Fig. 1,719.—General Electric vertical type induction motor; sectional view showing oiling system. It is provided with ball thrust bearings and top and bottom guide bearings, and a continuous flow of oil is maintained through all the bearings by means of a pump which is made integral with the motor. The ball thrust bearings are designed to support the weight of the armature only. In cases where the armature is direct connected a flexible coupling should be used to prevent additional weight coming on the thrust bearings. In operation, when the motor starts, the oil, revolving with the pan, flows against the stationary nozzle and is forced by its velocity at a high pressure through the oil pipe into the reservoir on top. It then flows down through the ball bearing and upper guide bearing, through a slot in the armature spider into the lower guide bearing and thence into the oil pan. Thus a continuous stream of oil is delivered through all bearings.
Ques. How is slip expressed?
Ans. In terms of synchronism, that is, as a percentage of the speed of the rotating magnetic field.
The slip is obtained from the following formula:
Slip (rev. per sec.) = Sf - Sa
or, expressed as a percentage of synchronism, that is, of the synchronous speed,
| (Sf - Sa) × 100 | ||
| Slip (%) | = | |
| Sf |
where