• Author / Uploaded
• Rajesh Pillai

• Categories
• Fusible (Eléctrico)
• Relé
• Rodamiento (Mecánico)
• Ingeniería de la Energía
• Máquinas

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Motor Protection

Introduction

z z

Many different applications Different motor characteristics

Difficult to standardise protection Protection applied ranges from FUSES

to

RELAYS

Introduction

COST & EXTENT OF PROTECTION

=

POTENTIAL HAZARDS

SIZE OF MOTOR, TYPE & IMPORTANCE OF THE LOAD

Motor Protection SYSTEM Voltage Dips Voltage Unbalance Loss of supply Faults

MOTOR CIRCUIT Insulation failure Open circuits Short circuits Overheating

LOAD Overload Locked rotor Coupling faults Bearing faults

Motor Protection Application Voltage

Rating

Switching Device

Protection

< 600V

< 11kW

Contactor

(i) Fuses (ii) Fuses + direct acting thermal O/L + U/V releases

< 600V

11 - 300kW

Contactor

3.3kV

100kW - 1.5MW

Contactor

6.6kV

1MW - 3MW

Contactor

6.6kV

> 1MW

Circuit Breaker

11kV

> 1MW

Circuit Breaker

Fuses + Electronic O/L + Time delayed E/F Options :- Stalling Undercurrent As above + Instantaneous O/C + Differential

Introduction 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

Mechanical Overload

Mechanical Overload OVERLOAD

HEATING

INSULATION DETERIORATION

OVERLOAD PROTECTION

FUSES

THERMAL REPLICA

Motor Heating MOTOR TEMPERATURE T = Tmax (1 - e-t/τ) TMAX

Time Rate of rise depend on motor thermal time constant τ

or as temp rise ∝ (current)2 T = KI2max (1 - e-t/τ)

Motor Heating I2 I22

T2 T1

I12 IR2

TMAX

t2 t1

Time

Time

t1

Thermal Withstand

t2

IR I1 I2

Current

Motor Cooling COOLING EQUATION : I2m' = I2m e-t/τr Current2 Im

Im' 0

t

Time

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

Motor Heating Temp

Trip Tmax T

Cooling time constant τr

t1

t1 = Motor restart not possible t2 = Motor restart possible

t2

Time

Emergency Restart

z

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 : z Use thermal protection characteristic z Use dedicated locked rotor protection

Locked Rotor Protection :- tSTART < tSTALL Thermal relay also provides protection against 3Ø stall. t

Thermal Cold Curve Cold Stall Withstand

tSL tST Start

IFL

Thermal Hot Curve IST ISL

I

Dedicated Locked Rotor Protection

Definite Time Thermal Cold tSL tS

Cold Stall Withstand

tSTART

O/C (IS)

(tS) T

Trip

tSL > tS > tSTART IS

IST ISL

Hot Stall Protection Tstart < Tstall Use of motor start contact to distinguish between starting and hot stall Time

Hot Stall Withstand start time

tSL (HOT) Full load Current

Io/c

Current

Locked Rotor Protection Start Time > Cold Stall Withstand z z

z

Motors with high inertia loads may often take longer to start than the stall withstand time However, the rotor is not being damaged because, as the rotor turns the “skin effect” reduces, allowing the current to occupy more of the rotor winding This reduces the heat generated and dissipates the existing heat over a greater area z Detect start using tachometer input

Stall Protection Tstart > Tstall Use of tachoswitch and definite time overcurrent relay. Time

Tacho opens at ∼ 10% speed TD < Tstall > Tacho opening

Start Time

Stall - Tstall

TD

Full load Current

Io/c

Current

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 ≈ NEGATIVE SEQ IMP CURRENT

STARTING CURRENT NORMAL RUNNING

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 STAR A

C

Normal starting current V Ι A = AN z With 1 phase open

DELTA

B

B

Ι' A

3VAN VAB = = 2z 2z = 0.866 x Ι A

1 1 (Ι' A + aΙ'B ) = (1- a)Ι' A 3 3 1 Ι1 = Ι A 2 1 1 2 Ι 2 = (Ι' A + a Ι'B ) = (1- a2 )Ι' A 3 3 1 Ι2 = Ι A 2 Ι1 =

A

z

z

C z

Normal =

3VAB z

1 Phase open 3 = VAB x 2z = 0.866 x normal 1 winding carries twice the current in the other 2.

Single Phase Stalling Protection

z z z

Loss of phase on starting motor remains stationary Start Current = 0.866 normal start I Neg seq component = 0.5 normal start I – Clear condition using negative sequence element

Typical setting ~ 1/3 I2 i.e. 1/6 normal start current

Single Phasing While Running

Difficult to analyse in simple terms z Slip calculation complex z Additional I2 fed from parallel equipment Results in :z I2 causes high rotor losses. Heating considerably increased. z Motor output reduced. May stall depending on load. z 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 z z z z

Reduced torque Increased stator current Reduced speed Failure to run-up

Form of undervoltage condition :z Slight but prolonged (regulation) z Large transient dip (fault clearance) Undervoltage protection :z Disconnects motor from failed supply z Disconnects motor after dip long enough to prevent successful re-acceleration

Undervoltage Considerations z

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)

z

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 z Instantaneous Earth Fault Protection z Instantaneous Overcurrent Protection z Differential Protection on some large machines

Stator Earth Fault Protection Rstab 50

(A) Residually connected CT’s

M

50

M

Note:

(B) Core Balance (Toroidal)CT

* In (A) CT’s can also drive thermal protection * In (B) protection can be more sensitive and is stable

50 Short Circuit z z z

Due to the machine construction internal phase-phase faults are almost impossible Most phase-phase faults occur at the machine terminals or occasionally in the cabling Ideally 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

TIME MPR FUSE M MPR ELEMENT

Ts

Is

Icont

CURRENT

Ts > Tfuse at Icont.

Differential Protection

High-Impedance Winding Differential Protection A

B

C

87 A

87 B

87 C

Note: Protection must be stable with starting current.

Self-Balance Winding Differential Protection A

87 A

B

87 B

C

87 C

Bearings

Bearing Failure

Electrical Interference Induced voltage Results in circulating currents May fuse the bearings Remember to take precautions - earthing Mechanical Failure Increased Friction Loss or Low Lubricant Heating

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 z

OUT OF STEP PROTECTION Inadequate 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 Method Detect AC Current Induced In Field Circuit. >Power Factor Method Detect Heavy Current At Low Power Factor.

Synchronous Machines

z

LOSS OF SUPPLY On 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