VFDs & Motor Control

What Is the Difference Between a VFD and a Soft Starter?

When choosing between a variable frequency drive (VFD) and a soft starter, consider the application, system requirements, and cost. Both reduce inrush current and torque during startup by gradually bringing the motor up to speed.

Soft Starters

A soft starter controls voltage during startup, gradually accelerating the motor to full speed before switching to line voltage. It is used when a controlled, gentle start is needed but continuous speed control is not required.

Variable Frequency Drives

A VFD controls the speed of an AC induction motor by changing the frequency of the supply voltage. It can regulate motor speed during startup, operation, and shutdown.

VFDs can also use sensor feedback through open-loop or closed-loop PID control to adjust motor speed as operating conditions change. This allows the motor to provide only the energy required by the application, which can reduce mechanical stress and extend equipment life.

Motor Protection

Both VFDs and soft starters include internal overload protection, helping protect the motor while reducing the need for separate conventional overload devices.

AIC can help determine whether a VFD, soft starter, or another motor-control approach is the best fit based on the motor, application, operating requirements, and control strategy.

What Are the Applications for VFDs and RVSSs?

Use VFDs in applications where you need total speed control, where energy savings is the end goal, or there is a need for custom control over the motor speed and torque. VFDs regulate the motor speed based on external signals or preprogrammed speed curves. Fans, pumps, conveyors, and compressors are some of the most common applications that use variable frequency drives.

Use soft starters if you need to reduce the power or load requirements during the start cycle of an operation, but the motor will run at full speed during operation. When equipped with a soft stop feature, some soft starters can also ramp down during the stop cycle.

A soft starter is typically used if you only need to reduce large startup inrush currents or if the mechanical system needs a gentle start to relieve torque spikes and tension.

AIC supports VFD and soft-starter applications including pumps, fans, conveyors, compressors, and other industrial equipment requiring controlled starting, speed regulation, or process control.

What Is a VFD or VSD?

A variable frequency drive, or VFD, is a motor controller that varies the frequency and voltage of the power supply to drive an electric motor.

VFDs control the ramp-up and ramp-down of a motor during the start or stop phases and allow the motor to run at a speed other than the design speed shown on the nameplate.

Advantages can include energy savings, extended machinery life, reduced peak current draw, and intelligent motor control.

What Are Other Terms for a VFD?

• VSD – Variable Speed Drive
• ASD – Adjustable Speed Drive
• VSI – Voltage Source Inverter
• CSI – Current Source Inverter
• Frequency Converter

When you use a frequency converter, a motor's voltage and frequency can be controlled over a wide range, which also controls its speed.

When used in closed-loop control with a reference signal and PID control loop, the motor provides the required amount of power for the process, but no more. This can reduce energy use and related costs.

What Are the Benefits of a VFD?

Benefits of using a variable frequency drive for motor control can include:

• Extended pump or fan life
• Energy reduction
• Simplified pump selection
• Closed-loop control
• Adaptable pump or fan curve
• Mitigating pressure and flow transients
• Reduction or elimination of inrush currents
• Reduction of full load amps
• Phase loss protection
• Overcurrent protection
• Ground fault protection
• The ability to oversize pumps
• Simplified instrumentation
• Additional monitoring potential
• Dynamic process control
• Reversing functions
• Safe Torque Off (STO)
• Early failure warnings

What Is an RVSS?

A reduced voltage soft starter (RVSS) gradually applies increasing voltage to the motor during startup, allowing the motor to accelerate smoothly instead of experiencing the sudden burst of power associated with inrush current.

Common applications include pumps, conveyors, some fan applications, and systems with high inertial loads. Applications prone to sudden or jerky starts can also benefit from a soft starter.

Advantages can include:

• Reduced energy use
• Reduced stress on the motor
• Reduced risk of power surges
• Increased starts per hour
• Adjustable acceleration times
• Lower risk of overheating
• Extended machinery lifespan
• Improved operating efficiency

What Is an Across-the-Line Starter?

An across-the-line starter connects incoming power directly to the load with no controlled ramp up to operating speed. It can also be referred to as a hard starter or full-voltage starter.

What Is a Reversing Starter?

A reversing starter is an across-the-line starter that can stop and start a motor and operate it in both forward and reverse. Two contactors are typically used, with the second wired to reverse motor rotation.

What Is a Combination Starter?

A combination starter integrates the motor starter, disconnecting device, and short-circuit protective device into a single unit.

What Is a Manual Motor Starter and Protector (MMP or MSP)?

A manual motor starter and protector is an electromechanical control and protection device for a motor circuit. These devices are primarily used to manually switch motors on and off while providing short-circuit, overload, and phase-failure protection.

Manual Motor Protectors may also be referred to as Manual Motor Starters (MMPs) or Motor Protection Circuit Breakers (MPCBs).

Benefits can include:

• Less machine downtime by protecting motors
• Reduced troubleshooting and maintenance costs
• Lower initial cost through reduced wiring and material costs
• Efficient planning and installation
• Compact design
• Fast, easy, and reliable connections