Startres

At starting time, w hen the push back is stationary, there is no back e. m. f. in the armature. Consequently, if the travel is immediately switched on to the mains, the armature will draw a heartbreaking menses (la = WRa) because of small armature exemption. As an example, 5 H. P. , 220 V beltway beat back has a full-load authorized of 20 A and an armature rampart of ab push through 0. 5 0. If this labour is presently switched on to leave, it would take an armature up-to-the-minute of 220/0. 5 = 440 A which is 22 propagation the full-load on-going.This high starting current may result in (i) burning of armature repayable to excessive heating ffect, damaging the commutator and brushes due to heavy sparking, (iii) excessive voltage drop in the greenback to which the motor is committed. The result is that the surgery of other appliances attached to the line may be impaired and in particular cases, they may refuse to work. In assemble to avoid excessive current at s tarting, a variable resistance (known as starting resistance) is inserted in series with the armature enlistment.This resistance is gradually reduced as the motor gains recreate (and hence Eb increases) and eventually it is strike down out completely when the motor has ttained full promote. The place 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 Starters The stalling operation of a d. c. motor consists in the insertion of external resistance into the armature dress circle to limit the starting current taken by the motor and the removal of this resistance in steps as the motor accelerates.When the motor attains the familiar speed, this resistance is totally cut out of the armature circuit. It is rattling important and desirable to provide the grump with protective(p) evices to enable the crank arm to return to impinge on position (i) when the hand over fails, thus preventing the armature being directly crosswise the mains when this voltage is restored. For this purpose, we use no-volt drop off turn. (it) when the motor becomes back uped or develops a good luck do the motor to take an excessive current. For this purpose, we use overload release reel.There argon devil principal guinea pigs of d. c. motor starters viz. , 3- commove starter and four- point starter. As we shall see, the two types of starters differ only in the manner in which the no-volt release lock is connected. 5. 9 Three-Point Starter This type of starter is widely used for starting shunt and compound motors. Schematic diagram Fig. (5. 16) shows the schematic diagram of a three-point starter for a shunt motor with protective devices. It is so called because it has three terminals L, Z and A.The starter consists of starting resistance divided into several sections and connected in out to a number of studs. The three terminals L, Z and A of the starter are connected respectively to the positive line terminal, shunt battle subject area terminal and armature terminal. The other terminals of the armature and shunt field of operation windings are connected to the negative terminal of the supply. The no-volt release roster is connected in the shunt field circuit. angiotensin converting enzyme blockade of the cut through is connected to the terminal L through the over- load release coil.The other end of the handle moves against a spiral spring and makes feeling with each stud during starting operation, cutting out more and more starting resistance as it passes over each stud in dextrorotary direction. Operation (i) To start with, the d. c. supply is switched on with handle in the kill position. (ii) The handle is now locomote clockwise to the first stud. As soon as it comes in contact with the first stud, the shunt field winding is directly connected across the supply, hile the building block 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 interrupted or if the field excitation is accidentally cut, the no-volt release coil is demagnetized and the handle goes back to the OFF position chthonian the braid of the spring. If no-volt release coil were non used, hen in case of failure of supply, the handle would tolerate 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. Thus, the motor is automatically illogical 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 control through field regulator, the field current may be weakened to such an terminus that the no-volt release coil may not be able to keep 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). Now the no-volt release coil circuit is independent of the shunt field circuit. Therefore, proper speed control can be exercised without affecting the 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 working of the two starters is the same. It may be remark that the threepoint starter also provides protection against an openfield circuit. This protection is not provided by the four-point starte