D. C. COMMUTATING MACHINES 219 In the alternating-current commutator motor, the field struc- ture as well as the armature must be laminated, since the mag- netic flux is alternating. The alternation of the field flux induces an e.m.f. of self induction in the field winding. In the shunt motor, this causes the field exciting current and with it the magnetic field flux to lag and thereby to be out of phase with the armature current which, to represent work, must essentially be an energy current, and thereby reduces output and efficiency and hence requires some method of compensation, as capacity in series with the field winding or excitation of the field from a quadrature phase of voltage. In the series motor the self-inductance of the field causes the main current to lag behind the impressed voltage and thereby lowers the power-factor of the motor. Thus, to get good power-factor, the field self-inductance must be made low, that is, the field as weak and the armature as strong as possible. With such a strong armature, and weak field, the commutating pole is not sufficient to control magnetic distortion by the arma- ture reaction, and complete compensation by a distributed compensating winding, as Fig. 102, page 190, is required. 79. When in the position of commutation the armature coil is short-circuited by the commutator brush, it encloses the full field flux and thus for a moment no e.m.f. is induced in the armature coil by its rotation through the field flux, and in the continuous current machine the coil is without voltage except whatever voltage may be intentionally produced by the com- mutating flux. In the alternating-current motor, however, the field flux induces voltage also in the armature coil by its alternation, and this voltage is a maximum in the position of commutation, and when short-circuited by the commutator brush tends to produce an excessive current and cause spark- ing. No position exists on the commutator of the alternating- current motor where the armature coil does not contain an induced e.m.f., but in the position midway between the brushes the e.m.f. induced by the rotation through the magnetic field is a maximum; in the position of commutation the e.m.f. induced by the alternation of the field flux is a maximum. To overcome the destructive sparking caused by the short circuit of the latter e.m.f. by the commutator brush is the problem of making a successful alternating-current commutator: 1. Inducing an opposite e.m.f. by a commutating field. As 220 ELEMENTS OF ELECTRICAL ENGINEERING the e.m.f. induced by the alternation of the main field is in quadrature with the main field, and the e.m.f. induced by the rotation through the commutating field is in phase with it, the commutating field must be in quadrature with the main field. By properly proportioning this commutating field, as in the series repulsion motor, completely sparkless commutation can be produced at speed. However, at standstill and low speeds this method fails, as the voltage induced by the rotation through the commutating field becomes zero at standstill. 2. Reducing the short-circuit current by high resistance leads between commutator and armature coil. This only mitigates the trouble, but due to the voltage drop in the lead resistance tends to increase sparking at speed. Also, the excessive con- centration of heat in the commutating leads in the moment of starting tends to destroy them if the motor does not quickly start. 3. Narrow brushes, to reduce the duration of short circuit. 4. Low impressed frequency, so as to give low values to the induced e.m.f. This is the cause of the desire for abnormally low frequencies, as 15 and even 8 cycles, in alternating-current railway electrification. 5. Low magnetic flux per pole. This is the reason why alternating-current commutator motors of large power usually have such a large number of poles. These very severe limitations of the design of alternating-cur- rent commutating motors are the reason why such motors have found only limited application, except in smaller sizes. 80. Alternating-current motors are usually single-phase, since the possibility of commutation control makes the single-phase easier than a polyphase design. In the single-phase motor, the magnetic field flux is constant in direction, and the direction in quadrature to the main field flux thus is available for pro- ducing a suitable commutating flux. In the polyphase motor, however, the magnetic flux rotates, assuming successively all directions, and thus no commutating flux can be used. For this reason, designs of polyphase commutator motors have been made in which the different (2 and 3) phases are kept separate, and spaces left between them for accommodating commutating fluxes. 81. Alternating-current commutator motors are used: 1. In railroading, for securing the advantage of the higher