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