VII. Variable Ratio Converters ("Split Pole" Converters) 98. With a sine wave of alternating voltage, and the com- mutator brushes set at the magnetic neutral, that is, at right angles to the resultant magnetic flux, the direct voltage of a SYNCHRONOUS CONVERTERS 253 converter is constant at constant impressed alternating voltage. It equals the maximum value of the alternating voltage between two diametrically opposite points of the commutator, or "dia- metrical voltage," and the diametrical voltage is twice the voltage between alternating lead and neutral, or star or Y voltage of the polyphase system. A change of the direct voltage, at constant impressed alter- nating voltage, can be produced — Either by changing the position angle between the commu- tator brushes and the resultant magnetic flux, so that the direct voltage between the brushes is not the maximum diametrical alternating voltage but only a part thereof, Or by changing the maximum diametrical alternating voltage, at constant effective impressed voltage, by wave-shape distortion by the superposition of higher harmonics. In the former case, only a reduction of the direct voltage below the normal value can be produced, while in the latter case an increase as well as a reduction can be produced, an increase if the higher harmonics are in phase, and a reduction if the higher harmonics are in opposition to the fundamental wave of the diametrical or Y voltage. Both methods are combined in the so-called " Regulating Pole Converter" or "Split Pole Converter," which is used to supply, from constant alternating voltage supply, direct voltage varying sometimes over a range of ± 20 per cent. In this type of converter, the field pole is divided into sections, usually two, a smaller one, the regulating pole, and a larger one, the main pole. By varying the excitation of the regulating pole from maximum in one direction, to maximum in the opposite direction, the direction of the resultant magnetic field flux, and the effective width of the field pole, and with the latter the wave shape, are varied. To keep the wave shape variation local in the converter, so as not to reflect it into the primary supply circuit, the proper transformer connection must be used. This is Y primary with preferably A or double delta (for three-phase and for six-phase) or Y and double Y or dia- metrical in the secondary. Vm. Starting 99. The polyphase converter is self-starting from rest; that is, when connected across the polyphase circuit it starts, acceler- 254 ELEMENTS OF ELECTRICAL ENGINEERING ates, and runs up to complete synchronism. The e.m.f. between the commutator brushes is alternating in starting, with the fre- quency of slip below synchronism. Thus a direct-current volt- meter or incandescent lamps connected across the commutator brushes indicate by their beats the approach of the converter to synchronism. When starting, the field circuit of the converter has to be opened or at least greatly weakened. The starting of the polyphase converter is largely a hysteresis effect and entirely so in machines with laminated field poles, while in ma- chines with solid magnet poles or with a short-circuited winding (squirrel-cage) in the field poles, secondary currents in the latter contribute to the starting torque, but at the same time reduce the magnetic starting flux by their demagnetizing effect. The torque is produced by the attraction between the alternating currents of the successive phases upon the remanent magnetism and secondary currents produced by the preceding phase. It is necessarily comparatively weak, and from full-load to twice full-load current at from one-third to one-half of full voltage is required to start from rest without load. Usually, low-voltage taps on the transformers are used to give the lower starting voltage. While an induction motor can never reach exact synchronism, but must even at no load slip slightly to produce the friction torque, the converter or synchronous motor reaches exact syn- chronism, due to the difference of the magnetic reluctance in the direction of the field poles and in the direction in electrical quadrature thereto; that is, the field structure acts like a shuttle armature and the polar projections catch with the rotating magnet poles in the armature, in a similar way as an induction motor armature with a single short-circuited coil (synchronous induction motor, reaction machine) drops into step. Obviously, the single-phase converter is not self-starting. At the moment of starting, the field circuit of the converter is in the position of a secondary to the armature circuit as primary; and since in general the number of field turns is very much larger than the number of armature turns, excessive e.m.fs. may be generated in the field circuit, reaching frequently 4000 to 6000 volts, which have to be taken care of by some means, as by breaking the field circuit into sections, or protecting against ex- cessive voltages by a squirrel-cage starting winding in the pole faces. As soon as synchronism is reached, which usually takes SYNCHRONOUS CONVERTERS 255 from a few seconds to a minute or more, and is seen by the ap- pearance of continuous voltage at the commutator brushes, the field circuit is closed and the load put on the converter. Ob- viously, while starting, the direct-current side of the converter must be open-circuited, since the e.m.f. between commutator brushes is alternating until synchronism is reached. When starting from the alternating side, the converter can drop into synchronism at either polarity; but its polarity can be reversed by strongly exciting the field in the right direction by some outside source, as another converter, etc., or by momen- tarily opening the circuit and thereby letting the converter slip one pole. Since when starting from the alternating side the converter requires a very large and, at the same time, lagging current, it is occasionally preferable to start it from the direct-current side as direct-current motor. This can be done when connected to storage battery or direct-current generator. When feeding into a direct-current system together with other converters or con- verter stations, all but the first converter can be started from the continuous current side by means of rheostats inserted into the armature circuit. To avoid the necessity of synchronizing the converter, by phase lamps, with the alternating system in case of starting by direct current (which operation may be difficult where the direct voltage fluctuates, owing to heavy fluctuations of load, as rail- way systems), it is frequently preferable to run the converter up to or beyond synchronism by direct current, then cut off from the direct current, open the field circuit and connect it to the alternating system, thus bringing it into step by alternating current. If starting from the alternating side is to be avoided, and direct current not always available, as when starting the first converter, a small induction motor (of less poles than the con- verter) is used as starting motor. Converters usually are started from the alternating side.