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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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In Primary Resistors (or rheostat) or reactors starter, Externel Resistor is Connected ______________
Externally connected through Slip Ring
between rotor winding & Slip
between Stator winding & Line
between Rotor winding & Line
between Stator winding & Line
In a primary resistor (rheostat) or reactor starter, external resistors are inserted directly between the 3-phase AC supply line and the stator windings of the induction motor. This reduced-voltage starting method limits the initial high starting current by introducing a voltage drop across the resistors. As the motor gains speed, these resistors are gradually cut out until full supply voltage is applied directly across the stator.
In a primary resistor (rheostat) or reactor starter, external resistors are inserted directly between the 3-phase AC supply line and the stator windings of the induction motor. This reduced-voltage starting method limits the initial high starting current by introducing a voltage drop across the resistors. As the motor gains speed, these resistors are gradually cut out until full supply voltage is applied directly across the stator.
Ist=x⋅Isc — Starting current with reduced voltage fraction x
Tst=x2⋅Tsc — Starting torque relation with fraction of rated voltage x
At starting, high inrush current is limited because the applied voltage per phase across the stator is Vstator=Vline−IstartRext. Since starting torque Tst∝Vstator2, reducing the stator terminal voltage reduces both starting current and starting torque simultaneously. Once the rotor accelerates to near rated speed, a short-circuiting contactor bypasses the external primary resistors, allowing the motor to operate under normal full-voltage conditions.
Used mainly for small squirrel-cage induction motors where smooth starting is required.
Provides smooth acceleration because external resistance can be reduced continuously or in steps.
Significant power loss occurs (I2R) across the primary resistors during the starting period.
Simple control circuit and smooth, stepless starting capability.
High power factor during the starting period compared to reactor starting.
High power dissipation and thermal loss in the starting resistors.
Low efficiency and significant reduction in starting torque (Tst∝V2).
Small squirrel-cage induction motors driving delicate machinery.
Applications where smooth start-up torque transition is required to avoid mechanical shocks.
| Feature | Primary Resistor Starter | Rotor Rheostat Starter |
|---|---|---|
Connection Location | Between Stator winding and AC line | Rotor circuit via slip rings |
Motor Type | Squirrel Cage & Slip Ring Motors | Slip Ring (Wound Rotor) Motors ONLY |
Starting Torque Effect | Reduces starting torque (Tst∝V2) | Increases starting torque and improves PF |
Option A and Option B refer to Rotor Resistance Starters used exclusively for Slip-Ring (Wound Rotor) Induction Motors.
Option D is incorrect as the primary resistor starter operates strictly on the stator side.
C is correct — Primary resistors are connected between the incoming AC power line and the stator winding terminals to drop voltage and limit starting current.
Remember that stator-side voltage control methods (like primary resistors and star-delta starters) reduce torque by the square of the voltage reduction factor (x2), whereas rotor resistance control increases starting torque while limiting current.