DC MACHINES course

Starting of Induction Motors It has been shown earlier that a plain induction motor is similar in action to a polyphase transformer with a short-circuited rotating secondary. Therefore, if normal supply voltage is applied to the stationary motor, then, as in the case of a transformer, a very large initial current is taken by the primary, at least, for a short while. It would be remembered that exactly similar conditions exist in the case of a d.c. motor, if it is thrown directly across the supply lines, because at the time of starting it, there is no back to oppose the initial inrush of current. Induction motors, when direct-switched, take five to seven times their full-load current and develop only 1.5 to 2.5 times their full-load torque. This initial excessive current is objectionable because it will produce large line-voltage drop that, in turn, will affect the operation of other electrical equipment connected to the same lines. Hence, it is not advisable to line-start motors of

Speed Control of Induction Motors* A 3-phase induction motor is practically a constant-speed machine, more or less like a d.c. shunt motor. The speed regulation of an induction motor (having low resistance) is usually less than 5% at full-load. However, there is one difference of practical importance between the two. Whereas d.c. shunt motors can be made to run at any speed within wide limits, with good efficiency and speed regulation, merely by manipulating a simple field rheostat, the same is not possible with induction motors. In their case, speed reduction is accompanied by a corresponding loss of efficiency and good speed regulation. That is why it is much easier to build a good adjustable-speed d.c. shunt motor than an adjustable speed induction motor. Different methods by which speed control of induction motors is achieved, may be grouped under two main headings : 1. Control from stator side (a) by changing the applied voltage ( b ) by changing the applied frequen

Double Squirrel Cage Motor The main disadvantage of a squirrel-cage motor is its poor starting torque, because of its low rotor resistance. The starting torque could be increased by having a cage of high resistance, but then the motor will have poor efficiency under normal running conditions (because there will be more rotor Cu losses). The difficulty with a cage motor is that its cage is permanently short-circuited, so no external resistance can be introduced temporarily in its rotor circuit during starting period. Many efforts have been made to build a squirrel-cage motor which should have a high starting torque without sacrificing its electrical efficiency, under normal running conditions. The result is a motor, due to Boucheort, which has two independent cages on the same rotor, one inside the other. A punching for such a double cage rotor is shown in Fig. 35.26. The oute r cage consists of bars of a high-resistanc e metal, whereas the inner cage has low-resistance copper bars.

Control Differential Transmitter It can be used to produce a rotation equal to the sum of difference of the rotations of two shafts. The arrangement for this purpose is shown in Fig. 39.20 ( a ). Here, a CDX is coupled to a control transmitter on one side and a control receiver on the other. The C X and C R rotor windings are energized from the same single-phase voltage supply. It has two inputs : Mechanical q and Electrical f and the output is Machnical (q - f). The mechanical input (q) to C X is converted and applied to the CD X stator. With a rotor input (f), the electrical output of the CD X is applied to the C R stator which provides the mechanical output (q - f). As shown in Fig. 39.20 ( b ), if any two stator connections between C X and CD X are transposed, the electrical input from C X to CD X becomes -q, hence the output becomes (-q - f) = - (q + f). Control Differential Receiver In construction, it is similar to a CD X but it accepts two electrical input angles an

Maximum Quantities It will now be shown from the circle diagram (Fig. 35.10) that the maximum values occur at the positions stated below : (i) Maximum Output It occurs at point M where the tangent is parallel to output line O ¢ A . Point M may be located by drawing a line C M from point C such that it V is perpendicular to the output line O ¢ A . Maxi mum output is represented by the vertical MP (ii) Maximum Torque or  Rotor Input It occurs at point N where the tangent is parallel to torque line O ¢ E . Again, point  may be found by drawing C N perpendicular  to the torque line. Its value is represented by N Q . Maximum torque is also known as  stalling or pull-out torque. (iii) Maximum Input Power It occurs at the highest point of the circle i . e . at point R where the tangent to the circle is horizontal. It is proportional to RS. As the point R is beyond the point of maximum torque, the induction motor will be unstable here. However, the maximum input is

Starting of Slip-ring Motors These motors are practically always started with full line voltage applied across the stator terminals. The value of starting current is adjusted by introducing a variable resistance in the rotor circuit. The controlling resistance is in the form of a rheostat, connected in star (Fig. 35.22), the resistance being gradually cut-out of the rotor circuit, as the motor gathers speed. It has been already shown that by increasing the rotor resistance, not only is the rotor (and hence stator) current reduced at starting, but at the same time, the starting torque is also increased due to improvement in power factor. The controlling rheostat is either of stud or contactor type and Slip-ring electric motor may be hand-operated or automatic. The starter unit usually includes a line switching contactor for the stator along with no- voltage (or low- voltage) and over-current protective devices. There is some form of interlocking to ensure proper sequential opera

Crawling It has been found that induction motors, particularly the squirrel-cage type, sometimes exhibit a tendency to run stably at speeds as low as one-seventh of their synchronous speed N s . This phenomenon is known as crawling of an induction motor. This action is due to the fact that the a.c. winding of the stator produces a flux wave, which is not a pure sine wave. It is a complex wave consisting of a fundamental wave, which revolves synchronously and odd harmonics like 3rd, 5th, and 7th etc. which rotate either in the forward or backward direction at N s / 3, N s / 5 and N s / 7 speeds respectively. As a result, in addition to the fundamental torque, harmonic torques are also developed, whose synchronous speeds are 1/ n th of the speed for the fundamental torque i . e . N s / n , where n is the order of the harmonic torque. Since 3rd harmonic currents are absent in a balanced 3-phase system, they produce no rotating field and, therefore, no torque. Hence, total motor torque