Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 7)

3 Starting and switching motors ............................................................................ 3-1
3.1 Selection criteria ................................................................................................... 3-1
3.2 Direct starting of squirrel-cage induction motors ............................................. 3-3
3.2.1 Starting time............................................................................................................ 3-3
3.2.2 Reversing starters................................................................................................... 3-4
3.3 Star-delta (Y- Δ, wye-delta) starting ..................................................................... 3-4
3.3.1 Normal star-delta starting ....................................................................................... 3-5
3.3.2 Motor connection for clockwise and counterclockwise direction of rotation............ 3-8
3.3.3 Influence of the third harmonic on motor protection relays ................................... 3-10
3.3.4 Uninterrupted star-delta starting (closed transition) .............................................. 3-11
3.3.5 Amplified star-delta starting .................................................................................. 3-12
3.3.6 Part-winding star-delta starting ............................................................................. 3-13
3.4 Auto-transformer starting .................................................................................. 3-14
3.4.1 Circuit and function ............................................................................................... 3-14
3.4.2 Rating of the starter .............................................................................................. 3-15
3.5 Starting via chokes or resistors ........................................................................ 3-15
3.5.1 Starting via chokes ............................................................................................... 3-15
3.5.2 Starting via resistors ............................................................................................. 3-16
3.6 Stator resistance soft starting ........................................................................... 3-16
3.6.1 Circuit and function ............................................................................................... 3-16
3.7 Pole-changing motors ........................................................................................ 3-17
3.7.1 Speed change by pole changing .......................................................................... 3-17
3.7.2 Ratings of starters for pole changing .................................................................... 3-18
3.7.3 Rating of the starter for steps with star-delta starting ........................................... 3-19
3.8 Starting wound-rotor motors ............................................................................. 3-20
3.9 Electronic soft starters....................................................................................... 3-22
3.9.1 Voltage ramp versus current limitation ................................................................. 3-23
3.9.2 Voltage ramp ........................................................................................................ 3-24
3.9.3 Kickstart ................................................................................................................ 3-24
3.9.4 Current limitation................................................................................................... 3-25
3.9.5 Soft stop................................................................................................................ 3-25
3.9.6 Soft starters for pump controls.............................................................................. 3-26
3.9.7 Motor braking........................................................................................................ 3-27
3.9.8 Positioning speed and controlled braking ............................................................. 3-27
3.9.9 Linear acceleration and deceleration by speed feedback..................................... 3-28
3.9.10 Direct start with full voltage ................................................................................... 3-28
3.10 Frequency converters ........................................................................................ 3-29
3.10.1 Principle of operation ............................................................................................ 3-29
3.10.1.1 Rectifier................................................................................................................. 3-29
3.10.1.2 Intermediate circuit ............................................................................................... 3-30
3.10.1.3 Inverter.................................................................................................................. 3-30
3.10.2 Operational performance ...................................................................................... 3-30
3.10.3 Change of sense of rotation and braking .............................................................. 3-31
3.10.4 Motor protection.................................................................................................... 3-31
4 Protection .............................................................................................................. 4-1
4.1 Protection requirements ...................................................................................... 4-1
4.1.1 Protection against electric shock ............................................................................ 4-1
4.1.1.1 Protection against direct contact............................................................................. 4-1
4.1.1.2 Protection against indirect contact .......................................................................... 4-2
4.1.1.3 Complementary protection...................................................................................... 4-3
4.1.2 Protection against overload and excess temperature............................................. 4-3
4.1.2.1 Different loading curves of various kinds of electrical equipment ........................... 4-3
LVSAM-WP001A-EN-P - April 2009 0-7