Ac Motor Protection: (3-phase Induction And Synchronous Motors)

ABB Oy, Distribution Automation Vaasa, Finland

AC Motor Protection (3-phase induction and synchronous motors)

Contents 1. AC motors, brief introduction 2. Introduction to motor protection 3. Thermal Protection, general

4. Thermal Overload Protection 5. Consecutive starts 6. Start-Up Supervision

7. Short-Circuit Protection 8. Unbalance Protection 9. Loss of Load 10. Earth Fault Protection 11. Miscellaneous

1. AC Motors • •

Mechanical construction, stator and rotor Asynchronous and synchronous

Image from Machine Elements in Mechanical Design, Robert L. Mott

1. AC motor construction There are only two main components: the stator and the rotor. The stator contains a pattern of copper coils arranged in windings. As alternating current is passed through the windings, a moving magnetic field is formed near the stator. Rotor (rotating part)

Stator (stationary part)

http://fi.wikipedia.org/wiki/Tiedosto:Asynchronmotor_animation.gif

1. Asynchronous (induction) motor The rotor is constructed of a number of conducting bars running parallel to the axis of the motor and two conducting rings on the ends. The assembly resembles a squirrel cage, thus this type of motor is often called a squirrel-cage motor. Magnetic field formed by the stator induces a current in the rotor, creating its own magnetic field. The interaction of these fields produces a torque on the rotor. Note that there is no direct electrical connection between the stator and the rotor.

Image from Machine Elements in Mechanical Design, Robert L. Mott

http://fi.wikipedia.org/wiki/Tiedosto:Asynchronmotor _animation.gif

1. Synchronous motor The synchronous motor operates at exactly synchronous speed with no slip. The rotor is of a constant polarity (either a permanent magnet or an energized electromagnet) and the windings of the stator are wrapped in such a way as to produce a rotating magnetic field. Such motors provide very little torque at zero speed, and thus need some kind of separate starting apparatus. Often a squirrel-cage rotor is built into the main rotor. When the motor reaches a few percent of synchronous speed, the rotor is energized and the squirrel cage becomes ineffective.

Image from Machine Elements in Mechanical Design, Robert L. Mott

Animation © Motorola, Inc.

2. Introduction to motor protection 2.1 2.2 2.3

Need for protection Protection relays Some important definitions

2.1 Protection objectives 

Protection must be able to operate for abnormal conditions 



Internal faults 

Insulation failure (e.g. winding short-circuits, earth-faults)



Bearing failure



Under-magnetisation (synchronous motors)

Externally imposed faults 

Overloading, insufficient cooling



Start-up stress, reversed sequence starting



Supply voltage unbalance or single phasing



Over- and undervoltage



Vibration



Etc.

2.1 Causes for motor damages in industrial drives

Long time overheating 26 % Insulation faults 30 % Rotor or bearing faults 20 % Faulty protection 5 % Other causes 19 %

2.1 Protective functions needed Faulty protection 5 %

Other causes 19 %

Rotor or bearing faults 20 %

Thermal overload protection & RTD Long time overheating 26 %

Short-circuit and earth-fault protection Start-up supervision and thermal sensor (RTD) unit

Insulation faults 30 %

Continuous self-testing of the protection relay Other protective functions

2.2 Motor Protection Relays 

SPAM 150C



REX521H07/H50



REF541/3/5



REM541/4/5



REF542Plus



REM610



REM615, 611, 620



REM630

2.3 Definitions 

Cold motor 



Warm (hot) motor  motor has rated operating temperature

motor temperature is same as rated ambient temperature (40°C)

2.3 Definitions 

Hot spots  at start-up and overload, some parts of the motor heats up faster than others. 

End of stator windings



Joints of the rotor rods and conducting rings

 temperature in this hot spots is higher than in the stator iron, but after start-up/overload the temperature will level out

Photo courtesy of Lincoln Motors, USA

2.3 How Stop/Start-up/Run conditions are defined Stop

= All phase currents are below 0.12 x IN.

