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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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DOL starter use for
phase Induction motor
DC series motor
DC Shunt Motor
All of above
phase Induction motor
A Direct-On-Line (DOL) starter is the simplest form of motor starter used to connect an induction motor directly to the supply line. It is suitable for small induction motors (typically up to 5 HP) where the high starting current does not cause significant voltage dips in the supply network.
A Direct-On-Line (DOL) starter is the simplest form of motor starter used to connect an induction motor directly to the supply line. It is suitable for small induction motors (typically up to 5 HP) where the high starting current does not cause significant voltage dips in the supply network.
Ist=(5 to 8)×Ifl — where Ist is starting current and Ifl is full load current
τst=(1.5 to 2.5)×τfl — where τst is starting torque
When the start button is pressed, the contactor coil is energized, closing the main contacts and applying full line voltage across the motor terminals. The motor draws a high inrush current (typically 5 to 8 times the full load current) to produce the necessary starting torque. The thermal overload relay integrated within the starter provides protection against excessive current heating during operation.
Used primarily for 3-phase squirrel cage induction motors.
Provides no protection against low starting torque, hence suitable for low-inertia loads.
The primary limitation is the high inrush current, which can destabilize the supply voltage.
Consists of a magnetic contactor and an overload relay.
Most economical and simplest starting method
Easy to install and maintain
Provides high starting torque
High inrush current can cause voltage fluctuations
Not suitable for high-capacity motors
High mechanical stress on the motor shaft due to abrupt starting
Small compressors and fans
Conveyor belts
Small centrifugal pumps
For motors above 5 HP, Star-Delta or Soft Starters are preferred to mitigate the inrush current.
Option B (DC series) and C (DC Shunt) use external resistance (rheostat) starters because DC motors have very low internal resistance and would draw massive current without current limiting.
A is correct — A DOL starter is specifically designed to connect 3-phase induction motors directly to the power supply.
Always remember that for motors above 5-7.5 HP, Indian electrical regulations (IE Rules) usually mandate starters that reduce starting current to prevent grid instability.