1. Motor Protection
By S.Sankaravel, EDRC, E&IC
LARSEN & TOUBRO LIMITED
ECC DIVISION – EDRC, HQ
CHENNAI

2. 2
 Introduction
 Voltage at motor start
 Thermal
 Stalling
 Unbalance current
 Single phasing
 Insulation failure
 Loss of load
 Starts management
 Synchronous motor protection
Contents

3. 3
Introduction

4. 4
 Many different applications
 Different motor characteristics
 Difficult to standardise protection
 Protection applied ranges from
FUSES to SOPHISTICATED SOLID
STATE RELAYS
Introduction

5. 5
COST & EXTENT POTENTIAL
OF PROTECTION HAZARDS
SIZE OF MOTOR,
TYPE & IMPORTANCE
OF THE LOAD
=
Introduction

6. 6
System
Voltage Dips
Voltage Unbalance
Loss of supply
Faults
Motor Circuit
Insulation failure
Open circuits
Short circuits
Overheating
Field Faults
(synchronous machine)
Load
Overload
Locked rotor
Coupling faults
Bearing faults
Introduction – Causes of Fault

7. 7
For INDUCTION MOTORS :-
The main types to be considered :-
 Over-temperature
 Thermal overload
 Stall / Locked rotor
 Phase unbalance and single phasing
 Short Circuit
 Earth Fault
 Undercurrent
Motor Protection

8. 8
Motor Protection Application
Voltage Rating Switching Protection
Device
< 600V < 11kW Contactor (i) Fuses
(ii) Fuses + direct acting
thermal O/L + U/V
releases
< 600V 11 - 300kW Contactor / CB Fuses
+ Electronic O/L
3.3kV 100kW - 1.5MW Contactor / CB + Time delayed E/F
Options :- Stalling
6.6kV 1MW - 3MW Contactor / CB Undercurrent
6.6kV > 1MW Circuit
Breaker As above
+ Instantaneous O/C
11kV > 1MW Circuit + Differential
Breaker

9. 9
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

10. 10
Motor Currents
INDUCTION MOTOR
Define Slip, S, as the per unit difference in speed between the
stator and rotor fields
Slip “S” = f - fr
f
Speed of stator field relative to rotor
f - fr = sf
fr
Stator
Field f

11. 11
Voltage At
Motor Starting

12. 12
About to Start
 Phase Loss
 Low Volts

13. 13
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

14. 14
Under voltage
 Causes low output torque
 Machine cannot reach rated speed
 Draws high stator current
 Use time delayed under voltage protection

15. 15
47
Three Phase Voltage Check
 V2<V1
 Prevents reverse operation of machine
 Vabc > Start Low V Setting
 Avoids excessive start times on DOL machines
caused by inadequate voltage

16. 16
Under voltage Considerations
 Reduced torque
 Increased stator current
 Reduced speed
 Failure to run-up
Form of under voltage condition :-
 Slight but prolonged (regulation)
 Large transient dip (fault clearance)
Under voltage protection :-
 Disconnects motor from failed supply
 Disconnects motor after dip long enough to prevent successful
re-acceleration

17. 17
Under voltage Tripping
Means of under voltage tripping :-
 AC holding coil for fused contactor
 Under voltage release
 Under voltage relay for shunt trip
 Definite time
 Inverse time
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

18. 18
Thermal Protection

19. 19
Mechanical Overload
OVERLOAD
HEATING
INSULATION
DETERIORATION
OVERLOAD PROTECTION
THERMAL REPLICA
FUSES

20. 20
Motor Heating
HEAT STORED
 INCREASES THE
MOTOR TEMPERATURE
HEAT DISSIPATED AT A RATE PROPORTIONAL
TO MOTOR TEMPERATURE
HEAT DEVELOPED AT A CONSTANT
RATE DUE TO CURRENT FLOW

21. 21
Motor Heating
MOTOR TEMPERATURE
T = Tmax (1 - e-t/)
or as temp rise  (current)2
T = KI2max (1 - e-t/)
Rate of rise depend on motor
thermal time constant 
Time
TMAX

22. 22
Motor Heating
Time
TMAX
T1
T2
t2 t1
I2
I2
2
I1
2
IR
2
Time
Current
IR I1 I2
t1
t2
Thermal Withstand

23. 23
Motor Heating









-
-
1n
t
2
eq
2
m
2
eq
2
Ι
Ι
Ι
Ι









2
2
2
2
1
-
K
a
-
K
1n
t
I2
 - I2
m = (I2
eq - I2
m) (1 - e-t/)
Rearranging this expression in terms of time
or alternatively

Time
I2
eq
I2

I2
m
tTRIP
Current2

24. 24
Motor Cooling
COOLING EQUATION :
I2
m' = I2
m e-t/r
After time ‘t’ equivalent motor current is reduced from Im to Im’.
Time
Im
Current2
Im'
t
0

25. 25
Motor Heating
t1 = Motor restart not possible
t2 = Motor restart possible
Time
Tmax
t2
t1
Trip
Temp
Cooling time
constant r
T

26. 26
Stalling Protection

27. 27
Stalling Protection
Required for :-
Stalling on start-up (excessive load)
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 (on high inertia drives)

28. 28
Where Starting Time is less than Stall Withstand
Time :
 Use thermal protection characteristic
 Use dedicated locked rotor protection
Stalling Protection

29. 29
Thermal relay provides protection against 3Ø stall.
Thermal
Stall
Withstand
Start
t
tSL
tST
IFL
IST
ISL
I
Stalling Protection

