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Transcript
Action of the Commutator Action of the Commutator We have seen that the e.m.f. induced in the simple coil rotating in a magnetic field is an alternating e.m.f. Hence if the two ends of the coil are connected to insulated slip rings mounted on the shaft, and the external circuit is connected to brushes which press on these slip rings, the electrical polarity of each ring will alternate as the coil rotates, so that the current set up will also be alternating. In order that the current may be unidirectional, it is necessary for one brush always to be positive and the other always negative. In other words, the negative parts of the induced voltage wave must be rectified. This is done by using one ring which is split into two halves, the halves insulated from one another, and each connected to one end of the coil. In this way the positive end of the coil is always under the same brush, because, as the polarity of one end is about to reverse that end passes to the next brush. Similarly, the negative end of the Coil is always under the other brush. The voltage across the brushes therefore always acts in the same direction, and the current produced in the external circuit always flows in one direction. This current will obviously pulsate with the variations in magnitude of the rectified voltage curve, so that, although it will be unidirectional, it can hardly be termed a continuous current. It must be remembered that this rectification of the current and voltage takes place in the external circuit only. The current and voltage in the rotating coil remain alternating We have seen that the e.m.f. induced in the simple coil rotating in a magnetic field is an alternating e.m.f. Hence if the two ends of the coil are connected to insulated slip rings mounted on the shaft, and the external circuit is connected to brushes which press on these slip rings, the electrical polarity of each ring will alternate as the coil rotates, so that the current set up will also be alternating. In order that the current may be unidirectional, it is necessary for one brush always to be positive and the other always negative. In other words, the negative parts of the induced voltage wave must be rectified. This is done by using one ring which is split into two halves, the halves insulated from one another, and each connected to one end of the coil. In this way the positive end of the coil is always under the same brush, because, as the polarity of one end is about to reverse that end passes to the next brush. Similarly, the negative end of the Coil is always under the other brush. The voltage across the brushes therefore always acts in the same direction, and the current produced in the external circuit always flows in one direction. This current will obviously pulsate with the variations in magnitude of the rectified voltage curve, so that, although it will be unidirectional, it can hardly be termed a continuous current. It must be remembered that this rectification of the current and voltage takes place in the external circuit only. The current and voltage in the rotating coil remain alternating М.А. Беляева и др. «Сборник технических текстов на англ. языке» М.А. Беляева и др. «Сборник технических текстов на англ. языке»