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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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Current controller
Current controller
Fig. 51 shows, for example, the diagram of the starting apparatus of a certain type of three-phase motors ranging from 1 to 22 H.P. When the motor is at rest, the pulley is on the idler A, the triple-switch C and the excitation circuit-breaker D are open. To start the motor the triple-pole switch C is closed on the first steps, and the motor then receives a current from the supply-transformer through a rheostat which limits the amount of current. The motor starts and attains synchronous speed in from 20 to 30 seconds. The field-excitation circuit is then closed, and the counterweight of the circuit-breaker is lifted, which enables the triple-pole switch to be brought to the second position. The motor then receives the current directly, and it attains full speed. The belt can then be transferred gradually from the idler pulley to the driving pulley B.
To stop the motor we proceed in inverse order. The belt is shifted to the idler pulley A, the counterweight is. dropped (which causes the three-pole switch to open), and finally, the circuit-breaker D of the field-excitation circuit is opened.
For motors of 25 to 50 H.P. the single resistance is replaced by a variable rheostat and the switches have but one step. Once the speed is attained the rheostats are short-circuited. For these large motors, separate exciters are generally used, as already seen (Fig. 52).
The operation of starting without load does not require more than 20 to 30 seconds, and it is effected with a current not exceeding the normal full load current. It will be observed that the process of starting involves the use of high-tension switches on the primary circuit, to isolate the transformer; also fuses on both the primary and secondary circuit.
The belt is shifted from the idler to the driving pulley in the ordinary way by a belt-shifter. Fig. 53 shows the manner in which this shifter should be placed to correspond properly with the direction of rotation and the direction of driving, for an observer placed at the commutator end. Figures A and B refer to the case where the direction of rotation (indicated by the arrows) is the same as the hands of a watch ; and Figs. C and D refer to the case where the rotation is in the opposite direction.
The use of a reactance-coil instead of a rheostat has the objection of increasing the lag of the current taken from the supply-source, which current already has too much lag. It is therefore preferable to replace it by an auto-transformer which enables the current of the motor to be increased at low voltage while reducing the amount of current taken from the line. Fig. 54 shows an example of this kind of arrangement which is in extensive use. The diagram represents the simplest case, i.e., that of a single-phase alternating current machine. It is a transformer with a single winding having taps connected to contacts, over which moves the switch b. The two ends, A, B, of the winding are connected to the source of current-supply, while the motor is connected to the end B and the switch b.
