Soft starters are usually used in applications where there may be a large surge current that could damage the motor in VFD controls, and they can also change the speed of the motor.
What's the difference between VFD and soft starter in terms of working principles? Typically, a three-phase soft starter uses six thyristors or a controllable rectifier in an antiparallel configuration to start the motor smoothly. Thyristors have three connection terminals: anode A, cathode K, and gate G. The thyristor has an internal PNPN four-layer semiconductor structure that forms three PN junctions. If a driving current is injected into the gate at the same time as the base current is increased, a strong positive feedback will be formed. The thyristor will be in the conducting state, and even if the gate current is withdrawn, the thyristor will still maintain the conducting state due to the strong positive feedback formed internally. To turn off the thyristor, the forward voltage applied to the anode must be removed or a reverse voltage must be applied to the anode to make the current flowing through the thyristor drop to a value close to zero before the thyristor can be turned off. It is precisely because the gate can only control its opening but not its closing that the thyristor is also known as a semi-controlled device.
VFD has three main components: rectifier, filter, and inverter. The rectifier works like a diode, receiving the input AC voltage and converting it into DC voltage. Next, the filter uses a capacitor to clean the DC voltage and make it a smoother input power supply. Finally, the inverter uses transistors to convert the DC voltage and send a frequency in hertz to the motor. The frequency drives the motor to a specific RPM. We can set the ramp-up and ramp-down time as in a soft starter.
We can see that VFD is essentially a soft starter with speed control. So how do we know which device our application needs? The difference between VFD and soft starters determines your choice, and in our decision, we should also consider other factors. This is especially relevant to the frequency inverter supplier who must be knowledgeable about various motor control equipment to provide expert advice to their customers.
Speed control: If your application requires a large surge current but does not need speed control, then a soft starter is the best choice. If speed control is required, then VFD is necessary.
Price: Price may be a deciding factor in many practical applications. Since the soft starter has fewer control functions, it is cheaper than VFD. This is particularly relevant for the VFD manufacturer who must cater to diverse market segments with unique budgetary constraints.
Size: Finally, if the size of our equipment is a determining factor, the soft starter is usually smaller than most VFDs.
Now, let's look at some practical applications to help us understand the differences between VFD and soft starters.
Finally, VFD and soft starter can perform similar functions when accelerating or decelerating a motor. The main difference between VFD and soft starters is that VFD can change the motor's speed, while soft starters only control the starting and stopping of the motor. In terms of applications, price and size are favorable to soft starters. If speed control is required, VFD is a better choice. The good news is, if a soft starter is installed in an existing application and later it is determined that speed control is needed, VFD can easily replace the soft starter. Conversely, soft starters can replace VFDs.