Start-up = The phase currents rise from a value below 0.12 x IN to a value exceeding 1.5 x IN in less than 60 ms. The starting situation ends when the currents fall below 1.25 x IN (more than 100 ms). Run

= The phase currents are over 0.12 x IN and the start condition is not active. < 60ms

Check the exact values from the relay manual

Overload

1.50 xIn 1.25 xIn 0.12 xIn Stop Start-up Running

Stop

3. Thermal protection, general 3.1 3.2

Motor thermal behaviour Direct temp.meas. / current based model

3.1 Motor Thermal Behaviour 

Heat is developed at a constant rate due to the current flow 

Light load => low current 





Electrical energy

=> small heat development

Rated => rated current

Motor Heat

Cooling air etc.



=> adequate heat development



=> temperature rises up to the motor operation temperature

Overload => high current 

=> high heat development



=> operation temperature will be exceeded



=> shortened life-time or damages

Mech. energy

3.1 Motor Thermal Behaviour 

Developed heat is proportional to the motor current (I2)



Dissipation rate of the heat is proportional to the motor temperature 



The higher the motor temperature is the faster heat is dissipated

Motor temperature will increase or decrease until the developed heat and the dissipated heat are in balance +

Developed heat  I2 R

-

Dissipated heat T

3.1 Motor Thermal Behaviour 

Heating follows an exponential curve 

Rate of temperature rise depends on motor thermal time constant t and is proportional to square of current

 I   K    I FLC

2

t      1  e t    

Load

t

 t

K e t t I IFLC

= constant = 2.7183 (Neper) = time = time constant = highest phase current = Full Load Current

3.1 Motor Thermal Behaviour 

Cooling also follows an exponential curve 

Rate of temperature drop depends on cooling time constant. (Can be different when the motor is stopped.)

Load

t

 t

3.1 Motor Thermal Behaviour 

Heating with different loads 

High load

Low load Time



Heating with different time constants  Small t Big t Time

3.2

Direct temperature measurement and current based thermal model

 I   K    I FLC

2

t      1  e t    

3.2 Direct temperature measurement 

Sensor types 



RTD (resistance temperature detector) 

Pt 100, Pt250, Ni 100, Cu 10 etc



Linear (or almost linear)

Thermistors (NTC or PTC) 

Typically only PTC (positive temperature coefficient) is supported



PTC curve is of S-shape  are like a switch which operates at designed temperature  cannot be used as thermometer

PTC

3.2 Direct temperature measurement 

Location 

Winding: embedded into the winding slot  this is overheating protection



Bearing



Other: ambient temperature, cooling air

3.2 Direct temperature measurement 

Disadvantages 



Slow, does not detect rapid change (start-up, heavy overload)

Advantages 

True information, i.e. detects also 

failures in the cooling system



reduced cooling because of a dirt etc

3.2 Current based thermal model 

 I   K    I FLC

2

t      1  e t    

Relay models the motor thermal behaviour based on the phase currents  this is overload protection



Disadvantages 



Does not see cooling problems

Advantages 

No direct temperature measurement needed



Fast => more “accurate" in starts and heavy overloads



Protection both for long time-constant (stator iron) and short time-constant (winding ends, rotor bars) parts in the motor

4. Thermal overload protection (current based) 4.1 4.2 4.3 4.4 4.5 4.6 4.7

Time/current and thermal capability curves Protection considerations Different approaches for finding settings SPAM 150C, REM610, 611, 615, 620 & 630 REM 54_, REF 54_, REX 521 (TOL3Dev) REF 542Plus Other settings

4.1 Time - current curves Motor start-up curves 10000

Time in sec.



1000

Start-up, U=90%

100

Start-up, U=100%

10

1 1

2

3

4

5

Stator current / rated current

6

7

4.1 Thermal limit curves Thermal capability, running Thermal capability for COLD motor (motor started from (rated) ambient temperature)

10000

Time in sec.



