What is a VFD?
VFD stands for Variable Frequency Drive. This device is used for running an AC motor at different speeds. You can also use a VFD to allow the speed of the motor to ramp up in order to give it a smoother startup.
VFDs work by adjusting the frequency of a motor and therefore the RPMs. Below is a step by step guide on how the VFD works.
- The VFD converts the alternating current to a direct current with diodes.
- A capacitor will clean the direct current.
- The transistors will work as switches and change the direct current back to an alternating current.
The use of transistors as switches is what allows the VFD to change the frequency that is being supplied to the motor. This allows the VFD to alter the speed of the motor.
What is a soft starter?
Soft starters reduce the inrush of current that is created when a motor is first started up. This inrush of current can be detrimental to the health and lifespan of your motor, so soft starters are important if you plan on turning your motor on and off repeatedly.
A soft starter will also reduce the initial torque of the motor. The lessened inrush of current allows the motor to startup safer and more smoothly than if you were to start the motor without it.
Soft starters can save your motor, devices, or appliances attached to the motor. Moreover, a lessened initial current can cost you less money in the long run.
Inside a VFD
There are only three main components inside a VFD:
- Rectifier: Acts as a diode, changing the alternating current and changing it to direct current.
- Filter: Uses capacitors to clean the DC, making it a source of cleaner and smoothing power.
- Inverter: Acts alongside transistors to alter the DC back to AC. Sends the motor a frequency measured in Hertz, which will then drive the motor to work with a specific RPM.
You can control the speeds of your VFD to alter the frequency that is sent to the motor. This allows you to change the RPMs depending on your application, giving you much more control over your motor.
Altering the speed can ramp up or down your motor, making the startup and cooling down periods much more controlled and safer than if you were to leave them uncontrolled.
Inside a soft starter
There are many types of soft starters out there, although we are looking at a typical single-phase soft starter. The soft starter will either use Silicon Controlled Rectifiers (SCR) or Thyristors to control the startup of a motor.
SCRs act as switches and turn on as soon as the motor is powered on. The SCRs prevent the majority of the inrush of current from passing and reaching the motor. Once the motor is ready to accelerate, the SCRs will relax and allow more of the current to pass through.
SCRs work on a ramp time so the current allowed to pass will slowly increase rather than all rush to the motor at once. Once the motor is turned off, the soft starter will work in a similar way to slowly ramp down the motor.
Differences between VFD and soft starters
To sum up, a VFD can be considered a soft starter with speed control. These two devices commonly get confused with one another, but you might have a need for one rather than the other.
So, instead of simply purchasing one and hoping for the best, let’s try to hone in on which will be best for you.
Control
How much control over your motor do you need? A VFD gives you much more control than a soft starter as you can adjust the speed of the device.
However, if you don’t need to control the speed but just want to lessen a large inrush of current from reaching your motor, a soft starter will suffice just fine.
Price
Price is an important factor for most people, as we always want to save money wherever we can. However, we also want the best for our motors and devices, so spending that extra money is acceptable as long as we’re getting exactly what we need.
As a soft starter does not offer as much control as the VFD, the price is almost always lower than opting for a VFD.
Size
The size of your device might also play a deciding factor in whether you opt for a VFD or a soft starter. Again, as they do not offer as much customization soft starters are often smaller in size than VFDs.
This means that soft starters can fit in smaller places than VFDs, which is helpful when you’re tight on space to install your device.
VFDs vs. soft starters in the real world
Cooling fan systems
A cooling fan turns on when the temperature of the air becomes too hot. As the cooling fan turns on, it will need to reach a certain speed to cool down the room quicker than it heats up again.
However, once the room temperature has cooled down, the fan will have to slow down to avoid making the room too cold.
A VFD would be much better than a soft starter in this instance because the fan would need to change speeds and therefore RPM. If the cooling fan only turned on to one speed instead of altering the RPM, a soft starter would be best.
Water treatment facilities
The second instance is inside a water treatment facility on a wastewater pump. The pump has a constant flow of water coming to it, needing to be pumped through the facility as quickly as it enters.
A soft starter would be best suited for the water pump because there is a large inrush of current on the motor anytime the pump needs to be started up.
As the pump will need to be used regularly and often, the soft starter will protect the pump and motor from the initial inrush of current.
If the demand for water exiting the facility was constantly changing, a VFD could be better as you can control the speed of the pump.
Summary
Essentially, a VFD is a soft starter with more control over the speeds. If you need to alter the speed of your application, a VFD is the best option for you.
However, if you do not need to alter the speeds and want a small and inexpensive device to control the inrush of current, a soft starter is the one for you.
