SYNCHRONOUS MOTORS AND CONVERTERS
Theory and Methods of Calculation and Testing

Stalling of a Synchronous Motor

Stalling of a synchronous motor

If a brake be placed on the pulley and if the load be gradually increased in such a way as to increase the mechanical power produced by the motor, the "lag" of the motor will be seen to increase at the same time as the current.

When this lag approaches a quarter of a period, i.e., half an interpolar space, the machine slows up all at once and stops as if held fast by the brake. We then say that it is "stalled," or " out of synchronism", or "out of step." The current in the circuit rises to a very high value as soon as the machine falls out of synchronism; and it becomes approximately equal to the short-circuit current in the circuit when the machine is stopped. In order to avoid accidents, it is necessary to introduce fuses in the circuit, or to provide some automatic disconnecting device, which will prevent the excessive load.

It is seen that what characterizes the synchronous motor is the increase of phase-lag with the load and the "stalling" of the motor or its falling out of step beyond a certain maximum load.

In a good motor, the limiting load should amount to at least 1.5 times, or, better, to twice the normal load. This limit is guaranteed by most makers of synchronous motors.

On the other hand, if the motor is run by a belt in such a way as to give it a "lead in phase" with respect to the machine or the circuit which supplies it with current, it can be found, by wattmeter measurements, that this power changes in sign, i.e., the motor acts as a brake and returns energy to the circuit instead of receiving it therefrom.

The phenomena become more complicated still on varying the E.M.F. of the motor or of the generator.

Case of Unequal Electromotive Forces

An interesting and characteristic property of synchronous alternating current motors, and which distinguishes them absolutely from direct current motors or from alternating current motors having commutators, is that they can be excited so as to give a voltage greater than that of the supply-circuit. For example, it is possible to feed, from a no-volt circuit, a motor which, driven by belt at the same speed, produces an E.M.F. of 120 to 150 volts at its terminals. But, if the E.M.F.'s are thus unequal, the current passing between the generator and the motor, when the latter is running without load, can, instead of being inappreciable, attain a considerable value.

Likewise, when the motor is running with load, the current is greater than that which corresponds to the work to be done. The same effects are produced when, instead of giving to the motor an excessive excitation, it is given an insufficient induced E.M.F. It is then observed, if the machines are alike, that the potential difference at the terminals assumes a third value, which is the mean of the two E.M.F.'s involved.

In both cases, the greater the inequality between the two E.M.F.'s the more the current measured will increase, by the change of excitation. If we plot a diagram, taking, as abscissae, the values of the excitation of one of the machines, and, as ordinates, the current passing through the circuit, the curve of variation of the latter, as a function of the former, has the form of a V more or less rounded at the bottom (Fig. 6). This form persists, although it may be less marked, when a constant load is placed on the brake. At the same time that the current increases, by reason of an inequality of the E.M.F.'s, it can be noted, by means of an apparatus for indicating phase-difference, that the current undergoes a change of phase, either forward or backward, with respect to the E.M.F. of the motor. This can be expressed in another way by saying that the machine consumes or produces wattless current, i.e., current which is "out of phase" being pi/2 behind or ahead of the E.M.F. This "wattless" current, which has the effect of increasing the "apparent" current, is thus named because it produces no work, the load on the brake remaining constant, by hypothesis.

Effects of Wattless current and Reactance

The effect of this wattless current is, therefore, to produce, in the motor, a supplemental positive or negative E.M.F., which adds itself to its own induced E.M.F., in such a way as to produce, at the terminals, a difference of potential equal to that of the generator. We can conclude from this, without further argument, that when the motor generates an E.M.F. which is too low, the current of the generator tends to over-excite it and that, in the contrary case, it tends to under-excite it.

The action of the current on the generator itself produces inverse effects.

The effects are more complicated still when resistances or inductances are added in the circuit between the machines, with the general effect of lowering the voltage. Synchronous operation remains possible, nevertheless, even when the resistance attains high values.

When the circuit includes reactance it is observed that, by overexciting the motor, the voltage will be raised at its terminals, and even at the terminals of the generator, so as to attain values which are higher than the E.M.F. of the generator, measured with open circuit. On the contrary, by under-exciting the motor, it is possible to produce increasing and rapidly exaggerated voltage-drop along the whole line.

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