Motors, no matter the type of application they are being used within, require a significant amount of energy to start up and accelerate to their maximum speed. Even standard EPAct motors that meet efficiency standards, and are started across the line, draw a large amount of full load amps (FLA) from electrical sources to start and maintain their desired torque and horsepower.
In applications with motors that require frequent start and stops — fans, pumps, conveyors, lifts, etc. — it’s essential to reduce the inrush current drawn by the motor in order to prevent overheating and increase the life of the motor. To accomplish this, automation specialists use one of two products: reduced voltage soft starters (RVSSs) or variable frequency drives (VFDs). Both RVSSs and VFDs reduce the voltage and inrush current supplied to motors during across on-the-line starting.
Which one you choose to use depends on your application as well as a few other factors.
When to Use RVSSs
Reduced voltage soft starters operations offer a low cost, simplified method of controlling inrush current and extending the life of a motor. These devices work best with large pumps, fans, centrifuges and other devices that range from 200-500 horsepower, require very few starts and stops, and generally run at full speed while operating. They are cost-effective, easy to operate, and efficient.
The main drawback to using RVSSs is in its performance and flexibility. RVSSs provide less reduction in energy usage than VFDs. They also do not have the same amount of performance parameters and tuning functions as VFDs.
Use an RVSS if:
- Speed control is not needed
- Consistent acceleration time isn’t an issue
- Current limitations prevent motors from starting at full voltage
When to Use VFDs
Variable speed drives are efficient, contain diagnostics capabilities (both analog and digital), and are able to precisely control a motor’s speed. When precision is required within an application, there simply is no alternative. While an RVSS applies line frequency, meaning operating speed is fixed, VFDs can vary the output frequency from zero to above base motor frequency, allowing for very tight regulation of speed. VFDs can also interface with plant automation and control networks and can eliminate the need for higher-level external controllers.
The drawback to VFDs is their cost and the expertise required to use them. Because they can be customized to work within process requirements and have multiple performance parameters, only a person trained in working with them will be able to get the most out of their performance.
Use a VFD if:
- Precise speed control is needed
- Consistent acceleration is needed
- Starting torque is a concern (VFDs have a higher torque per amp ratio)