The world of motor control can be a confusing mess. With VFD commonly being interchanged with other terms such as inverter, adjustable speed drive, adjustable frequency drive, AC drive, and microdrive, we decided to clear some things up.
In order to better understand what VFD (variable frequency drive) means, and why they may be the right fit for your project; we are starting off with a quick explanation.
A VFD is both an electrical and electronic-based frequency manipulation device with the purpose of:
- Taking AC power on the supply side
- Inverting that power to a DC voltage
- Storing that voltage in the VFD
- Using an internal technology of superfast switching technology called IGBT’S that will create a ‘sine wave-like’ form that can be manipulated by “altering the normal 60 HZ frequency” to an alternate value, thereby altering the speed of a 3 phase inductive or sometimes PM type motor. (HUH?)
Sounds simple right? At least the first three bullet points did… but this is where things can get a little tricky.
While it is true that a VFD does “invert an AC line current”, what is produced by the VFD is NOT actually a pure AC sine wave. What do we mean by this? This is where customers tend to experience some confusion. It is a common belief that a VFD produces a pure AC sine wave, just like a Rotary Phase Converter (RPC), which is not the case.
What the VFD actually provides is a simulated sine wave via (PWM) pulse width modulation. The PWM output is actually just manipulated wave of DC output. In this disguised format, something like an AC INDUCTION motor cannot distinguish between the AC and DC waves.
The whole purpose behind a VFD is nothing more than matching required speeds necessary for process applications. This could vary from conveying system, fan/blower systems for pressure or flow requirements, speed necessary for spindles on machining centers and many other types of processing application used in all kinds of industry.
Which applications should VFDs NOT be used on?
- Resistive loads (Welders, Ovens, Heaters, etc)
- Traditional 1 Phase motors with caps
- Equipment with a main control panel and (internal distribution) attempt to use a VFD as a power supply.
- Applying VFD to a machine with switches directly connected to motor (VFD needs to be directly connected to the motor) Open circuits create fires for instance.
In summary, VFDs are best for applications where motor speed control is needed and you can directly connect to an AC induction motor which can handle the wave form provided by the VFD controller. Alternatively, there are options such as Rotary Phase Converters and Soft Starters available for alternative applications.