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• Synchronous motors: An introduction
• Chapter I: Synchronous motors General Principles
• Efficiency Synchronous Motors and Experimental Properties
• Stalling of a Synchronous Motor
• Over-excited synchronous motor
• Necessity of synchronism and stability of synchronous operation
• Explanation of Single-Phase Synchronous Motors
• Equations of Synchronous Motors; Analytical Theory
• Symmetrical Polyphase Motors
• Equation of the Synchronous Motor by the Method of Complex Variables
• Excitation of Synchronous Motors
• Shunt-excitation
• Chapter II: Operation of synchronous motor
• Installation of Synchronous Motors
• Current controller
• Starting of Single-Phase Machine
• Starting of Machine with Laminated Field Poles

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Shunt-excitation

Shunt-excitation

Shunt-excitation has the advantage of being constant and easily regulated; but it has another objection, that of necessitating change of position of the diameter of commutation with respect to the phase of the current to be commutated. With normal load, the position of the brushes can be easily regulated in such a way that the commutation may occur exactly when the alternating current to be commutated is passing through the zero-point; but if the load increases, the lagging of the armature behind the field increases, without being followed by the phase of the current to be commutated. The same thing occurs during any accidental oscillation of the speed of the armature (rotor).

Now, as Fig. 20 shows, the mean strength of the commutated current is proportional to the algebraical sum of two areas, one of which (S₁) is positive, and the other (S₂) is negative. The former diminishes and the latter increases, when the lag increases, the effect being a rapid decrease in the excitation. If we let t0 = time at which the commutation occurs; if T=the period, and 2π^t0/_T = the lag in question, the exciting ampere-turns will be given by the following equation: i.e., they are reduced in the proportion indicated by the factor cos (2π^t0/_T) as compared with their value when the commutation occurs at the proper moment of passing through zero (at which time the given cosinus factor equals unity). It is necessary, therefore, to vary the position of the brushes with the load. This cannot be done, however, during the fluctuations of load, and it is conceivable that this decrease of excitation under the influence of the lag may then greatly reduce the stability of operation. It appears, therefore, that the method of excitation by commutated currents is a process open to criticism.

Advantage might be taken of the use of the transformer in Fig. 21 to compound the excitation in such a way as to cause the inducing flux to increase with the load, and thus produce an increase in stability. It would be sufficient, to this end, to wind on the same transformer a second primary coil with a large conductor through which the armature or rotor current passes.

Armature winding plus Continuous-current commutator

Another method of commutating the current, which is preferable to the preceding one, consists in combining with the armature winding a continuous-current commutator from which the exciting current is collected by means of two brushes. Such is the system of the Fort Wayne Company in America, and of the Societe l'Eclairage Electrique (Labour system) in France. The commutator need be only of small dimensions, proportional to the current which it delivers, unless it is also used in starting the motor. When the voltage of the armature is too high or when it is desirable to simplify the connections of the main circuit, it is sufficient to wind, on the armature, a small exciter winding connecting with a small commutator, which serves solely for the purpose of excitation. This method of excitation, which is the simplest of all, unfortunately still presents several objections. It is applicable only to motors having continuous current windings and a rotating armature; it cannot be adopted in the case of high voltage machines or alternators having a stationary induced winding; it does not lend itself to automatic regulation of the inducing field; finally, (and this is the most serious objection), when there are wattless demagnetizing currents in the motor, from any cause, the E.M.F. at the brushes and the excitation are both weakened in consequence. Now, that is precisely what occurs when the motor experiences a reduction in speed in consequence of an overload. The stability of operation, which decreases with the excitation, will, therefore, be diminished when the load increases. The oscillations due to improper operation are therefore exaggerated by this reaction.

The self-excitation of synchronous motors must, therefore, be studied with care, and its application is limited to small motors. For larger machines the use of a separate exciter is the most satisfactory plan.