Notes: Laboratory of Dynamo-electric Machinery for the Use of Students in the Lowell Institute School for Industrial Foremen, Massachusetts Institute of Technology

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Ranger Company, 1914 - Electric engineering - 126 pages
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Page 25 - ... speed as on a level. The series motor, on the other hand, exerts good torque at starting without excessive current, and on grades slows down and does its work at a moderate speed and current. This particular property of a series motor is due to the fact that the flux is a function of the load current. The speed of a motor is inversely proportional to its field strength and nearly proportional to the voltage at the terminals of its armature. The torque or turning moment exerted by the armature...
Page 37 - ... important part of any acceptance test. A motor or generator may be perfectly satisfactory in so far as efficiency, regulation, etc., are concerned, but may be absolutely incapable of standing continuous service at the load for which it is designed on account of the excessive temperatures reached by its parts.
Page 49 - Law, /a/?a represents the drop of potential through the armature due to the armature current, /,, flowing through the armature resistance, Ra. The electromotive force generated by the movement of a conductor across a magnetic field is proportional to the strength of the field and to the speed at which the conductor moves. Since the armature of a motor consists simply of a series of conductors revolving in a magnetic field, it follows that the back electromotive force set up in it as it revolves,...
Page 14 - The retention of unnecessary figures is not only misleading, but absolutely misrepresents the accuracy of any piece of work. A good general rule for ordinary engineering work is to reject all figures in final results beyond the first place in which variation occurs. For example, suppose a current of 50 amperes is to be read on an ammeter having one hundred divisions, each division representing one-half an ampere. If the conditions of the circuit are sufficiently constant to permit reading tenths...
Page 58 - This was a spermaceti (whale oil) candle weighing one-sixth of a pound, and burning at the rate of 120 grains per hour.
Page 14 - ... amperes. In the mean of a considerable number of single observations, one more figure should always be retained than is kept in the single observation. If two or more quantities having different accuracies are multiplied or divided, the number of figures retained in the result should be determined by the least accurate of the two quantities. For example: If the input to a motor is given as 51 amperes at 110.3 volts, the calculated input in watts should be written 5600 watts and not 5625.3 watts....
Page 49 - Hence, the motor electromotive force is known as the counter electromotive force of the motor. Then the net voltage acting on the armature of a shunt motor is equal to the difference between the applied voltage and the counter electromotive force. The counter electromotive force of a motor can be found by running it as a generator at rated speed and measuring the electromotive force developed with a voltmeter. Example 1. The armature resistance of a shunt motor is 0.05 ohm, the armature current...
Page 60 - Fig. 97 that the intensity of illumination varies inversely as the square of the distance of the surface from the light source. Thus E = CP/D...
Page 52 - TT-= = 12.2 per cent., llu % as compared with 45 per cent. by the previous method. Aside from the better speed regulation this last method involves no waste of energy in resistance, as does the former. If this last method is used, precautions must be taken to prevent injury to the field when its circuit is opened. If the machine is small there will be little danger. If it is large, however, a high resistance shunt should be placed about the field terminals to take the discharge from the magnetic...
Page 27 - The speed regulation, however, cannot be so good as that of the shunt motor; nor can the starting torque be so good as that of the series motor. The good qualities of each type of motor must be sacrificed to a very considerable degree in order that the motor, to a limited extent, may possess the qualities of both. A motor with a compound winding connected so as to aid its shunt field is called a cumulatively compounded motor.

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