What is a Reduced Voltage Soft Starter (RVSS)?

A soft starter, like its name, is a device that provides a “soft start” for the motor by gradually ramping up the voltage. Soft Starters are used to control torque and reduce starting current. Soft starters can be used to eliminate belt squeal, protect against water hammer, reduce voltage drop during motor starting, prolong equipment life, or reduce the size of a standby generator. They provide smooth controlled starting and stopping of mechanical equipment by reducing the torque of the motor, hence reducing stress on the motor and extending the lifespan of the system. A soft starter is also referred to as a Reduced Voltage Soft Starter (RVSS).

A motor typically requires 600% of its full load current to start and slams the load with a sudden torque of around 200% of the motor rating. It is not going to be a smooth start. The stress caused by the sudden change in torque will wear on the motor and driven equipment as well as stressing the power supply. This can cause failures of bearings, couplings, pulleys, belts, etc. Now imagine how this would affect equipment that needs to start and stop frequently.

Some advantages of soft starters are:

• About one quarter to one third the cost of a comparable VFD
• No harmonics to worry about
• Start and stop softly to reduce stress on the motor and connected equipment
• Reduce voltage drops and associated electrical problems

However, there is no control for motor speed.

Soft starters are readily available in both low voltage (208V, 230V, 480V, 600V) and medium voltage (2300V, 4160V, 6900V). WEG and SOLCON are two major manufactures of soft starters. At VFDs.com, we currently carry WEG soft starters (SOLCON medium voltage soft starters coming soon!) WEG’s medium voltage soft starter series – SSW7000 is a great option to provide smooth start / stop control and overload protection for three-phase medium voltage induction motors. The SSW7000 comes in 2 different sizes (Size A for IP41 enclosure, and Size N for NEMA 12 enclosure) and is available from 750HP to 4500HP. Here are some of its features:

• Flexible Torque Control (FTC)
• Accessories can be installed easily (plug & play)
• Operating interface (HMI) with graphic LCD
• IP41 or NEMA 12 protection
• Main and bypass vacuum contactors

What can you use soft starters for?

Common soft starter applications include: blowers, compressors, conveyors, chippers, fans, exhausters, pumps, rock crushers.

When do you need a soft starter?

If you don’t need speed control, but have limitations on line power or want the benefits a smooth starting and stopping, using a soft starter is a great solution to protect your motor and equipment.