Startres

At starting, when the motor is stationary, there is no back e. m. f. in the armature. Consequently, if the motor is directly switched on to the mains, the armature will draw a heavy current (la = WRa) because of small armature resistance. As an example, 5 H. P. , 220 V shunt 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 take an armature current of 220/0. 5 = 440 A which is 22 times the full-load current.This high starting current may result in: (i) burning of armature due to excessive heating ffect, damaging the commutator and brushes due to heavy sparking, (iii) excessive voltage drop in the line to which the motor is connected. The result is that the operation of other 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 resistance) is inserted in series w ith the armature circuit.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 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 Starters The stalling operation of a d. c. motor consists in the insertion of external resistance into the armature circuit 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 normal speed, this resistance is totally cut out of the armature circuit. It is very important and desirable to provide the starter with protective evices to enable the starter arm to return to OFF position (i) when the supply fails, thus preventing the armature being directly across the mains when this voltage is restored. For this purpose, we use no-volt release coil. (it) when t he motor becomes overloaded or develops a fault causing the motor to take an excessive current. For this purpose, we use overload release coil.There are two principal types of d. c. motor starters viz. , three-point 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 coil 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 field terminal and armature terminal. The other terminals of the armature and shunt field windings are connected to th e negative terminal of the supply. The no-volt release coil is connected in the shunt field circuit. One end of the handle 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 contact with each 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 moved 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 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 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 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 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 disconnected 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 tha t it carries the shunt field current. While exercising speed control through field regulator, the field current may be weakened 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). 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 th e same. It may be noted that the threepoint starter also provides protection against an openfield circuit. This protection is not provided by the four-point starte