Figs. 1,589 to 1,592.—Synchronous motor principles: II. The condition necessary for synchronous motor operation is that the motor be speeded up until it rotates in synchronism, that is, in step with the alternator. This means that the motor must be run at the same frequency as the alternator (not necessarily at the same speed). In the figures it is assumed that the motor has been brought up to synchronism with the alternator and connected in the circuit as shown. In figs. 1,589 and 1,590 the arrows indicate the direction of the current for the armature position shown. The current flowing through the motor armature induces magnetic poles which are attracted by the field poles, thus producing a torque in the direction in which the armature is rotating. After the alternator coil passes the vertical position, the current reverses as in fig. 1,591, and the current flows through the motor armature in the opposite direction, thus reversing the induced poles as in fig. 1,592. This brings like poles near each other, and since the motor coil has rotated beyond the vertical position the repelling action of the like poles, and also the attraction of unlike poles, produces a torque acting in the direction in which the motor is rotating. Hence, when the two armatures move synchronously, the torque produced by the action of the induced poles upon the field poles is always in the direction in which the motor is running, and accordingly, tends to keep it in operation.
either of which may revolve. The field is separately excited with direct current.
Figs. 1,593 and 1,594.—Synchronous motor principles: III. The current which flows through the armature of a synchronous motor is that due to the effective pressure. Since the motor rotates in a magnetic field, a pressure is induced in its armature in a direction opposite to that induced in the armature of the alternator, and called the reverse pressure, as distinguished from the pressure generated by the alternator called the impressed pressure. At any instant, the pressure available to cause current to flow through the two armatures, called the effective pressure, is equal to the difference between the pressure generated by the alternator or impressed pressure and the reverse pressure induced in the motor. Now if the motor be perfectly free to turn, that is, without load or friction, the reverse pressure will equal the impressed pressure and no current will flow. This is the case of real synchronous operation, that is, not only is the frequency of motor and alternator the same, but the coils rotate without phase difference. In figs. 1,593 and 1,594, the impressed and reverse pressures are represented by the dotted arrows Pi and Pr, respectively. Since in this case these opposing pressures are equal, the resultant or effective pressure is zero; hence, there is no current. In actual machines this condition is impossible, because even if the motors have no external load, there is always more or less friction present; hence, in operation there must be more or less current flowing through the motor armature to induce magnetic poles so as to produce sufficient torque to carry the load. The action of the motor in automatically adjusting the effective pressure to suit the load is explained in figs. 1,595 and 1,596.
The principles upon which such motors operate may be explained by considering the action of two elementary alternators connected in circuit, as illustrated in the accompanying illustrations, one alternator being used as a generator and the other as a synchronous motor.
Suppose the motor, as in figs. 1,585 and 1,586, be at rest when it is connected in circuit with the alternator. The alternating current will flow through the motor armature and produce a reaction upon the field tending to rotate the motor armature first in one direction, then in another.
Figs. 1,595 and 1,596.—Synchronous motor principles: IV—A synchronous motor adjusts itself to changes of load by changing the phase difference between current and pressure. If there be no load and no friction, the motor when speeded up and connected in the circuit, will run in true synchronism with the alternator, that is, at any instant, the coils A B C D and A°B°C°D° will be in parallel planes. When this condition obtains, no current will flow and no torque will be required (as explained in figs. 1,593 and 1,594). If a load be put on the motor, the effect will be to cause A°B°C°D° to lag behind the alternator coil to some position A"B"C"D" and current to flow. The reverse pressure will lag behind the impressed pressure equally with the coil, and the current which has now started will ordinarily take an intermediate phase so that it is behind the impressed pressure but in advance of the reverse pressure. These phase relations may be represented in the figure by the armature positions shown, viz.: 1, the synchronous position A°B°C°D° representing the impressed pressure, 2, the intermediate position A'B'C'D', the current, 3, the actual position A"B"C"D" (corresponding to mechanical lag), the reverse pressure. From the figure it will be seen that the current phase represented by A'B'C'D' is in advance of the reverse pressure phase represented by A"B"C"D". Hence, by armature reaction, the current leading the reverse pressure weakens the motor field and reduces the reverse pressure, thus establishing equilibrium between current and load. As the load is increased, the mechanical lag of the alternator coil becomes greater and likewise the current lead with respect to the reverse pressure, which intensifies the armature reaction and allows more current to flow. In this way equilibrium is maintained for variations in load within the limits of zero and 90° mechanical lag. The effect of armature reaction on motors is just the reverse to its effect on alternators, which results in marked automatic adjustment between the machines especially when a single motor is operated from an alternator of about the same size. In other words, the current which weakens or strengthens the motor field, strengthens or weakens respectively the alternator field as the load is varied.