DOL Starter Component Calculator
Calculate motor current, circuit breaker, thermal overload, and contactor sizes for DOL starters.
DOL Starter Component Calculator
Accurate sizing for Direct Online Starters
Results:
Motor Full Load Current (A): 0.00
Inv. Trip Breaker Size (A): 0.00
Thermal Overload Relay Size (A): 0.00
Min. OL Relay Setting (A): 0.00
Max. OL Relay Setting (A): 0.00
Magnetic Contactor Size (A): 0.00
Making/Breaking Capacity (A): 0.00
How to Use the Calculator
Input Parameters
- Enter Motor Power in Kilowatts (kW).
- Input the system Voltage in Volts (V).
- Provide the Power Factor (PF) of the motor (typically 0.8 to 0.9).
- Select the electrical system Phase (Single Phase or Three Phase).
Calculation & Results
- Click the "Calculate" button to get results.
- Results include Motor Full Load Current, Circuit Breaker sizes (Instantaneous and Inverse Trip), Thermal Overload Relay size and settings, Magnetic Contactor size, and Making/Breaking Capacity.
- Use the "Reset" button to clear all inputs and results.
Important Notes
- Ensure accurate input values for precise calculations.
- Calculations are based on standard industry practices.
- Always verify results with local electrical codes and manufacturer specifications.
DOL Starter Component Fundamentals
Motor Current Calculation
Understanding the motor's full load current (FLC) is crucial for sizing protective devices and control components. It varies based on motor power, voltage, and phase configuration.
Circuit Breaker Sizing
- Inverse Trip: Provides overload protection, typically set at 250% of FLC.
Thermal Overload Relay
Protects the motor from sustained overcurrents. Sized at 125% of FLC, with adjustable settings (70-100% of FLC) for fine-tuning.
Magnetic Contactor
Used for switching the motor ON/OFF. AC7b type is common for motors, rated at 125% of FLC. Making/Breaking capacity is critical for handling inrush currents.
Additional Required Components:
- 1mm flexible wire for control circuit wiring.
- One ON push button and one OFF push button.
- One 2 amps single pole circuit breaker for control circuit protection.
- Optional: One selector switch for Auto/Manual operation. In auto mode, the DOL switch turns ON automatically when power is restored. In manual mode, it's operated by ON/OFF push buttons.
Calculation Formulas
Motor Current Calculation (FLC):
Single Phase: FLC = (kW × 1000) / (Voltage × Power Factor)
Three Phase: FLC = (kW × 1000) / (1.732 × Voltage × Power Factor)
Circuit Breaker Size Calculation:
Inverse Trip: 250% of FLC
Thermal Overload Relay Size:
125% of FLC
Min. Setting: 70% of FLC
Max. Setting: 100% of FLC
Magnetic Contactor Size:
125% of FLC (AC7b type)
Making/Breaking Capacity: 8 × FLC
Example Calculation
Consider a 7.5 kW, 400V, Three Phase Motor with 0.8 Power Factor:
Motor Power (kW): 7.5
Voltage (V): 400
Power Factor (PF): 0.8
Phase: Three Phase
Motor Full Load Current (FLC):
FLC = (7.5 × 1000) / (1.732 × 400 × 0.8) = 7500 / 554.24 ≈ 13.53 A
Circuit Breaker Size:
Inverse Trip: 250% of 13.53 A = 33.83 A
Thermal Overload Relay:
Size: 125% of 13.53 A = 16.91 A
Min. Setting: 70% of 13.53 A = 9.47 A
Max. Setting: 100% of 13.53 A = 13.53 A
Magnetic Contactor:
Size: 125% of 13.53 A = 16.91 A
Making/Breaking Capacity: 8 × 13.53 A = 108.24 A
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