30. 30
If Stall Withstand Is Below
Thermal Curve
Separate stalling relay required :- Definite time O/C.
tSTART
Thermal
Stall
Withstand
tSL
tS
IS
IST
ISL
Definite Time
Trip
(tS)
T
O/C (IS)

31. 31
Stall Protection
Tstart < Tstall
Use of motor start contact and 2 stage definite time over current relay.
Current
+ -
TD1
MSD
TD1 O/C
TD2
TRIP
TD2
86
Time
TD1+TD2
start
time
TD1
tST
Cold Stall tSL (COLD)
TD2
Full load
Current
Io/c
Hot Stall tSL (HOT)

32. 32
 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
 Detect start using tachometer input
Stall Protection
Tstart > Tstall

33. 33
Stall Protection
Tstart > Tstall
Use of tacho-switch and definite time over current relay.
+ -
TD
O/C
TD
TRIP
86
TACHO
Time
Start
Time
TD
Full load
Current
Current
Io/c
Stall - Tstall
Tacho opens at
 10% speed
TD < Tstall
> Tacho opening

34. 34
Unbalance Current

35. 35
Motor Currents
NEGATIVE SEQUENCE CURRENT
Relative frequency of stator field = f + fr
But fr = (1-s)f
Therefore f + fr = (2-s)f
fr
Stator
Field f

36. 36
Operation on Supply Unbalance
At normal running speed
POSITIVE SEQ IMP STARTING CURRENT
NEGATIVE 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.


37. 37
Equivalent Motor Current
Heating from negative sequence current greater than positive
sequence
 take this into account in thermal calculation
Ieq = (I1
2 + nI2
2)½
where : n = pos seq imp : neg seq imp
 small amount of I2 gives large increase in Ieq and
hence calculated motor thermal state.

38. 38
Single Phasing

39. 39
Single Phase Stalling Protection
 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

40. 40
Single Phasing While Running
Difficult to analyse in simple terms
 Slip calculation complex
 Additional 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.

41. 41
Insulation Failure

42. 42
Insulation Failure
Results of prolonged or cyclic overheating
 Instantaneous Earth Fault Protection
 Instantaneous Over current Protection
 Differential Protection on some large machines

43. 43
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

44. 44
50
Short Circuit
 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

45. 45
50
Short Circuit
Sample Short Circuit Characteristic
0.01
0.1
1
0.8 1 1.2 1.4
Current (xI>>)
Time
I>>

46. 46
High-Impedance Winding
Differential Protection
A
B
C
87
A
87
B
87
C
Note: Protection must be stable with starting current.

47. 47
Self-Balance Winding Differential
Protection
A
87
A
B
C
87
A
87
B
87
C

48. 48
Instantaneous Earth Fault or
Neg. Seq. Tripping is not
Permitted with Contactors
TIME
Ts
Is Icont CURRENT
FUSE
MPR
ELEMENT
Ts > Tfuse at Icont.
TRIP
MPR
M

49. 49
Bearing Failure

50. 50
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

51. 51
Bearing Failure
Ball or Roller Bearings
 Immediate standstill
 Cannot protect bearing
 Stall protection for machine
Sleeve Bearings
 Failure rare
 Temperature rise, vibration, increase in current
 Temperature sensor in bearing
 Thermal overload for motor - does not protect bearing

52. 57
Loss Of Load

53. 58
Undercurrent Protection
 Detects loss of load
e.g. Pumps & Conveyors
 Submersible ‘down hole’ pump
 Cooled by pumped liquid
 Motor overheats if it runs dry even though current
reduces
 Setting current  40% IFL

54. 59
37
Loss of load
 In most applications it is desirable to stop the
motor if the mechanical coupling is lost.
 In addition a pump can be damaged if it
becomes unprimed
 No load current is normally about 50-60% of Ifl
 On lightly loaded machines under power provides
better discrimination between low load and load loss
 No load power about 10%
 May need to inhibit during start

55. 60
Starts Management

56. 61
Normal Shutdowns
 On machines where the thermal element is
limited during start, it is critical to ensure that
restarts do not damage the machine
 Selectable thermal start inhibit
 Jogging
 Selectable number of hot starts, cold starts,
period and inhibit time
 Selectable time between 2 starts

57. 62
49
Thermal start inhibit

PROHIBIT START
Motor halt
New
restart
 Prohibit START
Threshold

58. 63
66
Limited starts/period
Cold start Hot start Hot start
Treference
2 hot starts
Information on « Start Prohibited »
Tprohibit

59. 65
86
Lockout
 Some trips require maintenance before the
machine can be restarted. A latched trip may
be applied for the following conditions
 Short circuit
 Earth faults
 Loss of Phase

60. 66
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%

61. 67
Synchronous Motor
Protection

62. 68
Synchronous Machines
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
 Power Factor Method

63. 71
Synchronous Machines
OVER VOLTAGE
 Busbar & Motor Unloaded:
Motor terminal voltage may rise instantaneously
to 20 - 30% on loss of supply due to open
circuit regulation of the motor

64. 72
Synchronous Machines
UNDER FREQUENCY
 Motor loaded:
Decelerate fairly quickly & frequency of
terminal voltage will fall.

65. 73
Synchronous Machines
UNDERPOWER
 Only applicable when power reversals do not
occur under normal operating conditions
 Arranged to look into the machine; applicable
when there is a possibility of no load connected
on loss of supply.
 Time delay required to overcome momentary
power reversals due to faults

66. 74
Synchronous Machines
REVERSE POWER
 Only applicable when power reversals do not
occur under normal operating conditions
 Arranged to look away from the machine;
applicable where there is always load
connected.
 Time delay required to overcome momentary
power reversals due to faults

67. 75
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