Basic_Electrical_Engineering_4th_edition

The machinewillbehave as a shunt machine. The seriesmachinecharacteristicsfor speed vs la and torque vs la are shown in Fig. 6.19. Initially, under unsaturated condition when la is small, Tala2 and after large value ofla when the machine gets saturated T a la

Fig. 6.19. Speed V518 and torque V/a characteristics for series motor.

The speedtorque curve ofa de series motoris very suitable for electric traction and cranes. The starting torque is very high (TaF). Therefore, heavy masses can be accelerated very fast. The decrease in speed withincrease intorque is a stable operatingconditionforthis motor. When the motor has to go up hill, the torquerequired increases, hence the speed automatically reduces.

6.7.3DC Motor Starter

We know that the current drawn by a de motor is limited by its back emfas this emf opposes the supply voltage and hence the resultant voltage is responsible for the current. This back e.m.f. is proportional to the speed. Hence when the motor is to be started, initially the sped is zero and hence the back emfis zero and hence at start thecurrentis limited onlyby the armature resist-

ance which usually is in terms of friction of an ohm and the current I8T = RV where V is the

supply voltage andthe current becomes in terms ofhundreds ofamperes whichamay damage the armature winding. Hence a device known as a de motor starter is invariably used for starting these motors. The starting current can be reduced by inserting a resistance in series with the armature. As the motor is switched on to supply, the current is limited to suitable value by the external resistance andthemotorstartsrunning. Asthe motorpicksup speed, back emfis induced whichopposes the supplyvoltage andtheexternalresistanceis reducedgraduallyorin steps such that whenthe motorhas reached its normalrated speed the external resistance is completelycut off. The starterhas two additionalfeatures, the no-voltcoil and over load release coilto protectthe motorfrom damage when operatingunder abnormal conditions. Out ofthevarious starters avail­ able, three-point starter is more commonly used and is discussed next.

Three-PointStarter. The starterconsists ofa numberofresistances connected between studs. The arm ofthe starter makes contact with these studs and also to two metal arcs. These arcs are connected to the no-volt coil andtheoverloadreleaseto the starter arm. Initiallythe arm is in the extreme left position and all the resistance elements are in the circuit i.e., in series with the armature circuit. As the motor picks up speed, the arm is gradually moved towards right, cutting out the resistances in step. In the final position the arm is in the extreme right position whenthe whole ofexternal resistance has been cut out and the motor is directly connected on to the supply. The connection ofthe starter to the motor is shown in Fig. 6.20. The internal details ofthe starter are also shown.

Overload release

Fig. 6.20. Three point starter for d.c. shunt motor.

The starterhas two important featuresthe no-volt release andover load release. Suppose the motor is running under normal condition and suddenly the supply fails. If the starter arm remains in the extreme right condition when no external resistance is inserted in the armature circuit and inthe mean time the supply is restored, the armature ofthe motor will get damaged. However,hereno-voltrelease comes to therescue ofthe motor. As soon as thesupplyfails no volt coil gets de-energised and the starter arm goes back to extreme left position due to spring action.

Similarly whenunder operatingconditionifthe current drawn by the motor is more than 130% ofthe full load current the overload release attracts an armature which short circuits the no volt coil and the starter arm is brought back to its extreme leftposition (offposition) and the supply to the motor is disconnected. Thus the motor is saved from overheating.

6.8LOSSES IN DC MACHINES

Mainly following losses take place in a de machine :

1. Copper loss. These are the l2R loss and take place in armature conductors (la2Ra),

2.Rotational losses. In a de machine, the core loss is a true rotational loss. Except in the pole faces, the fluxinthe statorremains constant in steadystateoperation. The flux in the armature core is constantly alternating as the machine rotates and at zero speed the core loss is zero. The rotational loss then consist ofthe sum ofthe friction windage and core losses. In a generator orconstant speed motor these are considered part ofthe constant losses.

3.Brush drop loss. Brushcontactloss occurs due to voltagedropacross thebrush andis equal to 2Vdlawhere Vdis thevoltage drop across one brush.

4.Strayload loss. We calculate armaturecopperloss onthebasis ofderesistance whereas the current in the armature winding is alternating. An incremental loss should be included to account for skin effect, eddy currents in the conductors andcore loss due to