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Motors often require large amounts of energy when quickly accelerating to full speed. Soft starters and variable frequency drives can both be used to reduce inrush currents and limit torque—protecting your valuable equipment and extending the life of your motor by reducing motor heating caused by frequent starts and stops.
Choosing between a soft starter and a variable frequency drive often depends on the application, system requirements and cost (both for initial startup and over the life cycle of the system).
A variable frequency drive (VFD) is a motor control system or 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.
VFDs are also referred to as adjustable frequency drives (AFDs).
Reduces voltage drop normally associated with motor starting.
A soft starter is a solid-state system or device that protects AC electric motors from damage caused by sudden influxes of power by limiting the large initial inrush 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).
Reduces voltage drop normally associated with motor starting.
VFDs are used in applications where:
Soft starters are used in applications where:
VFDs convert input power to adjustable frequency and voltage source for controlling 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*F/P
N = speed (rpm)
F = frequency (Hz)
P = number of motor poles
For example, a four-pole motor is operating at 60 Hz. These values can be inserted into the formula to calculate the speed:
N = 120 x 60 / 4
N = 1800 (rpm)
Electrical soft starters temporarily reduce voltage or current input by reducing torque. Some soft starters may use solid-state devices to help control the flow of the current. They can control one to three phases, with three-phase control usually producing better results.
Most soft starters use a series of thyristors or silicon controlled rectifiers (SCRs) to reduce the voltage. 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.
VFDs offer the greatest energy savings for fans and pumps. Based on the affinity laws, flow changes directly with speed while power is reduced by the cube root of speed. If a fan’s speed is reduced by 1/2, the power/energy requirements will be only 1/8th that of full speed. As an example, a 500 HP fan load running mainly between 30% and 80% speed can see over $100K in annual energy savings when powered by a VFD.
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, as they usually take up less space than variable frequency drives.
Choosing a soft starter or a variable frequency drive often depends on your application. Soft starters are smaller and less expensive when compared with VFDs in larger horsepower applications. Larger VFDs take up more space and are usually more expensive than soft starters.
That being said, while a VFD is often more expensive up front, it can provide energy savings of up to 50 percent, thereby producing more cost savings over the life of the equipment.
Speed control is another advantage of a VFD, because it offers consistent acceleration time throughout the entire operating cycle of the motor, not just during startup. VFDs can also provide more robust functionality than soft starters offer, including digital diagnostic information.
It is important to note that a VFD 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.
