Motor Protection

Many different applicationsDifferent motor characteristics

Difficult to standardise protection

Protection applied ranges from

FUSES to RELAYS

Introduction

Introduction

COST & EXTENT POTENTIALOF PROTECTION HAZARDS

SIZE OF MOTOR,TYPE & IMPORTANCEOF THE LOAD

=

SYSTEMVoltage DipsVoltage

UnbalanceLoss of supplyFaults

Motor Protection

MOTOR CIRCUITInsulation failureOpen circuitsShort circuitsOverheating

LOADOverloadLocked rotorCoupling faultsBearing faults

Motor Protection Application

Voltage Rating Switching ProtectionDevice

< 600V < 11kW Contactor (i) Fuses(ii) Fuses + direct acting

thermal O/L + U/Vreleases

< 600V 11 - 300kW Contactor Fuses+ Electronic O/L

3.3kV 100kW - 1.5MW Contactor + Time delayed E/FOptions :- Stalling

6.6kV 1MW - 3MW Contactor Undercurrent

6.6kV > 1MW CircuitBreaker As above

+ Instantaneous O/C11kV > 1MW Circuit + Differential

Breaker

Protection must be able to :-Operate for abnormal conditions

Protection must not :-Affect normal motor operation

Considerations :-- Starting current- Starting time- Full load current- Stall withstand time (hot & cold)- Thermal withstand

Introduction

Mechanical Overload

Mechanical OverloadOVERLOAD

HEATING

INSULATIONDETERIORATION

OVERLOAD PROTECTION

THERMAL REPLICAFUSES

Motor Heating

MOTOR TEMPERATURE

T = Tmax (1 - e-t/τ)

or as temp rise ∝ (current)2

T = KI2max (1 - e-t/τ)

Rate of rise depend on motorthermal time constant τ

Time

TMAX

Motor Heating

Time

TMAX

T1

T2

t2 t1

I2

I22

I12

IR2

Time

CurrentIR I1 I2

t1t2

Thermal Withstand

Motor Cooling

COOLING EQUATION :

I2m' = I2m e-t/τr

After time ‘t’ equivalent motor current is reduced from Imto Im’.

Time

Im

Current2

Im'

t0

Motor Heating

t1 = Motor restart not possiblet2 = Motor restart possible

Time

Tmax

t2t1

Trip

Temp

Cooling timeconstant τr

T

Emergency Restart

In certain applications, such as mine exhaust and ship pumps, a machine restart is required knowing that it will result in reduced life or even permanent damage.

– All start up restrictions are inhibited– Thermal state limited to 90%

Start / Stall Protection

Stalling Protection

Required for :-Stalling on start-up (locked rotor)Stalling during running

With normal 3Ø supply :-

ISTALL = ILOCKED ROTOR ~ ISTART

∴ Cannot distinguish between ‘STALL’ and ‘START’ by current alone.

Most cases :- tSTART < tSTALL WITHSTAND

Sometimes :- tSTART > tSTALL WITHSTAND

Locked Rotor Protection Start Time < Stall Withstand Time

Where Starting Time is less than Stall Withstand Time :Use thermal protection characteristicUse dedicated locked rotor protection

Locked Rotor Protection :- tSTART < tSTALL

Thermal relay also provides protection against 3Ø stall.

Thermal Cold Curve

Cold StallWithstand

Start

t

tSL

tST

IFLISTISL

I

Thermal Hot Curve

Dedicated Locked Rotor Protection

tSTART

Thermal Cold

Cold StallWithstand

tSLtS

ISISTISL

Definite Time

Trip

(tS)T

O/C (IS)

tSL > tS > tSTART

Hot Stall Protection

Tstart < Tstall

Use of motor start contact to distinguish between starting and hot stall

Current

Time

starttime

Full loadCurrent

Io/c

Hot Stall Withstand

tSL (HOT)

Motors with high inertia loads may often take longer to start than the stall withstand timeHowever, the rotor is not being damaged because, as the rotor turns the “skin effect” reduces, allowing the current to occupy more of the rotor windingThis reduces the heat generated and dissipates the existing heat over a greater area

Detect start using tachometer input

Locked Rotor Protection Start Time > Cold Stall Withstand

Stall Protection

Tstart > Tstall

Use of tachoswitch and definite time overcurrent relay.

Time

StartTime

TD

Full loadCurrent

CurrentIo/c

Stall - Tstall

Tacho opens at∼ 10% speed

TD < Tstall> Tacho opening

Unbalanced Supply Protection

Operation on Supply Unbalance

Negative sequence impedance is much less than positive sequence impedance.

Small unbalance = relatively large negative sequence current.

Heating effect of negative sequence is greater than equivalent positive sequence current because they are HIGHER FREQUENCY.

