Understanding Direct Online (DOL) Starters for Motors

A Direct Online (DOL) starter is the simplest type of motor starter. When you use a DOL starter, you connect the motor terminals directly to the full power supply voltage. This means the motor gets the full voltage immediately when it starts.

How DOL Starting Works

The operation of a DOL starter is straightforward:

• A contactor is used to connect the motor directly to the three-phase (or single-phase) power supply.
• Overload protection (like thermal or electronic relays) is included to protect the motor from drawing excessive current during operation.
• Short-circuit protection (fuses or circuit breakers) is also part of the setup to protect against faults.

When the start button is pressed, the contactor closes, applying full voltage to the motor, and it starts running.

Limitations of Direct Online Starting

While simple and inexpensive, DOL starting has a major drawback: high starting current. When a motor starts from a standstill and is connected directly to the line voltage, it draws a very large current, often 5 to 7 times its full load rated current, or even higher.

This high starting current can cause several problems:

• Voltage Dip: It can cause a significant drop in the voltage of the power supply system, affecting other equipment connected to the same line.
• Stress on Motor: The high mechanical and electrical stress can reduce the lifespan of the motor windings and connected equipment (like gears, belts).
• Power System Impact: Utility companies often have regulations limiting the amount of starting current allowed, especially for larger motors, to maintain grid stability.

Motor Capacity for Direct Online Starters

Because of the issue with high starting current, DOL starters are typically only used for motors where this high current surge is manageable by the power supply and acceptable for the motor's size and application. This generally limits the use of DOL starters to smaller motors.

The common practice is to use Direct Online starters for motors with a capacity of less than a certain horsepower (hp) or kilowatt (kW) rating. This limit varies depending on local regulations, power system capacity, and specific application requirements, but a very common threshold for using DOL starters is for motors below 5 hp.

For motors larger than this typical threshold (e.g., 5 hp, 7.5 hp, 10 hp, and above), other starting methods that reduce the starting current are usually employed. These include:

• Star-Delta (Wye-Delta) starters
• Autotransformer starters
• Soft starters
• Variable Frequency Drives (VFDs)

These alternative methods reduce the voltage applied to the motor during starting, thereby limiting the starting current.

Analyzing the Options

Let's look at the given options in the context of our understanding of DOL starters and their limitations:

• Option 1: any Capacity - This is incorrect. DOL starting is not suitable for motors of any size due to the excessive starting current in large motors.
• Option 2: less than 5 hp - This aligns with the common practice of using DOL starters for small motors where the high starting current is acceptable.
• Option 3: more than 10 hp but less than 50 hp - This is incorrect. Motors in this size range almost always require a reduced voltage starting method to limit starting current and voltage dips.
• Option 4: more than 10 hp - This is incorrect for the same reason as Option 3. Motors above 10 hp generate very high starting currents with DOL starting.

Therefore, the most appropriate capacity range for which Direct Online Starters are generally used is less than 5 hp.

Revision Table: DOL Starters & Motor Size

Additional Information on Motor Starting Methods

Choosing the right motor starting method is crucial for the longevity of the motor and the stability of the power system. Here's a brief look at why other methods are preferred for larger motors:

• Star-Delta Starters: Start the motor in a 'star' configuration (lower voltage) and then switch to a 'delta' configuration (full voltage) once the motor speeds up. Reduces starting current to about 1/3 of DOL.
• Autotransformer Starters: Use an autotransformer to supply a reduced voltage to the motor during starting. Provides taps for different voltage reduction levels. Effective but can be bulky and expensive.
• Soft Starters: Use power electronics (like SCRs) to gradually increase the voltage to the motor. Allows for smooth acceleration and controlled starting current.
• Variable Frequency Drives (VFDs): Offer the most advanced control, allowing for precise speed and torque control throughout the entire operating range, including starting. They ramp up frequency and voltage smoothly, resulting in very low starting current. Suitable for applications requiring speed control or very gentle starts.

The choice of starter depends on the motor size, load characteristics, power supply limitations, cost considerations, and required control features.