1000

Thermal capability for WARM motor (long time motor temperature with rated load)

100

10

1 1

2

3

4

5

Stator current / rated current

6

7

4.1 Thermal withstand curves Thermal capability, running & locked rotor

10000

Time in sec.



1000

100

COLD motor

10

WARM motor

1 1

2

3

4

5

Stator current / rated current

6

7

4.1 Example 1

Squirrel cage motor HXR 500 • 900 kW • 3 kV • 200 A • 1492 rpm

4.1 Example 2

4.2 Protection consideration 

Thermal overload protection should operate 

Before the motor thermal limits for running motor are exceeded 





Locked rotor curves can be handled with start-up supervision

Note: relay thermal curve does not have to follow motor thermal limit curves, it only must be below

Relay should not operate 

At normal start-up conditions

4.2 Protection consideration Example 1: relay cold curve goes below start-up curves  Not good, motor can never be started (trips every time) 1000

Operation time (sec.)



100

10

1 1

2

3 Stator current / rated current

4

5

4.2 Protection consideration Example 2: relay warm curve is below start-up curves  OK, but motor can be started only after some cooling time 1000

Operation time (sec.)



100

10

1 1

2

3 Stator current / rated current

4

5

4.2 Protection consideration Example 3: relay curves are between start-up and motor thermal limit curves  OK, but how much protection margin do you want? 10000

1000 Operation time (sec.)



100

10

1 1

2

3 Stator current / rated current

4

5

4.3

Different approaches for finding settings

4.3 Motor thermal limit curves are known We have motor curves  relay thermal overload curves must be below running motor thermal limit curves (locked rotor curves can be taken care in start-up supervision) 10000

1000 Operation time (sec.)



100

10

1 1

2

3 Stator current / rated current

4

5

4.3 Motor locked rotor time We know locked rotor time, but no curves  Relay warm curve must be same or below given locked rotor time 10000

1000 Operation time (sec.)



Locked rotor time (warm) 100

10

1 1

2

3 Stator current / rated current

4

5

4.3 Process oriented approach 

Find smallest possible setting which gives the required number of starts without any problems. 

Must know: motor start-up time and current



Must know: how many starts is allowed



Settings allow you to “take out” from the motor all you need but nothing more



We simply assume that motor is big enough for this

4.4

Thermal overload protection in REM610, REM611, REM615, REM 620, REM630 & SPAM 150C

4.4 Thermal Overload Protection 

All these relays have basically same thermal overload protection



Single time-constant thermal model





But motor has many parts, with own time-constants using only one time-constant is not good enough especially with big motors



Good protection needs two time-constant 

Short time-constant: winding, rotor



Long time-constant: iron case

These relays uses a weighting factor –setting (p) 

Commonly p = 50%

 Practically gives same result as if using 2 time-constants

4.4 Thermal Overload Protection 

“Hot-spot” operation mode (p=50%) 

Motors designed for Direct-On-Line (DOL) starts



At start-up or any overload situation the relay follows the temperature of the “hot spots” (short time heating/cooling)



After start-up or overload the temperatures inside the motor will level out. Relay simulates this by “remembering only 50% of the thermal rise during the start-up/overload” % 100

Thermal capacity A

A 80 B 60

ΘA ≈ short time heating/cooling ΘB ≈ long time heating/cooling

B

I > I

I < I

Thermal level for eg. at start-up.

Thermal level at rated current.

