Page 110 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Starting torque

The torque developed by the motor at the instant if starting is called the starting torque.
Let E2 be the rotor emf per phase at standstill
X2 be the rotor reactance per phase at standstill
R2 be the rotor resistance per phase.
Therefore, Z2 = √(R2) ^2+(X2) ^2 rotor impedance per phase at standstill.
E2 R2
Then I2 = cos Ф2 =
Z2 Z2
Stand still or starting torque Tst = K1 E I cos Ф2
2 2
If the supply cottage is constant, then the flux Ф and hence E2 is constant.
Therefore, starting torque = K R Z where K is another constant
2 2 2 2
The starting torque of such a motor is increased by adding external resistance in the rotor circuit. The resistance is
progressively cut out as the motor gain speed. The resistance is progressively cut as the motor gain speed. Rotor
emf and reactance under running condition when the stator is stationary i.e. S =1 the frequency of the rotor emf
is the same as that of the stator supply frequency. The value of emf induced in the rotor at standstill is maximum
because the relative speed between the rotor and the rotating stator flux is maximum.
When the rotor starts running the relative speed between the rotor and the rotating stator flux is decrease. Hence
the rotor induced emf is also decreased. The rotor emf become zero if the rotor speed become zero if the rotor
speed become equal to the speed of stator rotating flux. Hence for a slip the rotor induced emf will be s times the
induced emf at standstill.
Under running condition Er = s E2.
The frequency of induced emf will likewise become Fr = s F2 where F2 is the rotor current frequency at stand still.

Resistance starter for 3-phase, slip-ring induction motor

Objectives: At the end of this lesson you shall be able to:
• explain the rotor resistance starters used for a 3-phase, slip-ring induction motor.

Slip ring induction motor are started with full line voltage across the stator winding. To reduce the heavy starting
current a star connected external resistance is added in the rotor circuit. The external resistances are cut out and
the rotor winding ends are shorted once the motor picks up speed. Now days semi-automatic starter is used. By
pressing the on button the contactor will close only when the shorting point A at the rotor resistance is in closed
position. This is possible only when the handle is in the start position once the motor starts running the handle of
the rotor resistance should be brought to run position to cut-out the rotor resistance.
The position of the handle clearly indicates that at the start position the contact s is in the closed position and at
the run position contact b is in the closed position but both cannot close at the same time. The on push button
needs to be held in the pushed position till the handle is brought to the run position. during the run position the
handle contact b closed the no-volt coil circuit and the pressure on the on button can be released.
If such a manual starter is used, there is a possibility that someone may apply full voltage to the stator when the
rotor resistance is in a completely cut-out position, resulting in heavy rush of the starting current and poor starting
torque. This could be eliminated by the use of a protective circuit in the resistance starter; thereby motor cannot
be started until and unless all the rotor resistances are included in the rotor winding. Such a semi-automatic
starter is shown in Fig 2.
By pressing the ‘ON’ button, the contactor will close, only when the shorting point ‘A’ at the rotor resistance is in a
closed position. This is possible only when the handle is in the start position. Once the motor starts running, the
handle of the rotor resistance should be brought to ‘run’ position to cutout the rotor resistance.

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CITS : Power - Electrician & Wireman - Lesson 70-75 CITS : Power - Electrician & Wireman - Lesson 70-75