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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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Chapter II: Operation of synchronous motor
Synchronous motors are not self-starting, the same as other electric motors, but they have to be started by some artificial means such as described hereinafter.
Starting by Direct Current
The simplest way, when there is a source of direct current and the synchronous motors are provided with a commutator for producing direct current for self-excitation, is to start the machine as if it wrere a D.C. motor, with the armature and fields connected in parallel. This plan can be especially adopted in transformer stations using motor-transformers, so as to start a set of motors operating dynamos as soon as one has been put in operation.
Starting with Alternating Current by Polyphase Motors
When there is no D.C. source, the alternating currents themselves must be used for starting the motor, either by mounting on the shaft of the machine a small induction-motor which serves to run it without load, or, more frequently, by using the machine itself as an induction-motor. (When a small motor is used its capacity should be about 10 per cent of that of the machine. It can be even less, if it can withstand overloading for a few seconds.)
When using the machine itself as an induction-motor, the first step is to suppress the regular excitation, which would prevent the armature from moving. The second step is to connect the armature with the source of alternating supply, taking care to lower the voltage sufficiently to prevent excessive current through the armature. The armature then behaves as if it were the primary winding of a revolving field machine in which the fields, and more especially the pole-pieces, play the role of secondary circuit. To obtain a torque equal to a quarter the normal torque, it usually requires, by this method, a current at least double the normal current. To complete this action, it is well to short-circuit the field-winding. Certain manufacturers have, for this purpose, arranged in the pole-pieces, as shown in Fig. 50, a series of slots containing windings or bars of copper, connected together like the armature of an induction-motor. In that case, the motor starts normally, and with a current which depends on the resistance. In any case this current is higher than in an induction-motor because the air-gap is greater and requires a stronger magnetizing current. For this reason, it is well to start the motors without load, in order to avoid excessive starting currents.
The production of the revolving field results from the eddy currents, but in the absence of these, the polepieces, even when laminated, give rise to hysteresis effects which are often sufficient for starting the motor without load. In certain motors having laminated poles the starting current is about double the normal full-load current.
This effect is explained by the fact that, during the rotation of the field, there is a stronger attraction in the direction of motion than in the opposite direction owing to the lag of the magnetization behind the changes of field producing it.
Synchronism
In proportion as the armature-speed increases, the magnetic pulsations produced by it in the pole-pieces become less numerous, as can be seen on connecting a lamp to the field-circuit and noting its variations of brightness. It is well to provide a centrifugal regulator which connects this lamp in circuit only when the speed approaches synchronism, because, at lower speeds, it would be subjected to excessive voltage. When synchronism is almost attained (the speed generally remains slightly lower,) the fields are excited when the phases come into opposition, the motor falls into step, and the current immediately diminishes, owing to the disappearance of the reactive current which was absorbed up to that time. The more carefully the time has been selected for closing the excitation-circuit, the more easily the motor will fall into step. It is well, as a rule, to include, in the circuit, a variable self-inductance, which has the effect of preventing excess of current and of damping objectionable harmonics.
When once the motor is in synchronism, it can be loaded progressively, by shifting the belt from the idler to the driving pulley.