Choosing Between Soft Starters & Adjustable Frequency Drives

Motors often require large amounts of energy when quickly accelerating to full speed. Using soft starters and adjustable frequency drives reduce in-rush currents and limit torque—protecting valuable equipment and extending the motor's life by reducing motor heating caused by frequent starts and stops. Choosing between a soft starter and an adjustable frequency drive (AFD) often depends on the application, system requirements and cost, both for initial startup and over the life cycle of the system.

Soft Starters

A soft starter is a solid-state device that protects alternating current (AC) electric motors from damage caused by sudden influxes of power by limiting the large initial in-rush of current associated with motor startup. They provide a gentle ramp up to full speed and are used only at startup (and stop if equipped). Ramping up the initial voltage to the motor produces this gradual start. Soft starters are also known as reduced voltage soft starters (RVSS). Soft starters are used in applications that have the following requirements:

• Speed and torque control during startup (and stop if equipped with soft stop)
• Reducing large startup in-rush currents with a large motor
• A gentle start for the mechanical system to relieve torque spikes and tension with normal startup (for example, conveyors, belt-driven systems, gears and pumps)
• Pumps that eliminate pressure surges in piping systems when fluid changes direction rapidly

How does a soft starter work?

Electrical soft starters temporarily reduce voltage or current input by reducing torque. Some soft starters may use solid-state devices to help control the current's flow. They can control one to three phases, while three-phase control usually produces better results. Most soft starters use a series of silicon-controlled rectifiers (SCRs) to reduce the voltage (see Figure 1). In the normal "off" state, the SCRs restrict current, but in the normal "on" state, the SCRs allow current. The SCRs are engaged during ramp up, and bypass contactors are pulled in after maximum speed is achieved. This helps to significantly reduce motor heating and improve the life of the systems.

Benefits of choosing a soft starter

Soft starters are often the more economical choice for applications that require speed and torque control only during motor startup. Additionally, they are often the ideal solution for applications where space is a concern because they usually take up less space than AFDs. Since soft starters use SCRs during normal operation, there is a reduction in heat emitted. This will significantly increase the reliability of the full system and maintain functionality. Also, soft starters can have the benefit of a pump algorithm to help eliminate the water hammer effect by using an S-curve to start and stop the pump. This can lengthen the pump system's life.

Adjustable Frequency Drives

An AFD is a motor control device that protects and controls the speed of an AC induction motor. An AFD can control the motor's speed during the start and stop cycle, as well as throughout the run cycle. AFDs are used in applications including ones that require complete speed control, have a goal of energy savings or need custom control.

How do AFDs work?

AFDs convert input power to adjustable frequency and voltage source for controlling the speed of AC induction motors. The frequency of the power applied to an AC motor determines the motor speed based on the following equation: N = 120 x f x p N = speed (rpm) f = frequency (Hz) p = number of motor poles For example, a four-pole motor is operating at 60 Hertz (Hz). These values can be inserted into the formula to calculate the speed: N = 120 x 60 x 4 N = 1800 (rpm) Figure 2 shows components of an AFD's function.

• AC supply: Comes from the facility power network (typically 480V, 60 Hz AC)
• Rectifier: Converts network AC power to DC power
• Filter and DC bus: Work together to smooth the rectified DC power and to provide clean, low ripple DC power to the inverter
• Inverter: Uses DC power from the DC bus and filter to invert an output that resembles sine wave AC power using a pulse width modulation (PWM) technique
• Pulse width modulation: Switches the inverter semiconductors in varying widths and times that, when averaged, create a sine waveform, as shown in Figure 3

Benefits of using an AFD

The benefits provided by AFDs in the appropriate systems are numerous:

• Energy savings
• Reduced peak energy demand
• Reduced power when not required
• Fully adjustable speed (pumps, conveyors and fans)
• Controlled starting, stopping and acceleration
• Dynamic torque control
• Provides smooth motion for applications such as elevators and escalators
• Maintains speed of equipment, making drives ideal for as mixers, grinders and crushers
• Offers versatility
• Provides self-diagnostics and communications
• Includes PLC-like functionality and software programming
• Digital inputs/outputs (DI/DO)
• Analog inputs/outputs (AI/AO)

Energy savings

AFDs offer the greatest energy savings for fans and pumps. The adjustable flow method changes the flow curve and drastically reduces power requirements. Centrifugal equipment, such as fans, pumps and compressors, follows a general set of speed affinity laws. The affinity laws define the relationship between speed and a set of variables including flow, pressure and power. Based on the affinity laws, flow changes linearly with speed while pressure is proportional to the square of the speed. The power required is proportional to the cube of the speed. The latter is most important because if the motor speed drops, the power drops by the cube. In this example, the motor is operated at 80 percent of the rated speed. This value can be inserted into the affinity laws formula to calculate the power at this new speed: Therefore, the power required to operate the fan at 80 percent speed is half the rated power.

Selecting Soft Starters

Choosing a soft starter or an adjustable frequency drive often depends on your application. Soft starters are smaller and less expensive when compared with AFDs in larger horsepower applications. Larger AFDs take up more space and are usually more expensive than soft starters. While an AFD is often more expensive up front, it can provide energy savings of up to 50 percent and produce more cost savings over the life of the equipment. Speed control is another advantage because it offers consistent acceleration time throughout the motor's entire operating cycle, not just during startup. AFDs can also provide more robust functionality than soft starters offer, including digital diagnostic information. It is important to note that an AFD 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 requires current limiting only during startup, a soft starter may be a better solution from a cost standpoint.