4.4 Thermal Overload Protection 

But first we must tell motor rated (FLC) current 

SPAM 150C 







PU scale 

I N 1 I NR  I NM I N 2

effects to ALL protection stages, except earth-fault

REM615/MTPPR 

I NM I N 2  I N 1 I NR

effects ONLY in thermal and undercurrent protection

REM610 

I 

Rated current 

effect only in the function itself

REM630/MTPPR

I NM I N 2  I N 1 I NR

Base value  setting (s)

IN1 = CT rated primary current INM = Motor rated current (FLC)

IN2 = CT rated secondary current INR = relay rated current

4.4 Thermal Overload Protection 

Heating time constant -setting 

SPAM 150C, REM610:

t6x



REM615, REM630:

Time constant start, Time constant normal (sec)





(sec) = op.time for cold motor at 6 x rated current

Time constant (t) = 32.15 x t6x

Cooling time constant - setting 

SPAM 150C, REM610:

Cooling factor Kc



REM615, REM630:

Time constant stop (sec)



Time constant stop(t) = Kc x 32.15 x t6x

4.4 Thermal Overload Protection New features in REM615, REM620, REM630 to give flexibility 

Separate time constants for normal run and starting



Overload factor setting (SPAM/610 has fixed 1.05)



Negative sequence factor (factory default = 0) if you want to emphasize effect of unbalance to the thermal overload



Initial thermal value (SPAM/610 has fixed 74%): Defines thermal level after power up or clearing of thermal content Cold Curve

Warm Curve

Motor rated

10000

1000 Operation time (sec)



100

10

1 0

100

200 300 Phase current (Amps)

400

500

600

4.5

Thermal overload protection in REM 54_, REF 54_ & REX 521 (TOL3Dev function block)

4.5 Thermal Overload Protection 

Motor rated (FLC) current 

Protective unit scaling

I N 1 I NR scaling   I NM I N 2 Where

IN1

= CT rated primary current

IN2

= CT rated secondary current

INM

= Motor rated current (FLC)

INR

= relay rated current

Scaling must be done in all phases (channels) and it effects to ALL functions using these channels

4.5 Thermal Overload Protection 

Protective unit scaling

4.5 Thermal Overload Protection 

Separete two time-constant thermal models for stator and rotor



Thermal curves ready for 4 type of motors, 2 type of generators and one transformer Warm motor start-up after 15 min standstill, an example 100 % 90 %

Stator Used thermal capacity [%]

80 % 70 %

Rotor 60 % 50 % 40 % 30 % 20 % 10 % 0% 0

500

1000

1500 Tim e [sec.]

2000

2500

4.5 Thermal Overload Protection 

Basic settings = “motor data” etc.



Advanced settings = “real settings” 

Advanced settings is automatically re-calculated by the relay whenever Basic settings has been changed

In the CAP tool, do not send the advance settings to the relay if the values are nonsense

4.5 Motor rated current, exercise 

Motor rated current is 345A, CT = 500/5A, we want short-circuit protection I>> = 3100A, calculate 

protected unit scaling factor for REM 543



I>> setting for REM 543



I setting for SPAM 150C



I>> setting for SPAM 150C

4.6

Thermal overload protection in REF 542Plus

4.6 Thermal Overload Protection 

Single time-constant thermal module, but three time-constant settings 

Time Constant Off = motor is standstill



Time Constant Normal = motor is running < 2×IMn



Time Constant Overheat = heavy overload or start-up

4.6 Thermal Overload Protection Warm

Cold

10000

Operation time (sec)

Time constant normal Time constant overload

1000

Time constant Off

100

10 0

1

2

3 Current (xIMn)

4

5

6

4.7

Other settings in thermal protection

4.7 Other settings: Cooling time-constant 

Cooling when motor is at standstill 

Methods of cooling (IC-code) standard IEC 34-6



Self-circulated cooling (IC_11)  stop the motor and cooling is 4..6 times slower because fan is stopped also



Heat exchangers cooling is 1..2 times slower



Setting names: cooling factor Kc, time constant stop/off

IC411

IC71W

IC86W

4.7 Other settings: Prior alarm level 



Setting considerations 

Should be higher than the thermal level of the motor which has been running for a long time with full load



Should give enough time to reduce the load



Generally, prior alarm level is set to 80 .. 95 % of the trip level

Undesirable alarms? 

At motor restart (compare alarm level to restart-inhibit margin)



At relay power-on 

Relay has no memory of the situation before power-off, therefore a hot (warm) motor condition is assumed. For example SPAM 150C begins from 74% after power-on.

