Direct online starting and star-delta starting of large induction motors pose a great problem to the grid as well as the motor itself due to sudden voltage dips and large fluctuating torque. Power electronics converter-based induction motor starters and controllers are becoming popular and are rapidly replacing conventional starters. Two commonly used new-generation motor control devices are the variable frequency drive (VFD) and the soft starter. But VFD and soft starter have lots of differences. Let’s compare.
Differences between VFD and soft starter
Choosing between a VFD and a soft starter often depends on the type of application, the mechanical system requirements, and the cost. But there are lots of other differences.
Definition of VFD and soft starter is different
A variable frequency drive (VFD) is a device that can control the speed of an electric motor by controlling the frequency and voltage of the electrical power supplied to it. When a drive starts a motor, it initially applies a low frequency and voltage to the motor thus allowing high inrush currents to be avoided.
The soft starter is a device used to limit the large initial inrush of current associated with motor start-up and provide a gentle ramp-up to full speed.
The soft starter can be referred to as a reduced voltage motor starter. Variable frequency drive can be referred to as variable speed drive, adjustable frequency drive, frequency inverter, and drive.
VFD can control the speed of a motor
The main difference between a variable frequency drive (VFD) and a soft starter is the control of the speed.
Variable frequency drives are used to control the speed of a motor by changing the frequency of the power. A VFD can control the speed at any point during the operation of the motor. The soft starter cannot control the speed of a motor. Soft starters are only used during startup to limit the inrush current. (and stop if equipped) The gradual start of the motor is produced by ramping up the initial voltage to the motor.
The working principle of the VFD is more complex
VFDs convert constant frequency and voltage input power to adjustable frequency and voltage sources for controlling the speed of AC induction motors.
Soft starters use semiconductor devices to temporarily reduce the motor terminal voltage. This provides control of the motor current to reduce inrush and limit shaft torque. The control is based on controlling the motor terminal voltage in two or three phases. By limiting the voltage, a reduced torque is provided to start the motor.
Soft starters utilize silicon-controlled rectifiers (SCRs) to perform phase angle control, while VFDs use insulated-gate bipolar transistors (IGBTs) to perform pulse-width-modulation (PWM) control.
The soft starter has fewer operating functions
VFDs can also provide more flexible functionality than soft starters, including digital diagnostic information. With a VFD, you can receive feedback using many types of communications, from hardwired I/O points, if you just want minimum feedback, to Ethernet, if you want to have all the data in the drive available to your controller.
The cost of a soft starter is cheaper than a VFD
In many applications, it is required to continuously regulate the speed of the motor, and a drive is then a very good solution. However, in many applications, a drive is used only for starting and stopping the motor, even though there is no need for continuous speed regulation. This will create an unnecessarily expensive solution if compared with a soft starter. In fixed-speed applications, soft starters are still the most economical and effective solution. However, while a VFD can be more expensive initially, it can provide significant energy savings which reduce operational costs over the life of the equipment for a lower total cost of ownership.
The market size of VFDs is huge
The market size of VFDs is extremely huge compared to soft starters. Most motors need speed control and VFD is a perfect solution. The global VFD market is projected to reach a market size of USD 27.57 billion by 2023. The global market for the soft starter is expected to reach $2.0B by 2023.
VFD needs more space in a control panel
The drive has a much bigger physical size and requires more space. Soft starters are often a solution for applications where space is a concern, as they usually take up less space than VFDs.
VFD generates more harmonics
Since the drive changes the frequency and creates the sinus wave, a drive will cause harmonics in the network. Additional filters and shielded cables are used to reduce these problems but the harmonics will typically not be eliminated. The soft starter does switch over to running across the line using bypass contacts. So, it will be a linear load while running across the line. This means it will not inject harmful harmonic distortion into your plant’s ac power grid.
A soft starter is lighter than a VFD
The drive is also much heavier than a soft starter. This makes it a less desirable solution on, for instance, ships where weight is important.
The heating and cooling systems of soft starters are not complex
Soft-starters are normally air-cooled due to their short operation time. For high powers or in case of reduced availability of cooling air, additional ventilators might be preferred. The integrated bypass function of a soft starter provides the opportunity to switch over to direct-online operation, after the initial start-up. By running partially direct-on-line, you win the advantages of reduced losses and need for heat dissipation, thus saving energy due to decreased cooling requirements.
VFDs operate on a continuous and enable more sophisticated control. However, they use insulated gate bipolar transistors (IGBTs) and so must be designed with sufficient cooling capability.
Forward and reverse operations can be done by VFD
On their own, soft starters cannot run motors in the reverse direction at full speed. However, forward and reverse operations can be achieved by using a forward and reverse contactor arrangement. Some soft starters also provide a part speed function that runs the motor at a slow speed in either the forward or reverse, without a reversing contactor. However, the reverse operation is limited to short periods at a fixed slow speed. On the other hand, VFD can operate in forward and reverse directions.
Maintenance of a soft starter is easier
Soft starters do not require so much maintenance. Keeping the fan vents clear and clean is enough. Depending on the drive, annual maintenance, or even less frequently, parts need to be inspected, cleaned, and or replaced. For example, on a drive operating 24 hours per day, in year 3, you should replace some parts of a VFD.
Energy can be saved by using VFDs
To determine energy saving when using VFD and soft starter, we need to calculate the average operation hour per day and energy consumption per day. Studies show that using VFDs in variable torque applications such as centrifugal pumps and fans can provide very fast payback in energy saving. Some applications can even pay back the entire cost of a typical VFD within months, especially if the local utility offers a VFD rebate program. VFDs may offer greater potential to reduce your electricity costs over the life of the equipment.
Applications are not the same
A VFD can perform the same function as a soft starter and can be used in applications requiring complete speed control, energy savings, and custom control. VFDs have also some microprocessor capabilities with inputs and outputs.
3 phase soft starter can be connected inside delta
3 phased controlled soft starters can be connected inside the delta. Drives are sized for line connection (three leads from the motor), based on the full load current of the motor.
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