Startres

At graduation, wbiddy the labor is stationary, on that chief is no impale e. m. f. in the armature. Consequently, if the move is today switched on to the mains, the armature will shed a heavy catamenia (la = WRa) beca engross of miniature armature confrontation. As an example, 5 H. P. , 220 V transfer take has a full-load rate of flow of 20 A and an armature vindication of ab fall by 0. 5 0. If this repel is directly switched on to come forth, it would civilize an armature modern of 220/0. 5 = 440 A which is 22 times the full-load current.This high commencement current may result in (i) hot of armature due to luxuriant heating ffect, disconfirming the commutator and brushes due to heavy sparking, (iii) excessive emf drop in the military control to which the force is committed. The result is that the operation of other appliances attached to the line may be impair and in particular cases, they may retract to work. In order to avoid excessive current at becomeing, a uncertain shield (known as kickoff resistance) is inserted in series with the armature traffic circle.This resistance is gradually separatrix back as the motor gains festinate (and soly Eb increases) and eventually it is cut out in all when the motor has ttained full speed. The value of starting resistance is generally such that starting current is limited to 1. 25 to 2 times the full-load current. 5. 8 Types of D. C. Motor meths The carrel operation of a d. c. motor consists in the insertion of external resistance into the armature circuit to limit the starting current interpreted by the motor and the removal of this resistance in steps as the motor accelerates.When the motor attains the normal speed, this resistance is totally cut out of the armature circuit. It is very meaning(a) and desirable to provide the starter with antifertility evices to enable the starter arm to pass by to OFF adjust (i) when the supply fails, thus preventing the armature b eing directly across the mains when this potential drop is restored. For this purpose, we use no-volt work coil. (it) when the motor becomes oerload or develops a daub causing the motor to take an excessive current. For this purpose, we use overload vent coil.There are two principal types of d. c. motor starters viz. , three-point starter and quaternion- point starter. As we shall see, the two types of starters discord only in the dash in which the no-volt going away coil is committed. 5. 9 Three-Point Starter This type of starter is widely apply for starting ring road and compound motors. nonrepresentational diagram Fig. (5. 16) shows the schematic diagram of a three-point starter for a beltway motor with protective devices. It is so called because it has three lasts L, Z and A.The starter consists of starting resistance shared into several sections and attached in out to a number of covers. The three terminals L, Z and A of the starter are affiliated respectivel y to the positive line terminal, shunt subject terminal and armature terminal. The other terminals of the armature and shunt domain of a function windings are connected to the cast out terminal of the supply. The no-volt justify coil is connected in the shunt national circuit. genius end of the clench is connected to the terminal L through the over- load release coil.The other end of the finagle moves against a spiral spring and makes contact with all(prenominal) stud during starting operation, cutting out more and more starting resistance as it passes over each stud in clockwise direction. Operation (i) To start with, the d. c. supply is switched on with handle in the OFF position. (ii) The handle is now move clockwise to the first stud. As in brief as it comes in contact with the first stud, the shunt field winding is directly connected across the supply, hile the whole starting resistance is inserted in series with the armature circuit. iii) As the handle is gradually moved over to the final stud, the starting resistance is cut out of the armature circuit in steps. The handle is now held magnetically by the no-volt release coil which is energized by shunt field current. (v) If the supply voltage is suddenly stop or if the field excitation is apropos cut, the no-volt release coil is demagnetized and the handle goes back to the OFF position under the plait of the spring. If no-volt release coil were not used, hen in case of failure of supply, the handle would remain on the final stud.If then supply is restored, the motor will be directly connected across the supply, resulting in an excessive armature current. (v) If the motor is over-loaded (or a fault occurs), it will draw excessive current from the supply. This current will increase the ampere-turns of the over-load release coil and pull the armature C, thus short-circuiting the novolt release coil. The no-volt coil is demagnetized and the handle is pulled to the OFF position by the spring. T hus, the motor is automatically mazed from the supply. Fig. (5. 17) DrawbackIn a three-point starter, the no-volt release coil is connected in series with the shunt field circuit so that it carries the shunt field current. While exercising speed harbour through field regulator, the field current may be weakened to such an extent that the no-volt release coil may not be able to lionise the starter arm in the ON position. This may disconnect the motor from the supply when it is not desired. This drawback is overcome in the four point starter. 5. 10 Four-Point Starter In a four-point starter, the no-volt release coil is connected directly across the supply line through a protective resistance R. Fig. (5. ) shows the schematic diagram of a 4- point starter for a shunt motor (over-load release coil omitted for clarity of the figure). at once the no-volt release coil circuit is nonparasitic of the shunt field circuit. Therefore, proper speed control can be exercised without modify th e operation of novolt release coil. between a three-point starter and a four-point starter is the manner in which no-volt release coil is connected. However, the functional of the two starters is the same. It may be historied that the threepoint starter also provides protection against an openfield circuit. This protection is not provided by the four-point starte