SYNCHRONOUS MOTORS AND CONVERTERS
Theory and Methods of Calculation and Testing

Synchronous motors: An introduction

Classification of Singlephase and Polyphase A.C. Motors

An alternating current motor comprises an inducing magnetic field and its induced circuit. The one turning with respect to the other. But, while the field of D.C. motors is always constant, that of A.C. motors may be either constant, alternating, or revolving. According to whether it be produced by a direct current, an alternating current, or a system of polyphase alternating currents serving to excite windings suitably interlaced. Therefore, as was proposed quite logically, in 1891, by E. Hospitalier, A.C. Singlephase and polyphase motors can be classified according to the nature of their magnetic fields, into three classes:

  1. Constant field motors
  2. Alternating field motors
  3. Revolving field motors

The first class constitutes the subject-matter of the present book, the second and third classes being reserved for another volume. As will be seen, these three classes each contain singlephase and polyphase motors.

Constant field motors

Constant field motors can also be characterized by the fact that the armature-rotation can be maintained at a single speed only, which is synchronous with the alternations of the currents employed. We will therefore call them, more frequently, in accordance with common custom, synchronous motors, in contradistinction to the two other classes, which may be characterized as asynchronous or non-synchronous. For the sake of greater precision, we will apply the latter qualification to motors of the last class only; and since alternating-field motors (with one single exception, which is of little importance) are characterized by the use of a commutator similar to that used with D.C. machines, we will give them the more distinctive name of Commutator Motors.

Synchronous Motors

A synchronous motor can be defined as being, merely, an alternator used as a motor. The transmission of power between an A.C. generator and an A.C. motor is, therefore, nothing more than a particular case of the coupling of two alternators in synchronous operation. Indeed, it is precisely through the study of the features of the coupling of alternators in parallel that the occasion presented itself of noting the phenomenon of the reversibility of alternators, that is to say, the possibility of using the same machine both as a motor and a generator, provided that it shall have been previously brought to a speed absolutely equal to that of the generator which supplies it with current.

We can easily understand the possibility of operating such a motor by comparing it to a motor with commutated current. It is known that if the current of a shuttle armature of the Siemens ("H") type is commutated at each half revolution, the motor-couple is always in the same direction when the machine is supplied by direct current. In an A.C. system, the same result is obtained without a commutator, because the direction of the supply-current changes at each half revolution, and this effect occurs only when the motion of the motor is synchronous, that is to say when the armature advances the distance of one pole during one alternation of the supply-current.

Although this property was noted as early as 1869 by Wilde, it passed unnoticed during more than ten years, and it has really been known only since the experiments of J. Hopkinson and Grylls-Adams, at the South Foreland Lighthouse, in 1883. The Memoir of Hopkinson (in which, without knowing the work of Wilde, he gives the explanation to which reference will be made later) , was epoch-making in the history of alternating currents.

In the South Foreland experiments, the alternators used were three similar de Meritens singlephase alternating current machines, all belt-driven from a common source of power. These machines could be easily coupled in parallel, as generators, by bringing them to the same speed before coupling them. The belt being then removed from one of them, it was observed that it continued to run synchronously by the action of the current of its neighbors, and that it could even develop a considerable amount of power, as measured by a friction brake, before losing its synchronism. These experiments were repeated a few years later by Mordey, on a much larger scale, with machines of low inductance presenting a much greater stability of operation and driven by independent prime movers. He was thus able to demonstrate the synchronizing power of the alternators on the motors or engines driving them, and even to cause one of the latter, with the power shut off, to be dragged by one of the alternators which it was driving. This gives the key to the principles involved in parallel working. He also showed, later, the possibility of accomplishing this coupling with machines connected by means of long lines of high resistance.

Synchronous singlephase motors

Synchronous singlephase motors have two great disadvantages: they are not self-exciting, and they cannot start alone, even without load. Zipernowsky was the first to overcome this difficulty by the expedient of adding a commutator to his motors, which enables them to be started with alternating current, and, after they have attained synchronous speed, to be excited by a portion of the alternating current which they consume.

These motors, manufactured by the firm of Ganz & Co., had a certain vogue, in consequence of the tests 4made of them at Frankfort, in 1899, by a Technical Commission. The efficiency was satisfactory, being 77 per cent for motors of 15 H.P., and 86 per cent for motors of 30 H.P.1 This system is no longer used at the present time, except for small powers (1 to 5 H.P.).

When the invention of polyphase currents became known, it led naturally to the idea of utilizing them for the transmission of power between two synchronous machines of the same type. Bradley, in America, and Haselwander, in Germany, took out patents, as early as 1887, the former on a two-phase synchronous motor, and the latter on a three-phase synchronous motor. In both cases the motor was produced by making taps on a Gramme ring and connecting these with insulated rings mounted on the armature-shaft. Non-synchronous motors were only invented in the following year, by Ferraris and by Tesla.

It was in 1891, at the Frankfort Exhibition, that synchronous polyphase motors, with flat ring or Gramme ring armatures, constructed by the firms of Schuckert and of Lahmeyer, and of sizes as large as 50 H.P., were seen, for the first time, alongside the first non-synchronous motors of Dolivo-Dobrowolski and of Brown. Since that time the principle of synchronous motor operation has been extended to ordinary polyphase alternators, with any winding whatever, stationary or movable, with poles alternating or not; and the only improvements that have been made have been in the means of their excitation or of starting them.

Over-excited Synchronous Motors

In 1890-1 Swinburne had had the idea of producing, by means of over-excited synchronous motors, the relatively considerable magnetizing current consumed by his "hedgehog" transformers. Under these conditions the motor played the same role as a condenser. This very interesting property was utilized industrially in 1893 in the Bulach-Oerlikon power-transmission installation, at the suggestion of Dolivo-Dobrowolski; and it was also used in the distributing system at Bockenheim, by Lahmeyer, to compensate for the wattless current of the non-synchronous motors, and even for raising the voltage of the generators. This method has come into extensive use at the present time, especially in the United States.

It constitutes an advantage in favor of synchronous motors, and it has prevented them from disappearing from the commercial field, after the development of motors of revolving field type, which are quite superior to them from other standpoints, notably in regard to starting power. It is desirable to utilize, wherever possible, both these types of motors in distributions of mechanical power. The synchronous polyphase motors are especially useful. They start readily without load, are self-exciting, and have an efficiency equal to that of alternators.

Objections to synchronous motors

The principal objection to synchronous motors (which is, however, in certain cases, an advantage) is the impossibility of modifying their speed of rotation (without modifying that of the generators). Revolving field motors are superior to them in that respect, but this advantage is obtained at the expense of efficiency. Commutator motors, of which no mention will be made here, are alone capable of running at all speeds, the same as direct current motors; and for this reason they present a certain amount of interest.

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