Understanding the Fundamentals of Reduced Voltage Soft Start (RVSS)

The Reduced Voltage Soft Start (RVSS) is one of many options for starting an electric motor along with the basic motor start and the more advanced Variable Frequency Drive (VFD).

In this article, we will discuss the operation of the RVSS, its applications, and advantages compared to other motor starter applications. If you want more information on motor starters, please read our blog, Understanding the Fundamentals of Motor Starters

What is an RVSS?

As the name implies, it is a Reduced Voltage Soft Starter. The RVSS was designed to control the starting voltage of a motor to provide smooth, controlled starting and stopping of a motor by reducing the torque of the motor.

Purpose of a RVSS

RVSS provides a smooth, controlled start and stop you can use to reduce stress on motors and mechanical systems, reduce voltage drops in facilities, and reduce electrical demand charges. The RVSS works by controlling the amount of voltage sent to the motor increasing the voltage until the motor reaches full speed, and when the motor is turned off the RVSS can decrease the voltage until the motor shuts down.

Many RVSS now have a bypass contactor built in. The purpose of the contactor is to reduce wear and tear on the RVSS by switching the power over to the contactor when the motor reaches full speed, thus not running the power through the RVSS 100 percent of the time, reducing the life of the RVSS. Thus, unit will shut down when not starting and stopping the motor, reducing wear and tear on the RVSS.

When RVSS were first introduced, they did not provide a contactor to switch the power over to when the RVSS reached full speed or a means of overload protection for the motor. Most modern RVSS now include integrated overload protection and bypass contactors. But if choosing an RVSS for your application, please be sure the product includes these functions.

How the RVSS Functions

Essentially, a soft starter works by controlling the amount of voltage running through the motor circuits. It does this by limiting voltage and torque into the motor. This in turn allows the soft starter to either reduce or increase the voltage and allows it to gradually either stop or start controlling the voltage to allow for a smooth progression of current.

A motor typically requires 600% of its full load current to start and will increase the current 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 downline components as well as stress the power supply.

This can cause breakdown of bearings, couplings, pulleys, belts, etc. The purpose of an RVSS is to reduce that shock on the system and ease the motor closer to 100% of torque load.

Advantages of RVSS

• Roughly about one-quarter to one-third the cost of a VFD.
• No concern the RVSS will cause harmonics on the facility electrical system.
• Soft start and stop reduces wear and tear on motor and downline equipment.
• Reduces voltage drops and associated electrical problems in your facility.

Is a RVSS also called a VFD (Variable Frequency Drive)?

NO. While an RVSS controls the voltage thus controlling the starting and stopping of a motor, an RVSS cannot control the speed of the motor. An RVSS is not a good fit in a high torque application compared to a VFD since a VFD controls the frequency into the motor and not the voltage.

If your application is a high torque application such as a loaded conveyor belt, VFD is a much better fit, since it can increase torque on start-up to give the motor the needed boost to start. The VFD is a better fit if you need to control motor speed. A VFD can control motor speed, where an RVSS does not have that capability.

Why use an RVSS?

If applied correctly, an RVSS is a solid, dependable device that can solve motor issues at a lower cost than a VFD. RVSS have shown they can also be long-lasting when utilized correctly. If your application fits, an RVSS will be a solid choice.

Palmer DCS offers a wide array of options for starting motors. To learn more about our capabilities please visit our website at https://www.palmerdcs.com/solutioncenter.

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