Page 92 - Industrial Power Engineering and Applications Handbook

Starting of squirrel cage induction motors 4/73

R Y B N
T T T I

m
I
G

I Minimum \ \ I
M
U
8

I Load torque I \\I
Speed -

60%
O/COn Off
trip
Note Current jumps to almost DOL current even at around 60%
speed, if switched to A position.

(a) Torque-current characteristics on a VA starting

Motor windings in A
Figure 4.3 Star-delta starting (b) Power and control circuit diagram

3 This type of switching requires six cable leads to the vulnerable as a result of greatly reduced torque and
motor as against three for other types of switching to necessitates a proper selection of motor.
accomplish the changeover of motor windings from Y
to A.
To determine the tapping of the autotransformer
Ratings of contactors Consider an autotransformer with a tapping at 40%. Then
by equating the powers of the primary and the secondary
Since all the contactors now fall in phase, they may be sides of the autotransformer (Figure 4.4)
rated for the phase current only, i.e. IrIa 58% of&
The star contactor is in the circuit only for a short period
and is generally chosen a size lower than the line and A
conlactors. or I,, = (0.4)' . V, IZ
For power and control circuit diagrams refer to Figure
4.3( b) . while the starting current on DOL:
I,,, = 2:3 . V,IZ
7
4.2.2 Autotransformer (AIT) starting
... IAT = (0.4)' . IDOL
For smoother acceleration and to achieve a still lower
starting current than above, this type of switching, although i.e. proportional to the square of the tapping, where
more expensive, may be employed. In this case also the IAT = starting current on an autotransformer switching
starting current and the torque are reduced in a square IDOL = starting current on a DOL switching
proportion of the tapping of the autotransformer. The Z = impedance of the motor windings referred to the
normal tappings of an autotransformer are 40%, 60% stator side per phase
and 80%. At 40% tapping the starting current and the
starting torque will be only 16% that of DOL values. At Generalizing the above equation, autotransformer tapping
40% tapping, therefore, the switching becomes highly for a particular starting current, IAT is