Page 58 - Industrial Power Engineering and Applications Handbook

Motor torque, load torque and selection of motors 2/39
Leakage
Leakage
fl, I“

(a) Deep bar (b) Taper bar (c) Double cage
Figure 2.6 Different types of rotor slots, making use of skin
effect
Speed -

Nr
Figure 2.5 Speed-torque characteristics of a double squirrel
cage motor

Performance
In such motors the pull-out torque is normally less than
the starting torque. This is because the pull-out torques
by the two cages occur at different speeds. Such motors
would possess a low power factor and efficiency compared
to an ordinary squirrel cage motor, because of the high Figure 2.7 Other designs of a few double cage slots
leakage reactance of inner cage and comparatively higher
12R losses. Such motors would have a slightly higher slip
than an ordinary squirrel cage motor due to higher rotor in inductive reactance (S.ssX2), which in an induction
resistance. motor varies with rotor frequency (i.e. speed). This effect
of change of resistance is termed the ‘skin effect’. For
Limitations more details, see Section 28.7. To make use of this effect,
the slot, irrespective of its configuration, may be made
During start-up since only the outer cage is in the circuit deep to create higher eddy currents and correspondingly
with a very high current, the motor is heated up quickly higher eddy current losses, to add to the effective resistance
by every start and may not be suitable for frequent starts of the rotor during start-up and to diminish this with
and reversals. speed. (See also Section 2.4.) In this way the depth, in
There are several other designs available to achieve a deep bars, and depth and taper, in tapered bars, can be
considerably high staring torque and yet overcome the varied to achieve the desired performance. For the same
above limitation. It is possible by employing a deep cage, torque characteristics either of these types of cages can
tapered cage or special types of rotor materials such as he employed which, for one characteri\tic, will require
brass and selenium to increase the starting resistance of the same area of cross-section but the depth will vary
the rotor circuit, and hence the starting torque. These depending upon the type. The deep bars will be deeper
methods are discussed briefly below. than a taper bar. Moreover, the taper slot will have a
better grip for rotor conductors during a run than a deep
2.3.2 Other designs of rotor cage parallel bar and also better cooling properties.

Use of skin effect Angle of skew in squirrel cage rotors
The basic concept used in the design and selection of The movement of rotor teeth around the stator produces
other types of rotors to provide better starting a clogging effect, resulting into vibrations and noise. To
characteristics is the high rotor resistance during start- reduce this effect, the common practice is not to provide
up. Other than the double cage rotors, this can also he the rotor slots parallel to the shaft axis but at an angle.
achieved by making deep or taper rotor bars as shown in This practice is known as ‘rotor bar skew’. A proper
Figure 2.6 (See also Figure 2.7). At different frequencies, skewing can also improve the starting torque and reduce
the rotor has different effective resistances, due to a change the starting current, in addition to the effects of space