Startres

At kickoff, w hen the force back is stationary, thither is no back e. m. f. in the armature. Consequently, if the motor is directly switched on to the mains, the armature go away draw a heavy current (la = WRa) because of small armature subway. As an example, 5 H. P. , 220 V shift motor has a full-load current of 20 A and an armature resistance of about 0. 5 0. If this motor is directly switched on to supply, it would grapple an armature current of 220/0. 5 = 440 A which is 22 times the full-load current.This high get-go current whitethorn result in (i) burning of armature due to ebullient heating ffect, damaging the commutator and brushes due to heavy sparking, (iii) excessive voltage ramble in the line to which the motor is attached. The result is that the effect of early(a) appliances connected to the line may be impaired and in particular cases, they may refuse to work. In order to avoid excessive current at starting, a variable resistance (known as starting resista nce) is inserted in series with the armature tour of duty.This resistance is gradually reduced as the motor gains speed (and hence Eb increases) and eventually it is cut out completely when the motor has ttained full speed. The pry of starting resistance is generally such that starting current is contain 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 travel to limit the starting current taken by the motor and the remotion of this resistance in steps as the motor accelerates.When the motor attains the conventionalism speed, this resistance is totally cut out of the armature circuit. It is very important and sexually attractive to provide the glass with protective evices to enable the meth arm to amends to bump off position (i) when the supply fails, thus preventing the armature being directly crossways the mains when this voltage is re stored. For this purpose, we use no-volt exsert kink. (it) when the motor becomes surchargeed or develops a fault causing the motor to take an excessive current. For this purpose, we use overload exsert curve.There are two principal types of d. c. motor dispatchers viz. , three-point starter and foursome- point starter. As we shall see, the two types of starters differ only in the musical mode in which the no-volt liberation coil is connected. 5. 9 Three-Point Starter This type of starter is widely used for starting electrical 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 turn of events of studs. The three terminals L, Z and A of the starter are connected separately to the positive line terminal, shunt case t erminal and armature terminal. The other terminals of the armature and shunt field twists are connected to the negative terminal of the supply. The no-volt release coil is connected in the shunt field circuit. One end of the grip is connected to the terminal L through the over- load release coil.The other end of the exert moves against a spiral spring and makes contact with separately stud during starting operation, cutting out more and more starting resistance as it passes over each stud in dextrorotatory direction. Operation (i) To start with, the d. c. supply is switched on with handle in the dark position. (ii) The handle is now moved clockwise to the first stud. As briefly 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 utmost stud, the starting resistance is cut out of the arma ture 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 clear up position under the pull of the spring. If no-volt release coil were not used, hen in case of failure of supply, the handle would remain on the net stud.If then supply is restored, the motor go forth 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 disconnected from the suppl y. 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 jibe through field regulator, the field current may be washed-out to such an extent 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). flat the no-volt release coil circuit is independent of the shunt field circuit. Therefore, strait-laced 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