The Asynchronous If the motor be driven by power from an outside source up to true synchronism, no current will flow in the secondary, and the primary current or field current will be wholly made up of the wattless exciting current, just as in a trans former at no load. The slip, or amount by which the motor speed at full load differs from synchronous speed, may be as little as 2 to 2% per cent of the speed of synchronism in large motors, and in small motors may be 5 per cent or more. If the motor above mentioned be forced above synchronism the motor becomes a generator, •provided the connection to the mains is left closed, and when a negative slip of the same amount as full load slip as a motor is reached, the generator will be giving out its full output at the same frequency as the exciting circuit. The possibilities of this system are interesting.
The Synchronous The synchro nous motor is merely an alternating current generator of special design. Both motors and alternators have a direct current field and an alternating current armature. The operation of a synchronous 'motor, when once brought up to speed and thrown into circuit, is the same as that of an alternator in with one or more alternators. When the back pressure of the motor is equal and directly opposed to that of the line no current can flow. The friction, however, causes the revolving element to lag slightly behind the line pressure, and a cur rent is driven through the motor by the. gener ator. This current increases directly with the lag behind the central-phase position caused by increased load. A good synchronous motor, while always revolving at the same polar speed as the alternator supplying the line current, will carry a load of five or six times full load be fore it breaks out of step, and becomes prac tically a short circuit on the system. The cur rent which passes through such a motor on short circuit, while held down by the induct ance of the windings, is yet sufficient to rapidly damage the insulation if not cut off. The great advantage of the synchronous over the induc tion type of motor is that the power factor can be raised or lowered at will. By raising the field strength of a synchronous motor the cur rent taken by the motor may be made leading and hence help keep up • the line voltage on a heavy inductive load. This is of the greatest importance in practice. It is good practice to set the field strength for a good power factor at full load. At light loads the motor is as sisting the generator to maintain the required pressure. Another advantage of the synchro nous motor is that it can easily be built for very high voltage, especially the revolving field type a 12,000 volt motor is not at all unusual practice — thus the use of transformers may be dispensed with.
The Rotary The rotary con verter is a specially designed direct-current generator provided, at proper points in the winding, with taps to collector rings, from which, if the machine is run as a motor from the direct-current side, an alternating current may be taken. Usually the alternating current is taken from the secondaries of suitable trans formers and supplied to the rings, driving the rotary as a synchronous motor, the direct cur rent being taken from the brushes on the com mutator. As the reaction of the incoming al ternating current about balances that caused by the outgoing direct current, the armature reac tion of such a machine is very small and the brushes can be always kept in one fixed posi tion. If the taps from the armature are taken
off at points differing 180 degrees from each other, electrically, we have a single-phase rotary. If connections are made 90 degrees apart we have a two-phase rotary, using four collector rings. Taking 120 degrees around the armature for our taps we have a three-phase rotary, using three collector rings. By adding to the number of taps and therefore to the num ber of rings we may have a six-phase rotary. The output of a rotary is greater than its out put as a direct-current generator, chiefly on account of the absence of armature reaction and because at certain positions the current flows straight from collector ring to commutator and thus avoids the loss due to heating. The rotary converter, with its step-down trans former, is the most efficient means we now have of transforming the high tension polyphase cur rents of our large central stations to direct current for the Edison system, and for railway purposes. This piece of apparatus is wound either shunt or compound, in accordance with the use for which it is intended. As in. the case of the synchronous motor, the rotary is a valuable help to the central station by running at a very high power factor. By overexciting the fields the current taken by the rotary be comes leading and helps to hold up the voltage of the central station in case of a heavy load of induction motors by means of the armature reaction of the generators. Owing to very high commutator speeds at the higher frequencies, rotaries are not much used on frequencies above 60 degrees. At this frequency they operate satisfactorily. At lower frequencies, however, rotaries are at their best, and will stand enor mous overloads, sudden changes in load and other disturbances, with perfect satisfaction. The voltage of the direct current end of a rotary is that of the peak of the sine wave of the alternating pressure, and thus a voltmeter across the collector rings would read !rrea where V 2 E is the direct-current electromotive force in single and two-phase rotaries. In the three phase system the ratio between the alternating current pressure and the direct current at the commutator brushes is Thus in the 2 V 2 Edison system operating at 250 volts we should have to transform down to 250X.612=153 volts at the secondary of the transformers. ‘Vhile rotaries can be started up without field, from the alternating current side, it is not good prac tice, excepting in certain special cases. Gen erally they are started up exactly like a shunt motor, synchronized, and then thrown upon the alternating current line. When a rotary is started up from the alternating current side, on closing the field switch it is impossible to tell what the polarity will be. Rotaries operate in parallel with perfect satisfaction, as a rule, on both the alternating current and the direct sides. The storage battery is always used in a large rotary installation to ensure against any possible contingencies. On compound rotaries the equalizer must be used, just as in the case of direct-current compound generators. See