Motor Differential Protection: Working Principle, 87M Scheme and Applications

Motor differential protection is a fast, selective primary protection technology that compares the inbound and outbound currents of a defined motor protection zone to detect internal stator winding faults. Classified as ANSI 87M in standard power system protection nomenclature, it is the most reliable protection solution for medium and large critical industrial motors.

Written by Leon Zhang, Protection Application Engineer · Technical review by Li Ming, Electrical Engineer · Last reviewed September 2026

Unlike overcurrent protection, which responds to current anywhere in the system, an 87M element looks only at what happens inside the motor.

A fault is cleared in one to two cycles instead of waiting for a grading step.

The function is routine in power stations, mining, water treatment and process plants, and in most EPC specifications for drives above a few hundred kilowatts.

Two 87M scheme families are in field use today, and a handful of settings decide how each one behaves.

The rest of this page covers that comparison, the CT rules, the fault types the element does and does not see, and how to choose a relay.

What Is Motor Differential Protection?

What Does 87M Protect?

Motor differential protection builds a closed zone between the line-side CTs and the neutral-side CTs.

In a typical six-CT arrangement the line-side set sits at the switchgear or starter and the neutral-side set at the motor terminals, so the feeder cable is inside the zone along with the winding and the terminal leads. Faults upstream of the line-side CTs are outside it.

The point is sensitivity. A winding fault drawing a fraction of rated current is invisible to a 51 element set above full load. The differential element sees it directly.

What Is ANSI 87M Protection?

Under ANSI/IEEE C37.2 device numbering, 87 is differential protection and the suffix marks the protected machine: 87M for motors, 87T for transformers and 87G for generators. They all belong to the same differential relay family.

Modern relays use a percentage restrained differential algorithm.

They sample both CT sets, compute the differential current, and stay stable through motor starting and external system faults. Restraint is what makes the element usable. Without it, CT ratio error alone would trip a healthy motor.

Motor Differential Working Principle: Current Comparison

How Does the 87M Comparison Work?

The motor differential working principle rests on Kirchhoff’s current law. In steady state the current entering the zone equals the current leaving it, so the difference is zero.

The comparison is made per phase, on phasor quantities:

I_d = | I₁ − I₂ | is the operate quantity.

The restraint quantity I_r is built from the same two currents, and vendors differ: some use the arithmetic sum | I₁ | + | I₂ |, others use the larger of the two. Tripping needs I_d above the pickup setting and above k · I_r. Two relays quoting the same slope can therefore behave differently, so read the vendor definition of I_r before comparing data sheets.

• Normal operation: the relay remains stable without tripping
• Internal fault: a stator short circuit or earth fault breaks the balance, I_d rises above the threshold and the relay trips

The zone is therefore fixed by where the CTs are installed.

That is what makes the element inherently selective, and also what limits it when a CT set is missing or wrongly placed.

Differential Current vs Restraining Current

Percentage restraint exists so the element ignores CT error and through-current during external faults. It works from two quantities:

• Differential current I_d: the current that fails to balance between the two CT sets. This is the operate quantity.
• Restraint current I_r: a measure of the current passing through the zone in normal operation and in external faults. Raising the slope of the I_d / I_r characteristic desensitizes the element as through-current grows.

The result stays locked under external faults and motor starting, yet remains sensitive to a genuine winding fault. Most of the setting work goes into pickup and slope, not into the differential equation itself.

The 87M Scheme: Six-CT vs Self-Balancing

Conventional Scheme: Six CTs and an Auxiliary Relay

The conventional 87M scheme uses six CTs in a three-phase full differential configuration.

It is the default answer for medium and high-voltage motors, and the one most commissioning teams have seen before.

Key configuration requirements:

• Three CTs on the line side and three on the neutral side, one per phase
• Same ratio, accuracy class and polarity convention on both sides
• Separate wiring runs for the two sides, so a fault on one does not disturb the other

It gives full phase fault coverage and stability, at the cost of a second CT set and two runs of wiring back to the relay.

Self-Balancing Scheme: Fewer CTs, Less Wiring

The self-balancing scheme works on a magnetic balance principle instead of a straight comparison of two CT sets. It cuts down the dual-side CT wiring that the conventional scheme needs.

