Page 59 - Industrial Power Engineering and Applications Handbook

2/40 Industrial Power Engineering and Applications Handbook
harmonics and slot losses. The angle of twist (skew) is a
matter of experience, by results obtained over the years.
The most common skew angles, for various combinations
of stator and rotor slots in practice, are given in Table 2.1.

Table 2.1 Typical angles of skew for cage rotors
Number Number Number Skew angle
of poles of stator of rotor (degrees)
slots slots
[! 14 26
16
20
2 28 16
{ 18 20
4 28 13 to 14
-
6 36 33 11 to 14 -
Speed --+
2.4 Effect of starting current on Slip N,
torque Figure 2.8 Starting (locked rotor) currents corresponding to
different starting torques
Ignoring the friction and core losses, the torque developed
in synchronous watts, due to the skin effect, will also diminish the full-load
power factor. (See the circle diagram, Figure 1.16,
corroborating this statement.) The T,, and I,, are, therefore,
a matter of compromise to achieve a good Tp, a better
I: R, power factor and a lower slip. Figure 2.9 shows for
or -3.-
S different starting torques the corresponding pull-out
torques and their occurrence of slip, maintaining the same

Since the stator current is a function of the rotor current,
the motor torque is proportional to the square of the
stator current. Generalizing,

(for
the
= [k)2 same rotor resistance R2)
TS,,
(2.3)
or = ( &)2 . 5 (for different rotor resistances)
Ts*2 R;

Analysing equation (2.2), the higher the starting torque,
the higher will be the starting current for the same motor
parameters (Figure 2.8). An attempt to keep the starting
current low and yet achieve a higher starting torque may
be feasible, but only up to a certain extent, by suitably
redesigning the rotor with a higher resistance (equation
(2.1)). However, the results of such an attempt may
adversely affect the other performance of the motor. For
example, the Tpo will be reduced due to a higher rotor
resistance and may occur at a higher slip, even if the
full-load slip is the same. The increased slot leakage, Figure 2.9 Effect of starting torque on T,, and slip