4.7 Other settings: restart inhibit 

How to prevent motor start when the temperature is too high? 



Exampe: motor temperature is high, but below relay trip level. If we start motor, the temperature will rise too high and relay trips.

Answer: set and use the “restart inhibit” correctly  Relay prevents starting untill the motor cools down enough

WAIT

4.7 Other settings: restart inhibit 

Typical error: restart inhibit is in series of OPEN circuit, when it should be in series of the CLOSE button

4.7 Other settings: restart inhibit 

Setting (inh ): 

100% - “consumption of one start-up” - margin

100%

Margin Consumption of a single start-up

inh

time

4.7 Other settings: restart inhibit 

Consumption of a single start can can be calculated, if the operation time for a cold motor with start-up current is known start  up time consumptio n   100% trip time

4.7 Other settings: restart inhibit 

Exercise 1, propose a suitable setting for restart inhibition 



Thermal level before start-up was 15% and immediately after 60%

Exercise 2, propose a suitable setting for restart inhibition 

Start-up time = 7.5 sec



Trip time = 22 sec

5. Consecutive starts

Cumulative start-up counter 

General 

Supplementary & Backup protection to the thermal overload



Limits the number of consecutive starts  protection against cumulative thermal stress caused by the starts



Does not Trip but inhibits restarting!

Cumulative start-up counter 

Number of consecutive starts? 

Typically 2 cold / 1hot (or 3 cold / 2 hot) starts within an hour



Typically 10 min cooling time after previous start/running before restarting. 



Check the motor manufacturer data, there can be surprises

Sometimes the process / devices connected to the motor limits the number of starts or set requirements for the minimum standstill-time 

speed reduction gear



etc

Cumulative start-up counter 

Operation of a Cumulative Start-Up Counter 

The motor start-up time is added to the counter



If the counter value exceeds the set inhibit-limit the motor is not allowed to be restarted 

Inhibition is activated at the begin of the last start-up. This does not abort starting!



The counter value is decreased with a fixed countdown rate



Only after the counter value falls below inhibitlimit the motor is allowed to be restarted

Cumulative start-up counter 

Settings 

The inhibit level tsi = (n-1)  ts +10% where n = number of allowed start-ups ts = starting time (in seconds)



Countdown rate Dts = ts / Dt where

ts = starting time (in seconds) Dt = count time for one start-up (in hours)



Compromising number of starts at nominal and under-voltage 

If calculated with nominal start-up time, fewer starts will be allowed at under-voltage situation

Cumulative start-up counter 

Exercise: calculate the settings 

Starting time = 60 sec.



3 cold start-up is allowed in 4 hours

6. Start-up Supervision = Locked Rotor (LR) protection

Start-up supervision with O/C protection Protection against prolonged start-up 

Definite time O/C protection  time setting must be long enough to allow starting at undervoltage

I> Time in sec.



1000

100

t> 10

1 1

2

3

4

5

Stator current / rated current

10

Start-up supervision, thermal stress principle Thermal stress is proportional to I2t 

Represents energy needed to get the rotor to run full speed



Represents thermal stress in case of stall

Time in sec.



1000

Max permitted locked rotor time

I2t

100

10

1 1

2

3

4

5

Stator current / rated current

10

Safe stall time less than start-up time Typically Exe -type of motor



Protection operation time set below the start-up time



Protection must be blocked by a speed switch when the rotor is speeding up Time in sec.



1000

100

Max permitted locked rotor time

10

1 1

2

3

4

5

Stator current / rated current

10

Safe stall time less than start-up time Have both 

Stall = for locked rotor (LRT) protection



I2t = for allowable running-up time (ART) protection (max. start-up time allowed for the motor in case the rotor starts to rotate, but because of external problem or too high mechanical load the start-up is prolonged)

Time in sec.