Operation on Supply Unbalance

At normal running speed

POSITIVE SEQ IMP STARTING CURRENTNEGATIVE SEQ IMP NORMAL RUNNING CURRENT

Negative sequence impedance is much less than positive sequence impedance.

Small unbalance = relatively large negative sequence current.

Heating effect of negative sequence is greater than equivalent positive sequence current because they are HIGHER FREQUENCY.

≈

Equivalent Motor Current

Heating from negative sequence current greater than positive sequence

→ take this into account in thermal calculation

Ieq = (I12 + nI22)½

where : n = typically 6

→ small amount of I2 gives large increase in Ieq and hence calculated motor thermal state.

Loss of 1 Phase While Starting

A

ANABA

ANA

x 0.866 2z

3V 2z

V '

open phase 1 Withz

V

currentstartingNormal

Ι

Ι

Ι

=

==

=

A2

A2

B2

A2

A1

ABA1

21

')a-(1 31 )'a '(

31

21

'a)-(1 31 )'a '(

31

ΙΙ

ΙΙΙΙ

ΙΙ

ΙΙΙΙ

=

=+=

=

=+=

STAR DELTA

normalx 0.866 2z3x V

open Phase 1z

3V Normal

AB

AB

=

=

=

A

z

z

z

BC

A

B

C

1 winding carries twicethe current in the other 2.

Single Phase Stalling Protection

Loss of phase on starting motor remains stationaryStart Current = 0.866 normal start INeg seq component = 0.5 normal start I

– Clear condition using negative sequence element

Typical setting ~ 1/3 I2i.e. 1/6 normal start current

Single Phasing While Running

Difficult to analyse in simple termsSlip calculation complexAdditional I2 fed from parallel equipment

Results in :-I2 causes high rotor losses.Heating considerably increased.Motor output reduced.May stall depending on load.Motor current increases.

Reverse Phase Sequence Starting

Protection required for lift motors, conveyors

Instantaneous I2 unit

Time delayed thermal trip

Separate phase sequence detector for low load current machines

Undervoltage Protection

Undervoltage Considerations

Reduced torqueIncreased stator currentReduced speedFailure to run-up

Form of undervoltage condition :-Slight but prolonged (regulation)Large transient dip (fault clearance)

Undervoltage protection :-Disconnects motor from failed supplyDisconnects motor after dip long enough to prevent successful re-acceleration

Undervoltage Considerations

U/V tripping should be delayed for essential motors so that they may be given a chance to re-accelerate following a short voltage dip (< 0.5s)

Delayed drop-out of fused contactor could be arranged by using a capacitor in parallel with the AC holding coil

Insulation Failure

Insulation Failure

Results of prolonged or cyclic overheating

Instantaneous Earth Fault ProtectionInstantaneous Overcurrent ProtectionDifferential Protection on some large machines

Stator Earth Fault Protection

M

50

M

50

Rstab

(A) Residually connected CT’s

(B) Core Balance (Toroidal)CT

Note: * In (A) CT’s can also drive thermal protection* In (B) protection can be more sensitive

and is stable

50Short Circuit

Due to the machine construction internal phase-phase faults are almost impossibleMost phase-phase faults occur at the machine terminals or occasionally in the cablingIdeally the S/C protection should be set just above the max Istart (I>>=1.25Istart), however, there is an initial start current of up to 2.5Istart which rapidly reduces over 3 cycles– Increase I>> or delay tI>> in small increments

according to start conditions– Use special I>> characteristic

Instantaneous Earth Fault or Neg. Seq. Tripping is not Permitted with Contactors

TRIP

MPR

M

TIME

Ts

Is Icont CURRENT

FUSE

MPRELEMENT

Ts > Tfuse at Icont.

Differential Protection

High-Impedance Winding Differential Protection

A

B

C

87A

87B

87C

Note: Protection must be stable with starting current.

Self-Balance Winding Differential Protection

A

87A

B

C

87A

87B

87C

Bearings

Bearing Failure

Electrical InterferenceInduced voltageResults in circulating currentsMay fuse the bearingsRemember to take precautions - earthing

Mechanical FailureIncreased FrictionLoss or Low LubricantHeating

Use of RTDs

RTD sensors at known stator hotspots

Absolute temperature measurements to bias the relay thermal characteristic

Monitoring of motor / load bearing temperatures

Ambient air temperature measurement

Synchronous Motors

Synchronous Machines

OUT OF STEP PROTECTIONInadequate field or excessive load can cause the machine to fall out of step. This subjects the machine to overcurrent and pulsating torque leading to stalling

>Field Current MethodDetect AC Current Induced In Field Circuit.

>Power Factor MethodDetect Heavy Current At Low Power Factor.

Synchronous Machines

LOSS OF SUPPLYOn Loss Of Supply Motor Should Be Disconnected If Supply Could Be Restored Automatically.Avoids Supply Being Restored Out Of Phase.

>Over voltage & Under frequency>Under power & Reverse Power