WEG White Paper: Choosing Between a Variable Frequency Drive or a Soft Starter

The following article is an excerpt from a white paper titled, “Choosing a Variable Frequency Drive or Soft Starter based on your application need” published by WEG Electric Corporation

Read the full White Paper

Overview

When accelerating an AC motor to full speed using a full voltage connection, a large inrush current may be required. Additionally, the torque of the AC motor is mostly uncontrolled and can shock the connected equipment, potentially causing damage. Variable Frequency Drives and Reduced Voltage Soft Starters and can both be used to reduce inrush currents and limit torque; thereby protecting expensive equipment and extending the life of the motor and coupling devices. Choosing between a variable frequency drive and soft starter often depends on the type of application, the mechanical system requirements, and cost (both for initial installation and over the lifecycle of the system).

Soft Starters

A reduced voltage soft starter helps protect the motor and connected equipment from damage by controlling the terminal voltage. This limits the initial inrush of current and reduces the mechanical shock associated with motor startup and provides a more gradual ramp up to full speed. Soft Starters are also beneficial to electrical systems with limited current capacity when using a soft starter for motor starting to limit the inrush current. By gradually increasing the motor terminal voltage the soft starter produces a more regulated motor acceleration up to full speed. Soft starters are also capable of providing a gradual ramp to stop where sudden stopping may create problems in the connected equipment.

Applications: Soft Starters are used in applications where:

• Speed ramping and torque control are desired when starting or stopping
• High inrush currents associated with starting a large motor need to be limited to avoid supply network issues or penalty charges
• A gradual controlled starting is needed to avoid torque spikes and tension in the mechanical system associated with normal equipment startup (e.g., conveyors, belt-driven systems, gears, couplings, etc.)
• Avoiding pressure surges or ‘hammering’ in piping systems when fluid changes speed too rapidly

Variable Frequency Drives

A variable frequency drive (VFD) is a motor control device that protects and controls the speed of an AC induction motor. A VFD can control the speed of the motor during the start and stop cycle, as well as throughout the run cycle. Variable frequency drives are also referred to as adjustable frequency drives (AFD).

Applications: Variable Frequency Drives are used in applications where:

• Complete speed control is required
• Energy savings is a goal
• Custom control is needed

Selecting the Correct Motor Control Equipment for Your Needs

Choosing a soft starter or variable frequency drive often depends on your application. Soft starters are smaller and less expensive when compared with variable frequency drives, especially in larger horsepower applications. Larger variable frequency drives take up more space and are usually more expensive than soft starters.

However, while a variable frequency drive can be more expensive initially, they can provide energy savings of up to 50 percent, therefore providing operating cost savings over the life of the equipment for a lower total cost of ownership.

Speed control is another advantage of a variable frequency drive, because they offer consistent acceleration time throughout the entire operating range of the motor, not just during startup. Variable frequency drives can also provide more flexible functionality than soft starters offer, including digital diagnostic information.

It’s important to note that variable frequency drives can initially cost two to three times more than a soft starter. Therefore, if constant acceleration and torque control is not necessary, and your application only requires current limiting during startup, a soft starter may be a more cost effective solution for your needs.

David Mintzlaff is Product Line Manager for Low Voltage Drives and Soft Starters at WEG Electric Corporation in Duluth, Georgia, USA. 

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ISC Companies is a top distributor of power transmission and electrical parts. We have many different models and brands in stock for fast delivery. Our sales professionals are happy to assist you in the selection process to ensure the correct part is specified for your application. Give us a call at 1-800-627-9434.