Motor Protection

Advertisements

MOTOR PROTECTION CALCULATIONS FOR SIEMENS SIPROTEC RELAYS

OC & EF PROTECTION

1. SHORT CIRCUIT PROTECTION: 50-1 & 50-2

Characteristics selected = definite time,Numerical relays sense harmonic effects during staring condition as well as under through fault condition.To avoid malfunctioning and filter such harmonic effects 60-80 mSec (>2 cycles) time delay shall be required.

2. EARTH FAULT PROTECTION: 51N & 50N

* Characteristics selected = Normal Inverse/DMT

NEGATIVE PH. SEQ. PROTECTION

3. NEGATIVE PHASE SEQUENCE PROTECTION: 46

     Characteristics selected = definite time / VI,Numerical relays sense harmonic effects during staring condition as well as under through fault condition. Also Motor can take Negative Ph. Sequence time for>10-15 Seconds. To Avoid malfunctioning & also to get sensitivity under actual abnormal conditions combination of VI & DMT characteristics settings are to be used.

Next page

Back

FAULTS & STATISTICS

FAULTS IN A MOTOR FEEDER

* THERMAL OVERLOAD
* LOCKED ROTOR
* OVER CURRENT (SHORT CIRCUIT)
* EARTH FAULT
* CURRENT UNBALANCE
* REPEAT STARTS
* UNDER CURRENT
* PROLONGED START
* INTER TURN SHORT
* LOSS OF EXCITATION
* UNDER EXCITATION
* UNDER VOLTAGE
* ROTOR EARTH FAULT

Insulation Class & Thermal Ratings

STATISTICS

Class of insulation
Max. winding temperature
A
105′ C
B
130′ C
F
155′ C
H
180′ C

Causes for Motor Failure – Historical data

Over Loads >>> 30%
Contamination  >>> 19%
Single Phasing  >>> 14%
Bearing Failure >>>  13%
Old age  >>> 10%
Rotor Failure >>>  5%
Miscellaneous  >>> 9%


Next page

Back

Starting characteristic

GENERAL

STARTERS FOR AC INDUCTION MOTORS FOR LT/HT MOTORS

DOL                                                         STAR DELTA
RESISTANCE                                         LIQUID ROTOR RESISTANCE
AUTO TRANSFORMER                        REACTOR
VARIABLE FREQ. DRIVE

PROTECTION

LESS THAN 10 kW BIMETAL RELAY
10 KW TO 50 KW BIMETAL + SINGLE PHASING
50 KW TO 100 KW THERMAL O/L + O/C +E/F+
UNBALANCE+LOCKED ROTOR
100 KW AND ABOVE COMPREHENSIVE MOTOR PROTECTION RELAY
THERMAL O/L+O/C+E/F+LR+
UNBALANCE+NO. OF STARTS+
UNDER LOAD+PROLONGED START
SYNCHRONOUS MOTORS
(1 MW AND ABOVE)
ABOVE PROTECTIONS+ U/V+
DIFFERENTIAL+ROTOR E/F+
UNDER EXCITATION+OUT OF STEP

MOTOR PROTECTION

TOPICS FOR DISCUSSION
  • AC INDUCTION MOTORS
  • STARTING CHARACTERISTICS
  • STARTERS
  • PROTECTION METHODS
  • FAULTS IN A MOTOR FEEDER
MOTOR PROTECTION – GENERAL

AC INDUCTION MOTORS – CONSTRUCTION

SQUIRREL CAGE
3Ph. WINDING IN STATOR 
COPPER BARS IN ROTOR
WOUND ROTOR
3Ph WINDING IN STATOR 
3 Ph. WINDING IN ROTOR 
(SHORTED INTERNALLY)
WOUND ROTOR WITH SLIP RING
3 Ph. WINDING IN STATOR 
3 Ph. WINDING IN ROTOR
SYNCHRONOUS MOTORS
3 Ph WINDING IN STATOR 
DC WINDING IN ROTOR
(TERMINATED TO SLIP RINGS)