This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1917 Excerpt: ...Fans and blowers frequently must be capable of speed adjustment. With d.c. motors, this is accomplished by field or armature rheostats or both, and with induction motors by means of resistance in the rotor circuit. Motors may be direct connected to centrifugal fans and to small propeller fans. For blowers and large ...
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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1917 Excerpt: ...Fans and blowers frequently must be capable of speed adjustment. With d.c. motors, this is accomplished by field or armature rheostats or both, and with induction motors by means of resistance in the rotor circuit. Motors may be direct connected to centrifugal fans and to small propeller fans. For blowers and large-sized fans, however, the speeds are so low that a belt or chain drive is preferable. 197. Compressors. The load on air compressors is generally intermittent. In some cases the machine is started and stopped as the demand for air changes, but generally the compressor is run continuously and an unloading valve is used to reduce the output of the compressor as required. When the compressor is started and stopped, the unloading valve is used to reduce the starting load. The starting conditions are therefore not severe in either case. Flywheels are used to equalize the load. 198. Elevators. These machines require a heavy starting torque and a constant running speed. One of the great problems in high-speed elevator work is to make accurate landings. This is very satisfactorily accomplished with d.c. motors by making them act as generators feeding into a resistance. With a.c. motors the control is not as satisfactory, because a friction brake Proportional to the cube of the speed. must be depended upon to bring the car to rest. The d.c. motors used have a heavy series winding which is employed in starting and is cut out when running, so the machine operates as a shunt motor. Slip-ring induction motors are generally used with a.c. systems. For high-speed service a two-speed slip-ring motor is used. Two separate windings are provided, one high-speed for starting and running and the other low-speed for stopping. By this means, the motor can be made to act ...
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