Otherwise called as Variable Speed Drives, AC drives, or inverters, Variable Frequency Drives, or VFDs, are responsible for controlling the speed and torque of an AC motor. It works by adjusting the voltage and input frequency to synchronous motors. While recent technologies in VFDs are capable of simplifying the entire process of adjusting the speed of AC motors with three phases, there was a point in history when it was a challenge to operate AC motors that come with a single or dual speed. What’s more, there are issues in terms of the construction of the AC motor, as well as the magnetic poles present, and these all affect the cost of VFDs in AC motors.
Factors to Consider with the Application of VFDs
It was only during the 1980s when the cost and dependability of VFD technologies have improved. Although VFDs are becoming more popular as the DC motor control, it is still very important that you have an in-depth understanding of how to configure, install and apply VFDs. After all, there are some issues involved in the application and operation of VFD such as over-voltage, overload, selection, grounding problems, electromagnetic field or electromagnetic interference and poor parameter settings or configurations.
Primarily, VFDs have the capacity to prevent overload and adjust deceleration/ acceleration. These can also minimize the in rush current in motors during startup, which is crucial when controlling the maximum power load to its corresponding peak demand. So, when you specify a VFD, you need to be able to choose the appropriate drive and know the application process involved. Be sure to take into account the load’s operating profile, whether it is a constant torque application or a variable torque application.
To prevent drive overload, you need to make it a point that the peak torque demand and maximum current requirements are considered when selecting the size of the VFD. This is important since considering the horsepower alone would not be able to meet the maximum demands on the motor. In the case of an oversized motor, it is expected to be less efficient in terms of its function, as compared to the right size of motor. However, this is where the benefit of using VFDs come in since they reduce inefficiency and any oversizing penalties instead of requiring you to oversize the motor and drive.
Heat dissipation and accommodation of other related components in a motor are made possible when you oversize the VFD. But you have to realize that while having an oversized drive can reduce negative impacts, this is unlikely to work in all instances. With this in mind, it may be better to just utilize dynamic braking units equipped with large resistors, as these can facilitate the conversion of the excess amount of energy into heat. Having an additional external braking resistor also enhances the braking torque of a VFD.
When it comes to overhauling, a typical problem that may arise during the deceleration process is an over-voltage drive fault. To prevent this concern, you may simply use the right size of braking resistor since excess energy coming from the motor only dissipates as heat with the help of the resistor. Another solution, although quite expensive, is the use of regenerative drives that bring back energy to the drive’s line size. In this case, VFDs come with their own DC bus attached to a common bus. This allows other drives to utilize excess power accordingly. In terms of cost, this option is more practical, particularly when there is a high level of excess power.
It is essential to specify the right accessories that can address any noise issues during the installation and application of VFDs. There are times that you can hear electrical noise at the load size or line side of the drive, and even the drive itself can generate the noise. Recent models of VFDs are barely affected by noise on the line side, but the drive can generate harmonic noise that may impact the rest of your devices.
There is also the concern that the VFD including the motor controlled by it can result to electromagnetic interference that will affect some sensitive devices in proximity to it. To minimize EMI, you can implement the right grounding techniques. You can also use isolation transformers and power filters to address EMF or EMI concerns from the drive.