Transcript starter
Elements of Electrical Design – 2150904 Active Learning Assignment “Topics” STARTERS Guided By: Jaimini Presented by : Ankit singh-130120109003 Abdul chhipa-130120109005 Raj makda-130120109017 Branch : Electrical Engineering Div : B Batch : B1 NECESSITY OF STARTER At starting ,the speed of motor is zero so that the back e.m.f. In the armature is zero. Armature resistance is so low, if it is connected to power supply directly ; huge current will pass thru armature. The huge current may damage the machine, major heat, very high speed in case of DC series motor. Ia = V/Ra FUCTION OF STARTER Start and stop the motor. Limit inrush current where necessary. Permit automatic control when required Protect motor and other connected equipments from over voltage, no voltage, under voltage, single phasing etc. Types of Starter for DC Motor Two point starter for DC series motor Three point starter for shunt motor Four point starter for compound motor THREE POINT STARTER FOR DC MOTORS: A 3 point helps in the starting and running of a shunt wound DC motor or compound wound DC motor by adding some external resistance to the armature circuit and decreasing the overall current to below the rated value . These devices are so named because they connect to the motor using three terminals: the armature (A) terminal connected to the armature winding; the line (L) connected to the positive supply pole; and the field (F) terminal linked to the field winding. 3 POINT STARTER 4 POINT STARTER Four-point starters are identical to three-point devices, but they add an additional terminal (N) that links the supply to the no voltage coil. These starters are typically employed in applications where the motor is expected to operate above normal speed. The N terminal is necessary because higher speeds reduce field currents through the F terminal so that the handle is unable to remain in the RUN position By establishing an independent current supply to the NVC, the motor can run continuously regardless of field current and will not be unnecessarily tripped by high-speed operation or speed changes. 4 POINT STARTER Types of Starter for Induction motors For slip-ring induction motors: Rotor rheostat starter For squirrel cage induction motors: D.O.L starter Primary resistance starter Auto transformer starter Star delta starter Other starters: Soft starters Starting of slip ring induction motor In case of slip ring induction motors it is possible to add external resistance in rotor phases as wound rotor has 3-phase star connected winding to limit the starting high current. Rotor resistance starter In rotor resistance starter the three terminals of the rotor winding are connected to a variable external resistances through slip rings. Full supply voltage is applied across the stator. Resistances are fully in the circuit at starting , so that the starting current is reduced. The external variable resistance connected in each phase of the rotor circuit not only reduce the current at starting but increases the starting torque also due to improvement in power factor. The rotor circuit resistance is gradually cut out , as the motor speeds up and during normal running condition ,the rotor circuit resistance is completely cut out and the slip rings are short circuited. D.O.L.(direct on line) starter A Starter Which Connects A Motor Directly Across The Line Is Called D.O.L. Starter. In This Method, The Motor Is Connected By Means Of A Starter Across The Full Supply Voltage. It Is Very Simple, Inexpensive, Easy To Install And Maintain. It Consist - START Button - OFF Button - Electromagnetic Contector - Overload Relay Switching By This Starter Is Directly From Line Without Any Provision To Control The Starting Current i.e. There Is No Device To Reduce The Starting Current In This Starter. WIRING DIAGRAM OF D.O.L STARTER L1,L2,L3 - LINES M – MAIN CONTACTS M(a) – AUXILIARY OR MAINTAINING CONTACT S1 – START PUSH BUTTON S2 – OFF PUSH BUTTON OLC – OVERLOAD RELAY COIL OL – OVERLOAD RELAY CONTECT C – MAGNETIC COIL OR OPERATING COIL OPERATION OF D.O.L STARTER 1) When START Button Is Pressed When Start(s1) Button is pressed, Path is L1 - S2 - S1 – C – OL - L2 Coil C Is Energized ,It Closes Contacts M And Connects The Motor Across The Line. Maintaining Contact M(a) Is Used To Keep The Holding Or Operating Coil Energized After The Finger Is Removed From START Push Button (S1). 2) When OFF Button Is Pressed When OFF(s2) Button is pressed, Path is Disconnected. Coil C Is De-Energized, The Main Contacts M Are Opened. Supply To Motor Is Disconnected And Motor Stops. . 2) Overload Protection The Motor Is Protected Against Overload By A Thermal Overload Relay Which Open Circuits the Control Circuit When Overload occure. The Normally Closed Contacts O.L Is Opened And The Contactor Coil C is De-Energized To Disconnect The Motor From The Supply. Rate Of Temperature Is Very High So Motor May Be Damaged If The Starting Period Is large. Which May Be Due To Excessive Load Or Excessive Voltage Drop In The Supply Lines. Only Used For Less Than 5 KW Squirrel Cage Induction Motor Advantages of d.o.l starter Most Economical and Cheapest Starter Simple to establish, operate and maintain Simple Control Circuitry Easy to understand and trouble‐shoot. It provides 100% torque at the time of starting. Only one set of cable is required from starter to motor. Motor is connected in delta at motor terminals. disadvantages of d.o.l starter It does not reduce the starting current of the motor. High Starting Current: Very High Starting Current (Typically 6 to 8 times the FLC of the motor). Mechanically Harsh: Thermal Stress on the motor, thereby reducing its life. Voltage Dip: There is a big voltage dip in the electrical installation High starting Torque: Unnecessary high starting torque, even when not required by the load. Auto transformer starter Circuit and function An auto-transformer starter makes it possible to start squirrelcage induction motors with reduced starting current, as the voltage across the motor is reduced during starting. In contrast to the star-delta connection, only three motor leads and terminals are required. On starting, the motor is connected to the tapping of the auto-transformer. Starting characteristics Motor terminal voltage less than line voltage (by transformer ratio). Motor current exceeds line current (by inverse of transformer ratio). Starting torque is reduced by the square of the terminal voltage. Applications: Where complete acceleration at reduced amperes is needed. Where line ampere reduction requirements are severe, and load torque is not minimal. Difference between d.o.l/ star delta/ autotransformer Sr. DOL Starter Star delta starter Auto transformer starter 1 Used up to 5 HP Used 5 HP to 20HP Used above 20 HP 2 Does not decrease the starting current Decreases the starting current by 1/3 times Decreases the starting current as required 3 It is cheap It is costly It is more costly 4 It connects directly the motor with supply for starting as well as for running It connects the motor first in star It connects the motor at the time of starting in delta according to the taping for running taken out from the auto transformer THANK YOU