Soft starter: A generation of electric motor power control

Published: 17th August 2006
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Now it is possible to apply the power of soft starting to even the smallest of applications. Over the years, the engineering and electronics industry has developed a range of soft starters for a range of applications. Many areas have benefited including marine, oil & gas and heavy industry. Now there is a new motor added to the family, the miniature analogue soft starter that incorporates a single thyristor to start a single phase squirrel cage induction motor.

What is a Soft Starter?

Technically, a "Soft Starter" (when in the context of to AC electric motors) is an electronic device that reduces the torque in the power train of a motor. In practice this is mechanically a clutch, fluid drive, magnetic coupling or shot coupling. Electrically a soft starter can be any control system that reduces the torque by reducing the voltage in, or a device that causes a change in the motor connection.

The soft starter can employ solid state devices to control the current flow and therefore the voltage applied to the motor. In theory, soft starters can be connected in series with the line voltage applied to the motor, or can be connected inside the delta loop of a delta connected motor, controlling the voltage applied to each winding.

A new range of AC electric motor soft starters provides the means to reduce the wear and tear caused by conventional direct-online starting, and reduce the peak demand currents that can impact electricity bills.

Two types of soft starters are offered: an intelligent soft starter with complete motor control devices such as these offer a high level of programmability to maximize the starting characteristics for particular applications or industry; many low-cost units are now available from a variety of manufacturers such as Solcon Industries, in forms configurations for simple stand-alone connection or retrofitting inside existing motor cabinets. This type of module for controlling three-phase AC induction motors prevent the torque surges that can cause mechanical damage, and the generation of shock waves in the system being driven - such as a "water hammer" in the case of a pump for example.

They also limit the large inrush currents that can cause voltage drops and impact electricity bills, by increasing the peak starting current that the electricity network has to supply.

Generations of Soft Starters

New generation Solcon soft starters tend to have the following features and characteristics:

Reduced inrush current & mechanical shock

Soft, smooth step-less acceleration & deceleration

Solcon Unique features: Pump Control Program, Dual Adjustment, Slow

Speed with Electronic Reversing, Energy Save, etc.

Torque and current starting & stopping characteristics

Third generation, Medium Voltage, digital soft starter, is designed for use with standard MV asynchronous and synchronous motors. This highly sophisticated soft starter ensures smooth, step-less acceleration and deceleration, eliminating high inrush current and damaging mechanical torque on startup and mechanical wear on stopping. The HRVS-DN is available as a stand-alone product or as part of a complete system including Line and/or Bypass Contactors.

For more information on HRVS-DN and additional products such as Load Break Switch, Motor Protection Relay, Digital Multimeter and more, visit

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