What it changes on site:

• Less on-site wiring, so fewer terminations to get wrong
• Magnetic balance is built into the relay, so the two CT sets do not have to be ratio matched by hand
• Better behaviour with unequal CT burden, which is where the conventional scheme usually fails

It suits compact motor rooms and projects where construction hours matter more than hardware cost. Technical data on the arrangement is on the self-balancing differential protection page.

How Does an 87M Relay Identify an Internal Motor Fault?

An 87M relay works through one closed loop, in order:

1. Sample both CT sets
2. Compare line-side and neutral-side currents
3. Compute the differential current
4. Apply the percentage restraint characteristic
5. Test against the pickup and slope thresholds
6. Issue the trip command to the motor breaker

Steps 3 to 5 are repeated once per sampling interval, typically 16 to 80 samples per cycle. The breaker opens some tens of milliseconds later, whatever the relay does.

What Faults Can 87M Protection Detect?

An 87M element answers some fault types cleanly and others not at all. The table below separates them, and names what decides the outcome in each case.

Fault or condition	87M response	What decides the outcome
Stator phase-to-phase fault	Trips, typically within 1–2 cycles at 50 Hz	Both CT sets in service, matched ratio and correct polarity
Stator phase-to-ground fault	Trips only if enough current enters the zone	Neutral grounding method. A resistance-earthed or unearthed system may not drive enough current through the fault
Turn-to-turn fault in one phase	Weak or no differential current	Number of shorted turns. Pickup is usually set well below full-load current, so a fault confined to a few turns can still fall short
External fault (through-fault)	Restrained, no trip	Slope setting and CT transient performance
CT saturation on one side	Spurious differential current is possible	CT class, connected burden and the DC component of the through-current
Motor starting current	No trip	Starting current passes through both CT sets and cancels, so it is not a differential condition

That last row is worth dwelling on.

A transformer 87T element needs inrush blocking because magnetizing current has no counterpart on the other side. A motor has no equivalent problem at start, which is one reason motor differential settings are simpler than transformer ones. Some relays still offer second-harmonic restraint on machine differential inputs; check the manual before assuming it is unused.

Key Note: the 87M element covers internal faults inside the CT zone only. It cannot replace overload, negative-sequence or external earth fault protection.

Why Use 87M Protection for Motors?

Fast Internal Fault Detection

An internal fault trips in one to two cycles.

A 50/51 element has to be graded against the upstream device, so it typically waits hundreds of milliseconds, and the winding absorbs that energy in the meantime.

High Sensitivity

A winding fault that draws a fraction of rated current never reaches a 51 pickup.

The differential element measures the difference directly, so it sees the fault while it is still a few turns.

Selective Protection

The zone boundary is the CT position, not a time-current curve. A fault anywhere else on the board trips something else, and this motor keeps running.

Protection for Large and Critical Motors

For a critical drive the arithmetic is simple.

A rewind or a replacement costs more than the CTs, the relay and the extra wiring, and the production loss usually costs more than both. That is why 87M tends to appear in the specification as soon as the motor rating goes up.

87M vs Overcurrent Protection (50/51)

87M and 50/51 are complementary, not interchangeable, and confusing the two is a common source of design error.

A full side-by-side comparison of the two is on the 87M vs overcurrent protection page. The summary fits in one table:

Protection Function	ANSI Code	Core Function	Protection Zone	Engineering Positioning
Differential protection (87M)	87M	Internal stator winding faults	Inside the CTs at the motor terminals	Primary unit protection
Overcurrent protection	50/51	Overload and external faults	The whole branch circuit	Main and backup protection
Thermal overload protection	49	Overload and cooling failure	A thermal replica of the motor	Long-term overload duty
Earth fault protection	50N/51N	Earth faults on the branch	The grounding loop	Backup for earth faults

Engineering Conclusion: 87M covers what is inside the CT zone, while 50/51 and the thermal elements cover everything else.

Neither one substitutes for the other, and a motor protected only by 50/51 keeps the blind spot it had before.

When Is 87M Protection Required?

Large Motors

Above roughly 200 kW the equipment value and the cost of a failure start to justify a dedicated differential element.

