Methods for starting of squirrel cage induction motors are by
(a) Primary resistors (or rheostat) or reactors.
(b) Auto-transformers (or autostarter).
(c) Star-delta switches.
In above methods, terminal voltage of squirrel-cage motor is reduced during starting.
a) By using primary resistors, applied voltage can be reduced by a fraction ‘x’
and torque by n2 because resistor decreases applied voltage which reduces
initial current and current varies directly as voltage. Torque varies as square of
applied voltage.
Auto-transformers
This method can be used for both star and delta connected motors with starting
connections. A reduced voltage is applied across motor terminals. When motor
has run upto say, 80% of its normal speed, connections are so changed that autotransformers are cut out and full supply voltage applied across motor. The
switch taking these adjustments from start to run may be air break (for small
motors) or oil immersed (for large motors) to reduce sparking
Star-delta starter
This method used where motors are built to run normally with a deltaconnected
wdg. It consists of a 2 way switch which connects motor in star for
starting and then in delta for normal running. When star-connected, applied
voltage over each motor phase is reduced by a factor 1/3 and hence torque
developed becomes 1/3 of that which would have been developed if motor
were directly connected in . The line current reduced to 1/3. Hence during
starting period of Y connected it takes 1/3rd as much starting current and
develops 1/3rd as much torque as would have been developed, were it directly
connected in delta.
Starting of slip-ring Motors
The controlling resistance is in the form of rheostat, connected in star, the
resistance being gradually cut-out of rotor circuit, as motor gathers speed. By
increasing rotor resistance not only rotor current is reduced at starting but at
the same time starting torque is also increased due to improvement in p.f. The
controlling rheostat is either of stud or contactor type and may be handoperated
or automatic.
Introduction of additional external resistance in rotor circuit enables a slip-ring
motor to develop a high starting torque with reasonably moderate starting current. Hence such motors can be started with load. This additional resistance
is for starting purpose only. It is generally cut out as motor comes up to speed.
gregorymasila answered the question on February 22, 2018 at 16:48
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