1. Motor Protection
Dr. Hendro Rijanto

2. Basic: - Adiabatic Heating (short time operation):
Thermal overload during the start phase
- Heating with thermal loss (long time operation):
Thermal overload during normal operation
- Cooling down
After the overload condition disappeared
- in rotation (I > 10% In)
- in standstill (I < 10% In)

3. During starting (short time) phase:
Number of start protection:
During the starting phase the magnitude of the current flowing through the stator and rotor winding can be very high, up to 8 In.
Consequently the number of starts must be limited by supervising the thermal condition. According to the operation guidelines:
3 starts in cold motor condition
2 starts in warm motor condition
Definition: Warm condition is equal to the filling status of the thermal storage (capacity), e.g. the thermal capacity is loaded more than 60%.

4. During starting (short time) phase:
Blocked rotor protection:
A blocking rotor is from electrical point of view a short circuited transformer.
The motor must be switched off again, if no decaying of the motor start current is observed. The blocked rotor protection can be carried out normally by an overcurrent definite time protection (51).
If the time duration of the starting phase, due to heavy load condition, exceeding the allowed blocked rotor time, it is recommended to supervised the rotor revolutions by an external device, which can be used to blocked the “blocked rotor protection”

5. During starting (short time) phase:
Motor start protection:
If heavy load condition during the starting phase must be taken into account, the adiabatic heating of the motor must be supervised to protect against overheated condition.
Because during the starting phase the current magnitude is more then the nominal motor current, the adiabatic heating can be calculated approximately as following:
I² t = Constant, whereby:
I Current of the Motor
t Allowed time duration for the adiabatic heating

6. During normal (long time) operation:

7. Thermal replica: Electrical diagram Analogy: (Thermal-) Energy
(Thermal-) Storage
(Thermal-) Loss
Analogy:
Energy stream  Current
Loss  Resistance (no need in case of adiabatic heating)
Storage (capacity)  Capacitance
Temperature  Voltage

8. Case study by means of electrical replica:
Input current increases from pre load current Ip to load current I:
Thermal replica:
pre load current  Temperature due to pre load condition
Load current  Temperature due to load operation

9. Temperature   Thermal energy  I ² 
Characteristic according IEC 255:
Ip pre load current
IB Basic current (motor nominal current)
k Factor for the start current
Characteristic in REF542plus:
I Operating current
Imn Motor nominal current
u Environment temperature
p Start temperature ( pre load )
Mn Motor nominal temperature
t Warn respectively trip temperature

10. Temperature   Thermal energy  I ² 
Characteristic according IEC 255:
Ip pre load current
IB Basic current (motor nominal current)
k Factor for the preload current
I Operating current
Imn Motor nominal current
u Environment temperature
p Start temperature ( pre load )
Mn Motor nominal temperature
t Warn respectively trip temperature
Characteristic in REF542plus

11. With both equations the setting factor k can be derived as following:

13. Setting Time Constant:

14. Exercise:
Rated motor current IMn 91A
Thermal class B
Blocking current IE
Blocking time (cold) tE 8sec
Current transformer rated current 100A/1A

15. Further protection in motor protection:
Unbalanced load protection
Differential protection
Stator earth fault protection
Under-/overvoltage protection
The same function can also be used for Generator Protection