Solcon USA Article

Across The Line Starting (Direct On Line) vs. Reduced Voltage Starters (Soft Starters)

Christopher Sanderson Solcon USA

Christopher B. Sanderson, Gulf Coast Regional Sales Manager

The first primitive induction motor was discovered by manually turning switches on and off, by Walter Baily in 1879. Since that time there has always been a need for motor control. There are various methods that can be used to start an AC inductive motor. In this article we will address two of the most commonly used methods of starting inductive motor starters.

The simplest method is by closing a contactor and allowing the motor to start at full voltage, or known as Across The Line (ATL). Today's motor control applications, such as ATL starters are simple, compact, and the least expensive style of electric motor starting. They are also known as "Direct On Line" (DOL) for which the full line voltage is applied over the motor terminals. The inrush currents are 6X to 8X times the motor's Full Load Amps (FLA) value, on average. Today's premium efficiency motors can have inrush currents greater than 10X times the motor's Full Load Amps. These characteristics cause voltage dips in the electrical network as well as mechanical wear and in some cases destruction of equipment, such as gearing, couplings, shafts, belts, or fragile parts or products. Although Across the Line starting is the most commonly used method, it is by no means always the best.

Disadvantages of Full Voltage ATL Starting

1. High inrush currents create stress on the motor's windings. This stress will cause the windings to move in the end turns of the stator. This will cause the insulation to break down. Eventually, phase to phase shorts will occur and result in early motor failure.

2. Full voltage starting will cause damage to belts, sheaves, gearboxes, and other mechanical components throughout the application drive train, thus causing downtime and replacement costs. For the most part, it is the down time that proves to be the most costly in any industry.

3. Full voltage starting can create line drops/voltage dips which may result in penalties from the utility company. The line drops that large motors can create may also cause problems with other applications throughout the plant.

4. Across The Line starting puts large amounts of stress on the contactor contacts which, in turn, require a relatively large amount of maintenance.

5. Poor motor protection with the use of overload with 20% accuracy.

6. No capability to control the deceleration.

Reduced Voltage Starters are usually used in applications that involve large horsepower motors. They are also referred to as "Soft Starters" because of their common association of slowly starting a motor. Soft starters reduce the amount of inrush current and reduce the mechanical stress on the motor load. For it addresses the inrush problem by drawing smaller increments of current and allowing the motor to get up to speed in smaller synchronous steps.

As soft starters are a method that controls the acceleration of an electric motor by means of controlling the applied voltage, they also reduce the amount of torque applied to the electric motor. At the time of starting the motor, the torque and the current can be adjusted by reducing the voltage.

A reduced voltage starter starts motors with reduced power supplied at startup. Like its name, a"Starter" helps in starting motors and can also accelerate, reverse, stop and protect them. The soft starter helps in reducing the power which can potentially damage the electrical and mechanical shocks on the system.

A soft starter can add many features and protections fairly easily, because they are generally monitored and controlled by a solid state microprocessor. They can offer choices of power monitoring, such as three-phase voltage, power usage, three-phase current, built-in bypass, power factors and a host of others. And an internal clock, a voltage meter and current transformers can be implemented by motor thermal capacity usage.

The advantages of Reduced Voltage Starters over Across The Line starting techniques:

  • Improved Efficiency
  • Controlled Acceleration and Deceleration
  • Optional Local and remote reset
  • Low Cost and size
  • Optional Field Communication (EX. MODBUS RTU, PROFIBUS, etc.)
  • Torque and Current Control

In summary, although Across the Line starting is the most commonly used method, it is by no means always the best. In many instances a solid state reduced voltage starter can help reduce the amount of electrical and mechanical stress on the motor and in turn help increase the life expectancy of the motor. Many maintenance organizations are looking for ways to increase the life of their equipment and avoid unexpected downtime. Unexpected downtimes are a strain on maintenance budgets and resources. As many of these maintenance organizations look to more preventative maintenance strategies, reduced voltage starters are playing a more crucial role in motor control starting.

To contact Mr. Sanderson, please e-mail him at

Solcon USA

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