COLLIERY GUARDIAN AND JOURNAL OF THE COAL AND IRON TRADES. Vol. CVII. FRIDAY, FEBRUARY 13, 1914. No. 2772. Speed Control of Electric Motors. [SPECIALLY CONTRIBUTED.] The question of speed control of electric motors has a peculiar interest for the colliery engineer, because he is continually confronted with the problem of driving electrically plant which, for maximum efficiency under different conditions, must run at different speeds. Of course, such interest is not confined to the colliery engineer. It is evident that there is such a variety of duties involving speed regulation that it is no exaggera- tion to say that the problem stimulates the interest of all electrical engineers. The colliery engineer, however, is particularly concerned owing to the importance of the problem in connection with the electrical driving of colliery ventilating fans. A mine ventilating fan runs, normally, at a constant speed. During any single shift its duty is usually constant; its speed, therefore, is also constant. The duty, however, may differ for two different shifts, and the week-end duty of a fan is usually different from its shift duty. In order to meet this varying duty efficiently and economically, it is most desirable to possess some kind of control over the fan speed. Of course, such a variation of duty can be met by throttling—i.e., by varying the equivalent orifice of the mine. Such a method, however, is extremely wasteful, and, in these days of keen competition, cannot be countenanced. Only by speed regulation can the conditions involved be satisfied. Owing to the growing tendency to electrify collieries, this question of speed regulation has become an important electrical problem, and, as such, it has received the serious attention of electrical manufac- turers. As a result, there are now on the market many ingenious and effective methods whereby speed control may be obtained. In this short article it is proposed to discuss these various methods only generally. It will be appreciated that an exhaustive theoretical explana- tion of the principles involved would entail the introduction of complicated technical considerations. This would be supererogatory, and it is therefore proposed to discuss the methods briefly, yet sufficiently comprehensively, to enable the reader to grasp their relative advantages and disadvantages. The electrical supply available in practically all modern collieries is three-phase alternating current. Owing to the ease with which it may be transformed from one voltage to another, to the consequent ease and efficiency of transmission to long distances, to the comparative simplicity and cheapness of alternating- current plant, and to many other advantages, alternating current is preferable to direct current for colliery work. Direct current, however, possesses the very important advantage of being much more suitable for driving variable speed plant.. The shunt-wound direct-current motor offers a reliable and efficient means of performing such a duty, and no alternating-current motor can compare with it in this respect. For this reason it is, under certain circumstances, advisable to convert from alternating to direct current, in order that direct- current motors may be utilised for obtaining the desired speed regulation. Either this may be done, or special alternating-current motors may be installed. There are thus two main alternatives open to the colliery engineer desiring to drive variable speed plant electrically. The one alternative involves direct-current motors and converting plant; the other involves more or less special alternating-current motors. Let us consider the two alternatives in detail. Direct-current Motors. As has already been pointed out, the supply available at most collieries is alternating current, and the adop- tion of direct-current motors for speed control, therefore, involves the installation of converting plant. The expense of such an equipment, however, should be incurred only if a wide, delicate, and frequent speed regulation is essential. Such conditions are met in electric winding problems, and for this reason direct- current winders of the Ward-Leonard or Ilgner type are now extensively used. For winding engines a very delicate speed control is imperative. Furthermore, the speed of a winding motor is varied almost continually. During the acceleration period of a winding cycle, the speed varies from zero to a maximum ; it then remains constant until the retardation period is reached, when it again falls to zero. This operation is repeated during every wind, and therefore a great many times during the course of, say, an hour. Hence, for winding engine work, speed control is of paramount importance, and it is due to this that direct-current winding engines are so largely used in this country. Winding engine work, however, represents practically the only variable speed duty met with in colliery practice which justifies the conversion of alternating into direct current. Most of the variable speed work encountered does not involve such exacting conditions or such delicate speed control. Usually, colliery variable-speed plant represents plant which runs for six or eight hours at a constant speed, but which may then be required to run for long or short periods at a lower speed. It would evidently be unwise to incur the expense of converting plant for the sake of obtaining such a speed variation with direct-current system generally must be ample enough to carry the field flux corresponding to full excitation, i.e., the motor must be designed on the basis of its lowest speed. In consequence a large amount of material is required, and so the machine is somewhat expensive. (&.) Series Rheostatic Control. — This method is adopted only with series-wound motors. The speed of a series-wound motor can be varied only by varying the voltage applied to it. As the voltage of the circuit supplying such a machine is invariably constant, external resistances are resorted to in order to obtain the requisite voltage reduction. These resistances, of course, are connected in series with the motor and form the controller. The fundamental connections are shown in fig. 2. This method is extremely wasteful, because the full motor current flows through the controller, so that the rheostatic losses are high. Partly owing to this impossibility of efficient speed control and partly to undesirable operating characteristics, series motors are seldom used in collieries. They are used only where a large starting torque is required and where the motors are never required to run on light load. A series motor must always run up to speed undei’ load, otherwise it will race. (c) Ward-Leonard Control.—This method is adopted only in special cases where a very wide and delicate speed control is necessary. The method is restricted in its application, because it involves the use of a special variable voltage generator for supplying the motor. The rq n s Shunt N o l"o k -AN\NV\/ Field Rheostat Fig. 1. X/artable VoUa