75 Years of Development

Leonard drives 75 YEARS OF DEVELOPMENT by K. A. YEOMANS, B.Sc, M.Sc.(Eng.), D.I.C., C.Eng., M.I.E.E. Although the Ward Leonard drive system is familiar to most engineers, its special features are not always appreciated,, When the system was ferst introduced in America 75 years ago, it was the subject of fierce criticism, and its early development for mine-winding and rolling-mill drives owes much to pioneering activities in Europe. Modern methods enable faster responses to be achieved, but no drive has shown itself more suitable than the Ward Leonard scheme for the purpose for which it was invented. IT is NOW over 75 years since the first paper then president of the ALEE, F. Sprague, convertor, using the basic principle that was presented by H. Ward Leonard whose remarkable pioneering efforts had the maximum efficiency of the motor is which described the drive system that given the world its first commercially achieved when 'voltage varies as speed has made his name famous. Although successful electric municipal-transport and torque as current'. By this means, a his system is now well known, few appre- system2 at Richmond, Va., USA, in 1887. high torque could be achieved for ciate that it is almost as old as the history starting without an excessive power of applied electrical engineering, and that New system demand. Regeneration of power at slow- the idea was first opposed by unusually The new system of connection was down, or when descending a gradient, fierce criticism of its technical and com- proposed as an answer to the current was also claimed as a feature. mercial value. problems in applying electric motors to Ward Leonard first described his traction duties. The series motor had been Merits system in an article in Electrical World successfully developed by Sprague for The merits claimed for the system in November 1891, and this was followed this application as a robust and reliable included the increased efficiency, the by a paper1 presented on the 8th June machine, capable of withstanding the reduced space, the size and cost of control 1892 at the 9th general meeting of the abuses and severe duties encountered in switches and rheostats, and the savings American Institute of Electrical Engi- municipal street-car practice; but the in power-station plant made possible by neers. The occasion was a 3-day meeting heavy currents drawn in starting or the steadier load. It was admitted that the held at Chicago by the then well estab- in crawling heavily laden up steep cost of equipment on the car would be lished institute, whose activities had been gradients caused severe fluctuations in greater, but it was estimated that the total more usually confined to New York. At the power-station loadings. In addition, cost of the transport system would be this meeting, a wide range of papers on power losses in the rheostats used for less. The system had already been tried electric-circuit theory, on power-station starting resulted in apparently un- and found successful for electric lifts and management, on electrotechnical educa- avoidable inefficiencies. Many engineers cranes. tion and on mining and traction applica- had attempted to apply variable-ratio From the beginning, the proposals tions was discussed. At this time, electrical transmissions between motors and axles came under heavy criticism. Time was engineering was developing rapidly, and to achieve better efficiency at starting too short for a full discussion on the paper the d.c. electric motor was finding in- and at low speeds, but none had suc- at that meeting, but, from the chair, creasing use for a variety of machine ceeded. Sprague voiced his reservations on the drives, cranes and electric traction. Ward Leonard's arrangement (Fig. 1) merit of the scheme, and later he left the The latter development owed much to the was proposed as a variable-ratio power chair to comment on its practicability. 144 Electronics & Power April 1968 1 Ward Leonard's drive was proposed in 1892 as a variable-ratio power convertor for 'a new system of electric propulsion1 H W While admitting it to be 'beautiful Massachusetts Institute of Technology uncertain of the reason for this, theoretically' and to 'accomplish every- at 22 years of age, and a year later he attributing it solely to the fact that the thing as desired from a physical stand- became associated with Thomas A. generator was separately excited. In fact, point', he did not consider it to be a Edison, as a member of a staff of four the usual difficulty in this respect arose practicable economic proposition for engineers selected to introduce the new from the practice of setting the brush axis street-car practice, for which he defended Edison power-station system. He was to commutate at full output with a the switched-field series motor that he had appointed superintendent of the Western highly saturated field. With this adjust- pioneered at Richmond. Electric Light Co. in Chicago at 26 ment, any alteration to main-field excita- years of age. tion to reduce the ouput resulted in Criticism The early experience of Ward Leonard neutral shift and sparking at heavy In New York about four months later, gave him an approach to drive problems currents. It was Ward Leonard's practice the discussion of the Ward Leonard paper that differed from that of his critics. He to set the generator brushes for no-load presented at Chicago was unusually was concerned much more with the neutral, probably to ensure good com- critical, the president (Sprague) again economics of the power-station operation mutation under regenerative conditions. vacating the chair to reiterate his earlier and distribution system than were the In discussion, Ward Leonard often adverse opinion. The opening speaker electric-railway engineers, whose main displayed a large degree of tol- claimed that Ward Leonard had been concentration of effort had been in the erance and patience, which was well unduly optimistic, and that the motor- design and operation of the street cars demonstrated at this meeting by a lengthy generator (m.g.) losses would be such themselves. duologue on commutation with one as to give only a marginal gain in efficiency The earlier electric railways were urban questioner who persisted in his —even at low-speed running—which was street-car systems with short trains of one doubts. Again, however, numerous objec- not worth the additional complication. or two cars, frequent stops and a compact tions were made to the new proposals, Comment was especially made on the distribution system. Electric railways, one member commenting that'. .. if Mr. heavy expense and maintenance charges .however, were being extended in range, Sprague, seven years ago, had objected of the m.g. set. and, in 1894, Ward Leonard presented to everything as the majority have done The criticism in the recorded account his paper—'How shall we operate an tonight, I do not think that we would be of the discussion3 on the paper would electric railway extending 100 miles from in the position with electric roads that we have discouraged most men, but Ward the power station?'5 It was proposed that are in today.' Leonard in his reply seemed still confi- power be transmitted at 20 kV a.c, with dent. His formal vocative to the meeting— variable-power convertors on the loco- New ideas 'Mr. chairman and gentlemen'—was motive to feed the separately excited d.c. It is likely that many of the objections perhaps unusual in that day, for it is traction motors. Unattended transformer were, in fact, raised by those who saw in hardly to be found recorded elsewhere in stations were used to feed the trolley the new scheme competition to their own the transactions of the ABEE. He denied wires at 500V a.c, and the convertor interests. There is little doubt that Ward that his efficiencies were optimistic, and m.g. set was to be driven by a single-phase Leonard himself had reached this opinion, revealed that they were based on practical a.c. motor. for in 1895, after a visit to France and tests. The increased costs of car equip- England,6 he made adverse comment on ment would also be lower than his critics Working model the relative lack of enterprise in trying had quoted, for it was intended to use At the meeting, the features of the new ideas in America and of the in- high-speed lower-cost machines for the Ward Leonard m.g. scheme were de- hibiting effect that the activities of large m.g. sets, the maintenance of which had monstrated by tests on a working model, companies had on engineering develop- been shown, in practice, to be negligible in which the ease of speed control and the ments. The AJDEE president, E. J. compared with that required for traction ability to regenerate and to reverse Houston, commented that '. If here motors which were, of necessity, exposed smoothly were shown. and there an inventor does not succeed to adverse working conditions. He again Discussion on the model, particularly in getting his particular system adopted made the point that his, and no other, on the absence of sparking at the gen- by a corporation, he may perhaps scheme would regenerate satisfactorily erator brushes, was lengthy. Most of the naturally feel that such corporations down to low speeds. questioners were puzzled by the excellence stand in the way of its introduction . H. Ward Leonard was born in 1861 in of generator commutation at low-voltage But they will be apt to buy anything of Cincinnati, USA.4 He graduated at the outputs, and even Ward Leonard seemed real commercial value.

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