The exact threshold in your project will come from the client specification, not from the motor, so read the specification before deciding.

Medium-Voltage and High-Voltage Motors

Once the motor is fed at 6 kV or 10 kV, differential protection is normal practice and appears in most utility and industrial specifications. See which motors need differential protection for the full screening rules.

Critical Process Motors

Where a single motor drives a continuous process, the loss when it stops usually dwarfs the protection cost. Power station auxiliaries, water treatment pumps, compressor drives and mining main drives all fall into that group.

When May 87M Not Be Necessary?

Small low-voltage motors and standby drives with low running hours rarely justify the extra CT set.

Overcurrent and thermal protection is enough there. The switch-off point is the cost of a failure, not the rating on the nameplate.

CT Requirements for an 87M Scheme

Nearly every 87M problem in the field traces back to the CTs, not the relay.

Ratio, polarity, class and burden all have to be right before the settings mean anything.

CT Configuration

One CT set on each side, all three phases on both, with no phase skipped.

A missing phase on one side leaves a permanent unbalance the relay cannot distinguish from a fault.

CT Ratio and Polarity

Both sets need the same ratio and the same polarity convention. A reversed set turns load current into differential current, and the relay will trip on load.

CT Accuracy and Saturation

Choose a class that stays linear under the worst through-fault you expect.

A CT that saturates on one side only produces a differential current during an external fault, which is exactly the event the restraint characteristic is supposed to ride through.

87M Operating Example: Three Conditions

Normal Operation

Line-side and neutral-side currents balance, I_d sits near zero, and the relay stays at rest.

Internal Fault

A phase-to-phase short circuit inside the stator breaks the balance.

I_d jumps to a multiple of the pickup setting, the slope characteristic is exceeded, and the relay trips the motor breaker within one to two cycles.

External Fault

A fault on the line side drives a large through-current past the motor.

Both CT sets see the same current, I_d stays small, the restraint term rises with the through-current and the element holds.

Motor Differential Applications: Where 87M Pays Off

Across motor differential applications the deciding question is always the same: what does a trip cost?

The table below lists the drives where that answer usually comes out in favour of 87M, along with the constraint that decides the design. Ratings are indicative.

Application	Typical drive (indicative)	What 87M adds	What to plan for
Power station auxiliaries (ID/FD fans, boiler feed pumps)	2–10 MW, 6.6–11 kV	Clears an internal fault fast enough to survive a bus transfer sequence	Two CT sets on a switchgear way that is often already crowded
Pumping and water treatment drives	200 kW–5 MW, LV or MV	Removes the need to grade downstream protection for faults inside the motor	Neutral-side CT leads running the length of the cable
Compressor and process train drives	1–10 MW, MV	Confines the trip to one drive instead of the whole board	Terminal box space for the neutral-side CTs
Mining conveyors, mills and crushers	1–15 MW, MV or HV	Copes with repeated starts and voltage dips, where 50/51 grading becomes unreliable	Vibration, dust and CT lead routing
Chemical and refinery critical drives	Any rating with a high downtime cost	A winding fault is cleared before it becomes a rewind or a fire	Spare relay and a routine secondary injection test schedule

Limitations and Application Considerations of 87M Protection

CT Installation Dependence

The protected zone ends where the CTs end. Anything outside them, including the cable upstream of the line-side CTs, is someone else’s problem.

Limited Inter-Turn Fault Detection

A fault across a handful of turns may produce less differential current than the pickup setting.

Negative-sequence or split-phase protection covers part of that gap, and neither closes it completely.

CT Saturation Risk

A severe external fault can saturate one CT set and create differential current that looks real.

Correct CT class and a slope setting that matches the actual CT performance are the defence.

Complementary Protection Required

87M is one element in a scheme, not the scheme.

It must be combined with overload, negative-sequence and earth fault protection to cover the conditions it was never designed to see.

