DC MACHINES course

Factors Controlling Motor Speed It has been shown earlier that the speed of a motor is given by the relation   It is obvious that the speed can be controlled by varying ( i ) flux/pole, F (Flux Control) ( ii ) resistance R a of armature circuit (Rheostatic Control) and ( iii ) applied voltage V (Voltage Con- trol). These methods as applied to shunt, compound and series motors will be discussed below. Speed Control of Shunt motors (i) Variation of Flux or Flux Control Method It is seen from above that N µ 1/F. By decreasing the flux, the speed can be increased and vice versa . Hence, the name flux or field control method. The flux of a d.c. motor can be changed by changing I sh with help of a shunt field rheostat (Fig. 30.1). Since I s h is relatively small, shunt field rheostat has to carry only a small V current, which means I 2 R loss is small, so that rheostat is small in size. This method is, therefore, very efficient. In non-interpolar machine, the speed can be

OBJECTIVE TESTS 1. The speed of a d.c. motor can be controlled by varying (a) its flux per pole (b) resistance of armature circuit (c) applied voltage (d) all of the above 2. The most efficient method of increasing the speed of a 3.75 kW d.c. shunt motor would be the ...........method. (a) armature control (b) flux control (c) Ward-Leonard (d) tapped-field control 3. Regarding Ward-Leonard system of speed con- trol which statement is false ? (a) It is usually used where wide and very sensitive speed control is required. (b) It is used for motors having ratings from 750 kW to 4000 kW (c) Capital outlay involved in the system is right since it uses two extra machines. (d) It gives a speed range of 10 : 1 but in one direction only. (e) It has low overall efficiency especially at light loads. 4. In the rheostatic method of speed control for a d.c. shunt motor, use of armature divertor makes the method (a) less wasteful

Thyristor Controller Starters The moving parts and metal contacts etc., of the resistance starters discussed in Art. 30.21 can be eliminated by using thyristors which can short circuit the resistance sections one after another. A thyristor can be switched on to the conducting state by applying a suitable signal to its gate terminal. While conducting, it offers zero resistance in the forward ( i.e. , anode-to-cathode) direction and thus acts as a short-circuit for the starter resistance section across which it is connected. It can be switched off ( i.e. , brought back to the non-conducting state) by reversing the polarity of its anode-cathode voltage. A typical thyristor-controlled starter for d.c. motors is shown in Fig. 30.49. After switching on the main supply, when switch S 1 is pressed, positive signal is applied to gate G of thyristor T 1 which is, therefore, turned ON. At the same time, shunt field gets established since it is directly connected across the d.c. supply. Conseq

Series Motor Starters The basic principle employed in the design of a starter for series motor is the same as for a shunt motor i.e., the motor current is not allowed to exceed a certain upper limit as the starter arm moves from one stud to another. However, there is one significant difference. In the case of a series motor, the flux does not remain constant but varies with the current because armature current is also the exciting current. The determination of the number of steps is rather complicated as illustrated in Example 30.55. It may however, be noted that the section resistances form a geometrical progression. The face-plate type of starter formerly used for d.c. series motor has been almost entirely replaced by automatic starter in which the resistance steps are cut out automatically by means of a contactor operated by electromagnets. Such starters are well-suited for remote control. However, for winch and crane motors where frequent starting, stopping, reversing and spe

Starter and Speed-control Rheostats Sometimes, for convenience, the field rheostat is also contained within the starting box as shown in Fig. 30.43. In this case, two arms are used. There are two rows of studs, the lower ones being connected to the armature. The inside starting arm moves over the lower studs on the starting resistor, whereas the outside field lever moves over the upper ones on the field rheostat. Only the out- side field arm is provided with an operating handle. While starting the motor, the two arms are moved together, but field lever is electrically inoperative because the field current flows directly from the starting arm through the brass arc to HOLD-ON coil and finally to the shunt field winding. At the end of the starting period, the starting arm is attracted and held in FULL-ON position by the HOLD-ON coil, and the contact between the starting arm and brass arc is broken thus forcing field cur- Speed-control Rheostats rent to pass through the field rheostat. T

Thyristor Speed Control of Separately-excited D.C. Motor In Fig. 30.32, the bridge rectifier converts a voltage into d.c. voltage which is then applied to the armature of the separately-excited d.c. motor M . As we know, speed of a motor is given by If F is kept constant and also if a is neglected, then, N µ V µ volt- age across the armature. The value of this voltage furnished by the recti- fier can be changed by varying the fir- ing angle a of the thyristor T with the help of its contol circuit. As a is in creased i.e. , thyristor firing is delayed more, its conduction period is reduced and, hence, armature voltage is decreased which, in turn, decreases the motor speed. When a is decreased i.e. , thyristor is fired earlier, conduction period is increased which increases the mean value of the voltage applied across the motor armature. Consequently, motor speed is increased. In short, as a increases, V decreases and hence N decreases. Conversely, as a decreases, V increases