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Selecting the right starter for an electric motor is a key decision to ensure proper startup and prevent damage to the system. In this guide, we explain the difference between soft starters and direct-on-line (DOL) starters, and when each should be used.
What Happens When a Motor Starts?
When an electric motor starts, it can draw between 8 to 10 times its rated current. This inrush current lasts only a few seconds but can cause several issues in the electrical installation:
- Voltage drops that affect other connected equipment.
- Tripping of circuit protection devices.
- Overheating of conductors and components.
A soft starter helps mitigate this issue by significantly reducing inrush current. Depending on the model and configuration, it can limit the starting current to about 2 or 3 times the motor’s rated current, resulting in a much more stable start for the electrical system.
What Is a Direct-On-Line Starter (DOL)?
A direct-on-line (DOL) starter applies full line voltage directly to the motor terminals at startup. This results in an immediate start with full torque and maximum inrush current. It’s ideal for applications where:
- High inrush current is not a concern.
- The electrical infrastructure can handle current spikes.
- A basic, cost-effective starting method is acceptable.
Typical examples include: small pumps, fans, compressors, and industrial equipment where mechanical shock is not a critical issue.
What Is a Soft Starter?
A soft starter gradually increases the voltage supplied to the motor during startup, allowing for a smooth acceleration and reducing both inrush current and mechanical stress. Additionally, it provides a soft stop (deceleration ramp), avoiding abrupt halts.
Recommended for applications where:
- Mechanical systems need protection from start/stop shocks.
- Prolonged motor and equipment life is important.
- Electrical supply has limitations for peak current.
Common in variable pressure pump systems, conveyors, and equipment with gearboxes or sensitive couplings.
Key Differences
| Feature | Direct-On-Line Starter ( | Soft Starter |
| Startup type | Instant, full voltage | Smooth, controlled ramp-up |
| Inrush current | High (8–10x rated current) | Reduced (2–3x rated current) |
| Mechanical stress | High | Low |
| Cost | Lower | Higher |
| Recommended applications | Simple, rugged equipment | Sensitive or high-value systems |
| Soft stop (ramp-down) | No | Yes |
How to Know Which One You Need
The right choice depends on the load type, mechanical sensitivity, and your electrical system’s capabilities. If the application can tolerate abrupt starts and the power system is robust, a DOL starter is a cost-effective and reliable choice. If the goal is smoother operation or the installation has limitations, a soft starter is the better option.
Additionally, for motor panels or starters above 25 HP at 220V or 50 HP at 440V, soft starters are strongly recommended. In these ranges, the inrush currents with DOL starters can be extremely high and may compromise the stability of the electrical system.
Conclusion
Both soft starters and direct-on-line starters serve specific purposes depending on the application. At Nassar Electronics, we offer both solutions built with premium components, IP66-rated enclosures, and customization options including voltmeters, ammeters, and advanced voltage protection.
Moreover, all of our pump control panels can be configured with soft starters for safer, smoother motor startups—particularly beneficial for high-horsepower systems.
View our Soft Starter Panel (TAS) here
View our Direct-On-Line Starter Panel (APV) here