Electrical Drives I Week 11: Three phase Induction otor Starting
Starting Problem Definition: ' I r Rs Vs 2 R ' r S 2 Xeq At S=0 and S=1, thus the current can be determined as: ' I r st Vs 2 ' Rs Rr Xeq S=1 S=0 wm Consequences: Consequences Torque pulsations Wear and tear of bearings, couplings and belts Water hammering and damage to fragile materials Continuous maintenance is required oad Problems otor Problems Overheating due to failures of Reduction in the motor s life time Electric Utility Problems Fatigue and stress causing insulation break ovement of stator and rotor bars and windings Voltage drop due to high current Starting current could not be tolerated by utility, causing trip of protection equipment
Induction otor: Starting Small motors can be started direct-on-line arge motors require assisted Starting arrangement chosen based on: oad requirements Nature of supply (weak or stiff) Some features of mechanism: otor T start must overcome friction, load torque and inertia of motor-load system within a prescribed time limit I start magnitude ( 5-7 times I rated ) must not cause machine overheating Dip in source voltage beyond permissible value
Induction otor Starting Starting Techniques Electro-mechanical Starters Electronic Starters Direct On-line Starter Star- Delta Starter Star- Delta with Reactor Starter AC Voltage Controller Starter V/F Starter Stator Resistance/ Reactor Starters Autotransformer Starter Voltage ramp Current imit
Reactor Reactor Reactor Practical Connections for Different Starting Techniques: 3 2 1 STOP START Direct On-line Starting otor 1 2 3 STOP START T2 T3 T2 T3 Stator multi resistance OTOR 1 2 3 STOP START T2 T3 T2 T3 OTOR Stator multi reactor 3 2 U V W 1 3 phase Induction motor STOP START T T Star-delta T 3 2 1 STOP START 1 2 3 N S S S T2 T2 S SCRs SB U V 3 phase Induction motor W 5 Star-delta with reactor Autotransformer otor AC voltage controller
Simulation Results for Different Starting Techniques: R1 > R2 > R3 R1 > R2 > R3 R1 > R2 > R3 1 > 2 > 3 1 > 2 > 3 1 > 2 > 3 Direct On-line Starting Stator multi resistance Stator multi reactor Star-delta + + + + + + V1 < V2 < V3 V1 < V2 < V3 V1 < V2 < V3 Star-delta with reactor Autotransformer AC Voltage controller Starting (V ramp) 6 AC Voltage controller Starting (I limit)
Induction otor Starting Star-delta starter Special switch used Starting: connect as star () Stator voltages and currents reduced by 1/ 3 T e V T 2 T e reduced by 1/3 When reach steady state speed Operate with delta ( ) connection Switch controlled manually or automatically
Induction otor Starting Autotransformer starter Controlled using time relays Autotransformer turns ratio a T Stator voltages and currents reduced by a T T e V T 2 T e reduced by a T 2 Starting: contacts 1 & 2 closed After preset time (full speed reached): Contact 2 opened Contact 3 closed Then open contact 1
Induction otor Starting Reactor starter Series impedance (reactor) added between power line and motor imits current When full speed reached, reactors shorted out in stages
Induction otor Starting Soft Start For applications which require stepless control of T start Semiconductor power switches (e.g. thyristor voltage controller scheme) employed Part of voltage waveform applied Distorted voltage and current waveforms (creates harmonics) When full speed reached, motor connected directly to line
Induction otor Braking Regenerative Braking: otor supplies power back to line Provided enough loads connected to line to absorb power Normal I equations can be used, except s is negative Only possible for > s when fed from fixed frequency source Plugging: Occurs when phase sequence of supply voltage reversed by interchanging any two supply leads agnetic field rotation reverses s > 1 Developed torque tries to rotate motor in opposite direction If only stopping is required, disconnect motor from line when = 0 Can cause thermal damage to motor (large power dissipation in rotor)
Induction otor Braking Dynamic Braking: Step-down transformer and rectifier provides dc supply Normal: contacts 1 closed, 2 & 3 opened During braking: Contacts 1 opened, contacts 2 & 3 closed Two motor phases connected to dc supply - produces stationary field Rotor voltages induced Energy dissipated in rotor resistance dynamic braking