A motor doesn’t need to slam from 0 to full speed every time it starts.
For many industrial applications, the choice between a Direct-On-Line (DOL) starter and a Soft Starter comes down to starting current, mechanical stress, application requirements and cost.
A DOL starter is simple, economical and perfectly suitable for many motors.
But where starting current, mechanical shock or controlled acceleration is a concern, a Soft Starter can provide a much smoother start.
The important question is:
Which starting method is right for your application?
A Direct-On-Line (DOL) starter connects the motor directly to the supply when the motor starts.
The motor therefore receives full supply voltage immediately and accelerates as quickly as its load and motor characteristics allow.
DOL starting is one of the simplest and most economical methods of starting an electric motor.
Advantages of DOL Starting
- Simple electrical design
- Low installation cost
- Straightforward operation and maintenance
- Suitable for many smaller motors and uncomplicated applications
- Full starting torque is available immediately
The downside
The simplicity of DOL starting comes with a trade-off.
A motor can draw several times its normal running current during a DOL start. The sudden application of full voltage can also produce mechanical shock through couplings, belts, gearboxes, pumps and other driven equipment.
For some installations, that is perfectly acceptable.
For others, it can become a problem.
A Soft Starter controls the voltage supplied to the motor during starting, allowing the motor to accelerate progressively rather than receiving full voltage instantly.
This produces a much smoother starting process and can reduce the electrical and mechanical impact associated with starting.
Soft Starters are commonly used on applications such as:
- Pumps
- Fans
- Conveyors
- Compressors
- Blowers
- Other fixed-speed industrial machinery
A Soft Starter is particularly useful where the motor normally operates at one fixed speed, but the starting process needs to be controlled
| Feature | DOL Starter | Soft Starter |
|---|---|---|
| Starting method | Full voltage applied immediately | Controlled voltage ramp |
| Starting current | High | Reduced |
| Mechanical Shock | Higher | Lower |
| Starting acceleration | Very rapid | Controlled |
| Motor Speed Control | No | No |
| Installation complexity | Low | Higher |
| Equipment cost | Lower | Higher |
| Best suited for | Simple fixed-speed applications | Fixed-speed applications requiring controlled starting |
Neither method is automatically better.
The correct choice depends on the motor, driven equipment, starting conditions and installation.
A DOL starter can be an excellent solution when:
- The motor is relatively small
- Starting current is acceptable for the electrical installation
- Mechanical shock is not a significant concern
- The driven equipment can tolerate the rapid acceleration
- Low equipment cost and simplicity are priorities
For a straightforward fixed-speed motor application, there is often no reason to make the system more complicated than necessary.
Sometimes DOL is simply the right answer.
A Soft Starter becomes more attractive when the starting process itself creates problems.
Examples include:
Pumps
Controlled acceleration can help reduce pressure surges and water hammer in suitable pumping applications.
Conveyors
A smoother start can reduce mechanical shock through belts, gearboxes and couplings.
Fans and blowers
A controlled acceleration can reduce the impact of starting larger rotating equipment.
Compressors
A Soft Starter can provide controlled motor acceleration where a full-voltage start is undesirable.
In these situations, the additional cost of a Soft Starter can be justified by reducing starting stress and improving the overall starting behaviour of the system.
This is where the choice often gets confused.
A Soft Starter controls the starting and stopping process.
A Variable Speed Drive (VSD) controls the motor’s speed during operation.
A Soft Starter is therefore generally the better fit when:
The motor needs to run at one fixed speed, but you want a controlled start and/or stop.
A VSD becomes the better choice when:
You need to vary the motor speed during normal operation.
A Soft Starter is not simply a cheaper VSD. They solve different control problems.
A simple way to approach the decision is:
The motor and electrical installation can comfortably handle direct-on-line starting, and simplicity and low cost are the main priorities.
The motor will operate at a fixed speed but you need to reduce starting current, mechanical shock or starting-related problems.
You need to control the motor’s operating speed, process flow, pressure or torque after startup.
The right solution is therefore not necessarily the most sophisticated one.
It is the one that matches the application.
Need Help Choosing the Right Motor Starting Solution?
Choosing between DOL, Soft Starter and VSD depends on more than motor kW alone.
Starting current, load characteristics, mechanical equipment, operating conditions and the required level of control all need to be considered.
IODrive Systems supplies Variable Speed Drives, Soft Starters and related motor-control equipment for industrial applications
Need Help Selecting the Right Solution for your Motor ?
Contact IODrive Systems and let’s look at the application.
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Still deciding between a Soft Starter and a VSD? See our guide to VSD vs Soft Starters to understand which technology is better suited to your application.
