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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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Installation of Synchronous Motors
In the particular case where it is possible to start the generator and the motor at the same time, the operation of starting becomes easier and there is no necessity of suppressing the excitation of the motor. The motor starts of itself by giving it a slight impulse, it falls into step at the very first period, owing to the low reactance at low speed, and its motion then accelerates synchronously with that of the generator, increasing gradually to full speed.
Observations on the E.M.F. Induced in the Poles
During the starting period, the fields are the seat of an important alternating flux whose frequency attains, at first, the same frequency as the alternating currents themselves. The result is the production of a very high E.M.F. in the field-winding, which is generally made of fine wire. If, therefore, this winding remains on open circuit, its insulation should be particulary good, and contact should not be made with its terminals. It is preferable to short-circuit this winding. This alternating E.M.F. naturally vanishes when synchronism is attained. It is obvious that the armature-brushes on the D.C. side must also be disconnected, since, otherwise, the pulsating current would produce a braking effect which would prevent the motor from starting.
Accessory Starting Apparatus. Installation of Synchronous Motors
The starting process involves the use of two kinds of accessory apparatus, i.e., phase-indicators and current-controllers.
It is also well to provide fuses and automatic circuit-breakers which break the main circuit whenever there is no excitation.
Phase-Indicators
Phase-indicators play the same role in this case as in coupling alternators in parallel. They are of the same kind and are connected in the same way. Their use is recommended for large-size motors.
When a synchronous motor is started by a direct current or by an auxiliary motor, a lamp of higher voltage than that of the supply-source may be connected in series with the motor. Owing to its high resistance, this lamp burns feebly at first under the action of the E.M.F. of the source of supply. In proportion as the motor increases in speed, if the field-winding is connected with a D.C. source, the E.M.F. which acts on the lamp, being the resultant of the generator and motor E.M.F.'s, will produce beats or pulsations which will gradually become less frequent. The proper time to connect the armature directly to the circuit is when the beats have become extremely slow, and when the lamp goes out, thereby indicating that the E.M.F.'s are in opposition.
When the motor is started without field-excitation, as an induction motor, this method is no longer applicable and the phase-lamp (or else several lamps in series) must be connected to the terminals of the fieldcircuit, which is then subjected to the inductive action of the variable flux of the armature. The lamp burns brightly at first, then its bright ness diminishes gradually, and the extinction of the lamp shows that synchronism is approaching; the direction which the E.M.F. will have when the circuit is closed is not determined, but this is generally of very slight importance.
With a well-constructed synchronous motor there is no need of a phaseindicator, because, when once it has attained its highest speed as an induction-motor (which may sometimes require several minutes), the powerful reactions which occur when the excitation-circuit is closed are sufficient to insure the machine falling into step.
Current-controllers
It is necessary, in order to reduce the amount of current taken from the line during the starting period, to employ a current-controller, owing to the absence of counter E.M.F. in the armature. Use may be made for this purpose of a simple rheostat or a choke-coil. Rheostats are more frequently used, on account of their simplicity. Switching appliances are used for the purpose of connecting them into circuit and then short-circuiting them, an armature circuit-breaker being interposed to break the circuit in case the field-excitation fails.