Because of the very rapid reversals in direction of the torque thus set up, there is not sufficient time to overcome the inertia of the armature before the current reverses and produces a torque in the opposite direction, hence, the armature remains stationary or, strictly speaking, it vibrates.
Figs. 1,597 and 1,598.—Synchronous motor principles: V. The effectiveness of armature reaction in weakening the field is proportional to the sine of the angle by which the current lags behind the impressed pressure. If a motor be without load or friction, its armature will revolve synchronously (in parallel planes) with the alternator armature. In the figures let ABCD represent an instantaneous position of the motor armature when this condition obtains; it will then represent the phase relationship of impressed and reverse pressures for the same condition of no load, no friction, operation. Now, if a light load be placed on the motor for the same instantaneous position of alternator armature, the motor coil will drop behind to some position as A", fig. 1,597 (part of the coil only being shown). The reverse pressure will also lag an equal amount and its phase with respect to the impressed pressure will be represented by A". The armature current will ordinarily take an intermediate phase, represented by coil position A'B'C'D', inducing a field strength corresponding to the 9 lines of force OF, O'F', etc. The current being in advance of the phase of the reverse pressure A", the armature reaction weakens the field, thus reducing the reverse pressure and allowing the proper current to flow to balance the load. The amount by which the field is weakened may be determined by resolving the induced magnetic lines OF, O'F', O"F", etc., into components OG, GF, O'G', G'F', O"G", G"F", etc., respectively parallel and at right angles to the lines of force of the main field. Of these components, the field is weakened only by OG, O'G', O"G", etc. Since by construction, angle OFG = AOA', and calling OF unity length, OG = sine of angle by which the current lags behind the impressed pressure. The construction is shown better in the enlarged diagram. For a heavier load the armature coil will drop back further to some position as A"', fig. 1,598, and the lag of the current increase to some intermediate phase as A"B"C"D". By similar construction it is seen that the component OG (fig. 1,597) has increased to OJ (fig. 1,598), this component thus further weakening the main field, by an amount proportional to the sine of the angle by which the current lags behind the impressed pressure. The increased current which is now permitted to flow, causes the induced field to be strengthened (as indicated by the dotted magnetic lines M, M', M", etc.), thus increasing the torque to balance the additional load.
Now if the motor armature be first brought up to a speed corresponding in frequency to that of the alternator before connecting the motor in the circuit, the armature will continue revolving at the same frequency as the alternator.
The armature continues revolving, because, at synchronous speed, the field flux and armature current are always in the same relative position, producing a torque which always pulls the armature around in the same direction.
A polyphase synchronous motor is self starting, because, before the current has died out in the coils of one phase, it is increasing in those of the other phase or phases, so that there is always some turning effort exerted on the armature.
The speed of a synchronous motor is that at which it would have to run, if driven as an alternator, to deliver the number of cycles which is given by the supply alternator.
Figs. 1,599 and 1,600.—Synchronous motor principles: VI. A single phase synchronous motor has "dead centers," just the same as a one cylinder steam engine. Two diagrams of the motor are here shown illustrating the effect of the current in both directions. When the plane of the coil is perpendicular to the field, the poles induced in the armature are parallel to field for either direction of the current; that is to say, the field lines of force and the induced lines of force acting in parallel or opposite directions, no turning effect is produced, just as in analogy when an engine is on the dead center, the piston rod (field line of force) and connecting rod (induced line of force) being in a straight line, the force exerted by the steam on the piston produces no torque.
For instance a 12 pole alternator running at 600 revolutions per minute will deliver current at a frequency of 60 cycles a second; an 8 pole synchronous motor supplied from that circuit will run at 900 revolutions per minute, which is the speed at which it would have to be driven as an alternator to give 60 cycles a second—the frequency of the 12 pole alternator.