87M Protection Functions in a Motor Protection Relay

An integrated motor protection relay carries 87M as the core function and adds the elements that cover everything the differential zone cannot see. A typical machine protection package includes:

• Core: 87M differential element (internal fault primary protection)
• Current protection: 50/51 overcurrent protection, 50N/51N earth fault protection
• Thermal and unbalance protection: 49 thermal overload protection, 46 negative sequence protection
• Working condition protection: 48 motor starting/stalling protection
• Auxiliary protection: 27/59 voltage protection, 81 frequency protection

How to Select an 87M Relay

Six factors decide which 87M relay fits a project. Work through them in order, because the first two rule most models in or out before the rest matter:

Motor Voltage and Rated Current

Voltage level and rated current set the required CT secondary rating and the relay input range.

An input sized for a 5 A CT will not read a 1 A CT correctly, and the sampling error lands straight in the differential calculation.

Motor Power and Criticality

High-power and critical drives justify a relay with a proper slope characteristic and full disturbance recording. A standby drive rarely does.

CT Configuration Conditions

If there is no room for a second CT set, or the neutral-side leads would run too far, the self-balancing arrangement is the practical answer. Otherwise the conventional six-CT scheme is easier to test.

Required Protection Functions

Check that the relay also offers the functions the specification asks for: 49 thermal, 46 negative sequence, 48 stall, 50N/51N earth fault and 27/59 where relevant. Missing one function usually means a second device.

Communication and Integration

For SCADA integration, confirm the protocols you actually need.

IEC 61850 Ed.2, Modbus TCP and DNP3 are the common choices, and the ability to pull disturbance records remotely matters more than the protocol list on a data sheet.

Protection Setting and Coordination

Check that pickup, slope and time delay can all be set independently, and that grading against the upstream device is possible. A relay that meets every other requirement but cannot be coordinated will still trip the plant.

For medium and high-voltage critical motor projects, the motor differential protection relay page lists the function configuration and engineering parameters in full.

Motor Differential Protection FAQ

1. What is motor differential protection?

Motor differential protection (ANSI 87M) compares the current entering the motor with the current leaving it.

Any difference above the pickup setting means the difference is flowing somewhere inside the zone. It is fast and sensitive, and it covers only faults inside the CTs.

2. What is ANSI 87M protection?

ANSI 87M is the device number for motor differential protection under ANSI/IEEE C37.2.

The 87 family covers differential protection in general; the M refers specifically to motors, as T does to transformers and G to generators.

3. How does motor differential protection work?

Based on Kirchhoff’s Current Law, it samples dual-side CT currents in real time, calculates differential current, and triggers tripping when the internal fault causes the current difference to exceed the threshold, while remaining stable during external faults.

4. What is a motor differential protection scheme?

A scheme is the combination of CT placement, wiring and relay algorithm that implements the comparison.

Two are in common use: the conventional six-CT arrangement and the self-balancing arrangement, which needs a different CT configuration on site.

5. What faults does 87M protection detect?

It detects phase-to-phase and earth faults inside the stator, plus any unbalance between the two CT sets.

It is weak on inter-turn faults involving very few turns, because the differential current they produce can stay below the pickup setting.

6. Do all motors need differential protection?

No. Medium and high-voltage drives, large motors and critical process drives normally get 87M protection.

Small low-voltage non-critical motors do not need it, and overcurrent plus thermal protection is adequate for them.

Conclusion

Motor differential protection (ANSI 87M) is the standard primary protection for internal faults in medium and large industrial motors.

It compares the currents either side of the zone and applies percentage restraint, so it trips in one to two cycles on a genuine winding fault and stays locked on everything else.

It is never the whole scheme. 87M has to be coordinated with overcurrent, thermal overload, negative sequence and earth fault protection, or the motor simply moves its blind spot rather than losing it.

For a medium or high-voltage motor project, our 87M relay page covers the configuration options and the engineering support we provide with them.

Related reading: 87M protection settings, which motors need differential protection, and the motor protection relay range.

Technical References and Data Sources

Source	What it covers	Type	Edition
ANSI/IEEE C37.2	Device function numbers and acronyms; definition of 87M	Standard	2022
IEEE C37.96	Guide for AC motor protection: protective functions and application criteria	Guide	2012
IEC 60255-187-1	Functional requirements for restrained and unrestrained differential protection of motors, generators and transformers	Standard	2021
IEC 60255-1	Common requirements for measuring relays and protection equipment	Standard	2022