Fig. 1,755.—Western Electric squirrel cage armature of high speed induction motor for centrifugal pump service. This armature is an example of heavy duty construction. The inductors are welded to the short circuiting end rings, the latter being located beneath the inductors, as shown. Fan vanes are provided at one end for ventilation. In the field construction, the core laminations are assembled in a closed box frame, and clamped by heavy rings while under hydraulic pressure. The stator coils are form wound and subjected to a special insulating process, which renders them especially impervious to moisture, and capable of operating without breakdown in locations which are too damp for ordinary motors. The bearing brackets are of rigid mechanical construction, and the pulley end bracket and bearings of all sizes are split to facilitate removal of the rotor and complete inspection. These machines range in size from 50 to 200 horse power, the rugged construction adapting them to heavy and severe service, such as is met with in mining, the construction of dams, canals, aqueducts, tunnels, etc.
Fig. 1,756.—Wagner squirrel cage armature for polyphase induction motor, as employed on motors of from 5 to 25 horse power. The features of construction as seen in the illustration are bar inductors, ventilating passages through the core laminæ, riveted connection between inductors and end rings ventilating vanes on end plate, extra large end rings. The object of making the rings unusually large is to make the resistance of the rings lower than is desirable for some classes of service, in order to obtain motors having minimum slip, increased efficiency, and maximum overload capacity under normal operation. When the torque required by some very unusual and entirely abnormal installation exceeds that of the average conditions, it is an easy matter to reduce the section of the end rings, by turning them down in a lathe, thereby increasing the resistance and starting torque.
Field Windings for Induction Motors.—The field windings of induction motors are almost always made to produce more than two poles in order that the speed may not be unreasonably high. This will be seen from the following:
If P be the number of pairs of poles per phase, f, the frequency, and N, the number of revolutions of the rotating field per minute, then
| 60 × f | ||
| N | = | |
| P |
Thus for a frequency of 100 and one pair of poles, N = 60 × 100 ÷ 1 = 6,000. By increasing the number of pairs of poles to 10, the frequency remaining the same, N = 60 × 100 ÷ 10 = 600. Hence, in design, by increasing the number of pairs of poles the speed of the motor is reduced.