1000

Stall

I2t

100

Max permitted locked rotor time

10

1 1

2

3

4

5

Stator current / rated current

10

7. Shot-circuit protection

High-set Overcurrent Protection 

Interwinding short-circuit protection for the motor



Phase-to-phase short-circuit protection for the feeder cable



Typical setting 1.5 x start-up current



Fast operation needed!

High-set Overcurrent Protection 

Doubling effect 

I>> setting can be set for automatic doubling during a motor start-up. Setting is selected 75% - 90% of start-up current



High-set Overcurrent functions as a fast run-time stall protection

I

Phase current I>> setting

Start-up

Run-time STALL

time

High-set Overcurrent Protection 

But when you have more than one OC stages 

Use one stage only for short-circuit, fast op.time



Use another stage for run time stall (=jam protection)





Blocked (or setting doubling) during start up



Op.time can be longer, for example 2 sec

REM615/630 has JAMPTOC function, which basically is OC function with blocking during start up

8. Unbalance protection

Unbalance protection 

Unbalance causes negative sequence component of phase current => causes a 2fN rotating flux to the rotorcircuit => the rotor temperature starts to rise.



Squirrel-cage induction motors can withstand unbalance pretty good, but a full broken phase condition is not allowed (about as severe situation as locked rotor)



Two principles are used: NPS and Min-Max



Inverse time characteristics gives selective operation when multible motorfeaders.

Unbalance Protection 

3 phase currents are required



In case of 2 x CT 

The sum of two phase currents is connected to the relay



NPS function block will work nicely, but if possible set for two-phase operation mode



E/F current will cause errors L1

L1

L1+L3

-(L1+L3) L3

L3

Unbalance Protection 

Small voltage unbalance can cause high current unbalance DI  DU  

I Start-up I FLC

If, for example, the start-up current is 5 x FLC, a 2% voltage unbalance causes 2% x 5 = 10% current unbalance

Positive Phase Sequence Reactance I Startup DI   Negative Phase SequenceReactance I FLC DU

Unbalance protection, NPS principle 

Negative Phase Sequence Current (NPS, I2) is calculated from the phase currents vectors 

Inverse operation, a good estimation of the time constant K is K

175 Istart  2

where Istart = start-up current of the motor x FLC

Unbalance protection, Min-Max Principle 

Phase discontinuity protection (SPAM 150C) 

The unbalance DI is calculated

DI  

IL max IL min  100% IL max

If the phase currents are less than the FLC

IL max IL min DI   100% IFLC 

DI and negative phase sequence (NPS) are not the same! 

For example 12% NPS sensitivity requires a setting of 12%3 = 20.7%

9. Loss of Load = Undercurrent protection

Undercurrent Protection 

Operates upon a sudden loss of load



Can be used in applications where the loss of load indicates a fault condition





submersible pumps, when cooling is based on the constant flow of liquid



conveyor motors, broken belt

Automatically blocked when the phase currents falls below blocking level (8..12%)

10. Earth-fault Protection

Earth Fault Protection 

Protection typically nondirectional, definite time



Core Balanced CT (ring CT, toroidal CT) 

a must for sensitive protection



recommended CT ratio 50/1 or higher 

CT construction



Secondary side voltage



Efficiency

Cable terminal box

Isolation

Sheath earthing

Cable Sheath

Earth Fault Protection 

When using sum connection of CTs (known as Holmgren circuit) an apparent EF-current will occur 

differencies of CTs



saturation of CTs mainly at start-up because of the DC-component



Minimum recommended setting 10% of the CT rated primary current

Earth Fault Protection 

Apparent E/F current will occur at startup due to the saturation of CT when sum connection of CTs is used.

Earth Fault Protection 

Avoiding problems of apparent E/F current 

Stabilising resistor



Uo condition (directional protection)



Long operation time, or



Temporary blocking of operation at start-up

11. Miscellaneous

Contactor controlled drives 

Relay must have normally closed trip contact



Contactor cannot break high currents 

High-set O/C protection should be set out-ofuse



E/F protection should be inhibited on high overcurrents