Fig. 2,874.—Sectional view of General Electric vertical synchronous converter. In this construction, the field frame carrying the poles is mounted on cast iron pedestals and is split vertically. This allows the two halves of the frame to be separated for inspection or repairs of the armature. The armature, including commutator and collector rings, is mounted on a vertical stationary shaft, which is rigidly supported from the foundation. The thrust of the armature is carried on a roller bearing attached to the top of the shaft and upper side of the armature spider. The under side of the lower plate of the roller bearing is made spherical and fits into a corresponding spherical cup on the end of the shaft, making the bearing self aligning. The armature spider has a babbitted sleeve along the fit of the vertical shaft, which acts as a guide bearing and has to take only the thrust due to the unbalancing effect of the rotating parts. A circulating pump furnishes oil to the roller bearing, the oil draining off through the guide bearing. A marked advantage of this type of construction is the accessibility of the commutator for adjustment of the brushes, etc., as there is no pit or pedestal bearing to interfere.
NOTE.—Some converters are provided with a small induction motor for starting mounted on an iron bracket cast in the converter frame, and whose shaft is keyed to that of the converter. Allowing for a certain amount of slip in the induction motor, the field of this machine must possess a less number of magnet poles than the converter in order to enable the latter machine to be brought to full synchronism. To start the induction motor, it is simply necessary to apply to its field terminals the proper alternating voltage. The bracket, and therefore the motor, is usually mounted outside the armature bearing on the collector side of the converter.
Fig. 2,875.—Resistance measurement by "drop" method. The circuit whose resistance is to be measured, is connected in series with an ammeter and an adjustable resistance to vary the flow of current. A voltmeter is connected directly across the terminals of the resistance to be measured, as shown in the figure. According to Ohm's law I = E ÷ R, from which, R = E ÷ I. If then the current flowing in the circuit through the unknown resistance be measured, and also the drop or difference of pressure, the resistance can be calculated by above formula. In order to secure accurate determination of the resistance such value of current must be used as will give large deflections of the needle on the instruments employed. A number of independent readings should be taken with some variation of the current and necessarily a corresponding variation in voltage. The resistance should then be figured from each set of readings and the average of all readings taken for the correct resistance. Great care must be taken, however, in the readings, and the instruments must be fairly accurate. For example, suppose that the combined instrument error and the error of the reading in the voltmeter should be 1 per cent., the reading being high, while the corresponding error of the ammeter is 1 per cent. low. This would cause an error of approximately 2 per cent. in the reading of the resistance. In making careful measurements of the resistance, it is also necessary to determine the temperature of the resistance being measured, as the resistance of copper increases approximately .4 of 1 per cent. for each degree rise in temperature. Use is made of this fact for determining the increase in temperature of a piece of apparatus when operating under load. The resistance of the apparatus at some known temperature is measured, this being called the cold resistance of the apparatus. At the end of the temperature test the hot resistance is taken. Assume the resistance has increased by 15 per cent. This would indicate a rise in temperature of 37½ degrees above the original or cold temperature of the apparatus. Suppose then that in measuring the cold resistance, results are obtained which are 2 per cent. low, and that in measuring the hot resistance, there be 2 per cent. error in the opposite direction. This would mean that a total error of 4 per cent. had been made in the difference between the hot and cold resistances, or an error of 10 degrees. The correct rise in temperature is, therefore, about 27½ instead of 37½ degrees. In other words, an error of 2 per cent. in measuring each resistance has caused an error of approximately 36½ per cent. in the measurement of the rise in temperature. The constant .4 which has been used above is only approximate and should not be used for exact work. For detail instructions of making calculations of resistance and temperature, see "Standardization Rules of the A.I.E.E."
Ques. Describe the usual wiring for the installation of a rotary converter in a sub-station.
Ans. Commencing at the entrance of the high pressure cables, first there is the wiring for the lightning arresters, then for the connection in circuit of the high tension switching devices, from which the conductors are led to bus bars, and thence to the step down transformers.