According to Weiner, the slip varies according to the following table:

SLIP OF INDUCTION MOTORS
Capacity of motor H. P.Slip at full load per cent. Capacity of motor H. P.Slip at full load per cent.
Usual limits Average Usual limits Average
⅛ 20 to 40 30 15 5 to 11 8
¼ 10 " 30 20 20 4 " 10 7
½ 10 " 20 15 30 3 " 9 6
1 8 " 20 14 50 2 " 8 5
2 8 " 18 13 75 1 " 7 4
3 8 " 16 12 100 1 " 6 3.5
5 7 " 15 11 150 1 " 5 3
7½ 6 " 14 10 200 1 " 4 2.5
10 7 " 12 9 300 1 " 3 2

Fig. 1,736.—Sector method of measuring the slip of induction motors. A black disc having a number of white sectors (generally the same as the number of poles of the induction motor) is fastened with wax to shaft of the induction motor, and is observed through another disc having an equal number of sector shaped slits (that is a similar disc with the white sectors cut out) and attached to the shaft of a small self-starting synchronous motor, which is fitted with a revolution counter that can be thrown in or out of gear at will; then the slip (in terms of Nr) = N ÷ (Ns ÷ Nr), in which: N = number of passages of the sectors; Ns = number of sectors; Nr = number of revolutions recorded by the counter during the interval of observation. For large values of slip, the observations may be simplified by using only one sector (Ns = 1), then N will equal the slip in revolutions.

Ques. Describe one way of measuring the slip.

Ans. A simple though rough way is to observe simultaneously the speed of the armature and the frequency, calculating the slip from the data thus obtained, as on page [1,315].

This method is not accurate, as, even with the most careful readings, large errors cannot be avoided. A better way is shown in fig. 1,736.

Fig. 1,737.—Detail of Westinghouse squirrel cage armature for induction motor. This is an example of cast on construction similar to that of Morse-Fairbanks (see figs. 1,752, 1,753 and 1,915). The inductors are embedded in a special cement.