Discussion on “Control of electric winding and hoisting engines”

Abstract

ROSENBERG: CONTROL OF ELECTRIC [Dec. 6th, shaft-horsepower works out higher. The power consumption will be of greater importance the higher the price of the kilowatt-hour in the particular case, and will, therefore, depend upon the local price of fuel and labour. As to the question whether the current consumption or the capital expenditure should be kept low, it will prove a decisive factor whether the shaft in question is intended to be worked for many years, or whether the working will cease presumably after a few years' time. If power is taken from a supply company, the selection of the system, and the question whether equalising apparatus should or should not be used will greatly depend upon the method of charging for the current consumption. If current is charged on a flat-rate basis, an equaliser would, in general, be out of place, because it not only increases the initial costs, but also, owing to the inevitable losses, the total current consumption. A charge on the maximum demand system, on the other hand, will turn the balance in favour of equaliser systems. The question also whether any inducement is offered to the power consumer for a good power factor might turn the balance in a given case from the direct-coupled slow-speed alternating-current winding motor to the Ward-Leonard system with high-speed motor-generator. Safety apparatus which, in one case, due to the local conditions and regularity of the supply, would seem an unnecessary luxury, will be of vital importance in other cases. So each of the systems has its own merits which will call it to the front according to the circumstances of the case. DISCUSSION BEFORE THE MANCHESTER LOCAL SECTION. Mr. Peck. Mr. J. S. PECK : The subject of the braking of induction motors is of special interest. At first sight it would appear that by exciting the primary of an induction motor with direct current so as to turn it into an alternating-current generator, and by varying the resistance in the secondary circuit we could obtain almost any desired braking effect. But the author points out the difficulties in changing the connections, the magnetic stresses between stator and rotor, and, most serious of all, the fact that for each speed a certain definite secondary resistance is required for maximum braking effect. On account of this last feature it would always be difficult for the driver to know whether he should increase or decrease the secondary resistance in order to reduce the motor speed. Consequently there would be danger of the motor reaching an excessive speed, with perhaps serious consequences. It is to be hoped, however, that the scheme hinted at by the author for indicating or fixing automatically the proper resistance for any speed may be developed further. With the usual alternating-current winding engine two methods of braking are available : (i) mechanically by means of the brake blocks, and (2) electrically by means of counter current. Also in lowering it is possible to let the motor run above synchronism, and so return power to the line ; but t...