

Motor protection relay testing is a comprehensive process validating every aspect of performance — from basic hardware to complex algorithms and communication integration.

Test Type	Purpose	Key Validation Points
Functional Testing	Verify basic hardware operations	Power supply, I/O, display, LED indicators
Protection Function Testing	Validate each protection element with simulated fault signals	Trip accuracy within ±2%; time delay ≤±3% or ≤±40ms
Accuracy Testing	Verify measurement precision across full range	Current/voltage ±0.5% FS; frequency ±0.01Hz
Communication Testing	Ensure SCADA/DCS integration	Modbus, IEC 61850, Profibus; register mapping; packet loss <0.1%
Environmental Testing	Confirm ruggedness in industrial conditions	-40°C ~ +85°C, humidity, vibration, EMC immunity
Insulation Testing	Validate dielectric strength for safety	Insulation resistance ≥100MΩ; hipot 2kV/1min
FAT (Factory Acceptance Testing)	Witnessed testing for project orders	Protection demo, communication handshake, signed report

How to Select a Motor Protection Scheme

Selecting a motor protection scheme follows a systematic approach based on motor characteristics, application criticality, and system coordination.

• Motor Data & Starting Conditions – Evaluate nameplate data (power, voltage, FLA, NEMA design) and starting method (DOL, soft-starter, VFD). Starting current (5–8× FLA) and frequency determine thermal settings and trip class selection.
• Application Criticality – Non-essential motors need basic overload + short-circuit protection. Standard industrial motors add phase loss, unbalance, and ground fault. Critical and large motors (>2000kW) require differential (87M), RTD thermal monitoring, and communication — often with dual-relay redundancy.
• Device Architecture – Choose MPCB for compact fixed applications; contactor + overload relay for retrofit flexibility; or electronic relays for programmable protection and diagnostics.
• Protection Functions by Motor Size – Small LV: overload + short-circuit. Standard LV: add unbalance + ground fault. MV (200–2000kW): add locked rotor, start supervision, voltage protection. Large MV (>2000kW): differential (87M) + comprehensive backup.
• Coordination – Set overcurrent pickup at 115–125% of FLA, time dial above starting curve, instantaneous pickup at ~8× FLA, and limit starts to 2–3 cold starts/hour with adequate cooling intervals.

Basic Motor Protection Setting Principles

Protection settings determine when the relay trips — too low causes nuisance tripping; too high risks motor damage. Correct settings balance protection with operational continuity.

Why Settings Matter

Every motor has unique ratings, starting profiles, and thermal limits. Settings must be tailored to each motor for reliable protection.

Key Parameters for Setting Calculation

Parameter	Description	Why It Matters
FLA	Motor full-load current from nameplate	Base value for all settings
CT Ratio	Primary-to-secondary ratio	Converts primary current to relay input level
Starting Current	Typically 6-8 × FLA	Overcurrent must ride through start without tripping
Starting Time	Duration to reach rated speed	Determines trip class; must exceed normal start
Motor Thermal Capability	Overload/locked rotor withstand limit	Relay curve must fall below damage curve
Protection Coordination	Time-current grading with upstream/downstream devices	Ensures selective tripping

Setting Principles

• FLA is the foundation — all settings expressed as percentage of FLA
• Starting current and time set minimum pickup and start supervision to avoid nuisance tripping
• Thermal capability defines overload limit — relay curve must operate below damage curve but allow starting
• Coordination ensures relay operates before upstream protection but after downstream devices

FAQ

Q1：What are the most common motor protection faults?

A：Common faults include overload, phase loss, phase unbalance, locked rotor, ground faults, short circuits, and undervoltage/overvoltage conditions.

Q2：What protection functions are required for a medium-voltage motor?

A：MV motors (1kV–15kV) require thermal overload (49), overcurrent (50/51), locked rotor/start supervision (51LR/48), unbalance (46), earth fault (50N/51N), under/over voltage (27/59). Differential (87M) is strongly recommended for motors above 2000kW.

Q3：What ANSI functions are commonly used for motor protection?

A：49 (Overload), 50/51 (Overcurrent), 46 (Unbalance), 37 (Undercurrent), 27/59 (Under/Over Voltage), 50N/51N (Ground Fault), 48 (Start Supervision), 51LR (Locked Rotor), 66 (Starts/Hour), 86 (Lockout), and 87M (Differential).

Q4：What protection is required for a 10kV motor?

A：Differential (87M) as primary; thermal overload (49) with RTD; locked rotor/start supervision (51LR/48); unbalance (46); earth fault (50N/51N); under/over voltage (27/59); start limitation (66). Typical settings: differential 20-40% of rated current; overcurrent 115-125% of FLA; instantaneous ~8× FLA.

Q5：What is the difference between overload and locked rotor protection?

A：Overload (49) uses thermal modeling for prolonged overcurrent (inverse-time, seconds to minutes). Locked rotor (51LR/48) detects stall/failed start at 6-8× FLA with time delay allowing normal start — tripping only if current persists beyond allowable start duration.

Q6：Why is negative-sequence protection important for motors?

A：Unbalanced supply generates negative-sequence current, creating reverse rotating field that heats the rotor excessively. Standard thermal protection may not fully detect this. Negative-sequence (46) provides dedicated protection (typically 10-20% of FLA setting).

Q7：How are motor protection schemes selected?

A：Evaluate motor data and starting conditions; assess application criticality; choose device architecture (MPCB, contactor+overload, or electronic relay); define functions by motor category — small LV: overload+short-circuit; standard LV: add unbalance+ground fault; MV: add locked rotor+start supervision+voltage; large MV (>2000kW): differential (87M)+comprehensive backup.

Q8：How are motor protection relay settings calculated?

A：Settings are calculated using FLA as base; overload based on thermal curve and trip class; overcurrent at 115-125% of FLA with time dial coordinated above starting curve; instantaneous at 6-8× FLA; locked rotor above normal start profile; unbalance at 10-20% of FLA; earth fault at 5-15% of FLA; differential at 20-40% with dual-slope and start blocking. Settings must fall between starting curve and damage curve, coordinating with upstream/downstream devices for selectivity.