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DOL starter use for induction motor up to
HP
HP
15 HP
50 HP
HP
A Direct-On-Line (DOL) starter is the simplest method for starting a 3-phase induction motor, connecting the motor terminals directly to the supply line. Due to the high inrush current (typically 5 to 8 times the full load current), it is typically restricted to motors with power ratings up to 5 HP (approx 3.7 kW) to prevent excessive voltage dip in the supply system.
A Direct-On-Line (DOL) starter is the simplest method for starting a 3-phase induction motor, connecting the motor terminals directly to the supply line. Due to the high inrush current (typically 5 to 8 times the full load current), it is typically restricted to motors with power ratings up to 5 HP (approx 3.7 kW) to prevent excessive voltage dip in the supply system.
IstтАЛ=(5┬аto┬а8)├ЧIflтАЛ тАФ represents the starting current magnitude relative to full load current
TstтАЛтИЭV2 тАФ represents the starting torque relationship with supply voltage
When the start button is pressed, the contactor coil is energized, closing the main power contacts and connecting the motor directly to the supply. Because no resistance or reduced voltage is inserted into the circuit during start-up, the motor develops maximum starting torque immediately. However, this causes a significant surge in current, which can damage smaller supply networks if the motor size exceeds certain limits.
Used for small capacity motors where the starting current does not disturb the power system voltage stability.
DOL starters have the simplest circuit design and lowest cost.
The starting current is uncontrolled, leading to potential mechanical stress.
Commonly used in household pumps, fans, and small workshop machinery.
Simplest and most economical starting method
Provides high starting torque
Easy to install and maintain
High inrush current causes voltage drops in the supply lines
Not suitable for large motors due to heavy thermal stress
High mechanical stress on the motor and coupled loads
Small centrifugal pumps
Domestic fans
Small workshop lathes
Conveyor belts with light loads
Standard industrial practice often limits DOL usage to 5 HP (3.7 kW) for 415V supply.
Above this limit, Star-Delta or Soft Starters are preferred to reduce current magnitude.
Option B (10 HP), C (15 HP), and D (50 HP) represent limits for Star-Delta or other reduced-voltage starters.
A is correct тАФ DOL starters are primarily used for low-capacity induction motors up to 5 HP to minimize the impact of high starting current on the electrical network.
Always remember that for larger motors, we shift to Star-Delta starters to reduce starting current to 1/3 of the DOL value.