21/02/2017
The modern process control demands more versatility in the equipment operation. Sometimes the process will demand more draft from a fan, or reduce a belt speed, others increase the feed from a rotary valve. These are some examples of the use of the Variable Frequency Drives in the industry.
VFD is the acronym for Variable Frequency Drive and there are multiple applications for a VFD in the industry. Their use has been increasingly spreading in different areas as the technology and price make them more accessible for the general costumers.
How a VFD actually works? It controls the speed of the motor by modifying the base line frequency.
The motor speed is determined by the equation:
Ns = 120 (f/p)
Where Ns is the synchronous speed, f is the line frequency (60Hz America, 50 Hz Europe) and p is the number of pole pairs that the motor has.
Without a VFD the standard (60 Hz) motor speeds for two poles is 3,600 RPMs, four poles 1,800RPMs, six poles 1,200 RPMs, etc. So, how can we modify the speed of the motor? What the VFD does is to decrease or increase the line frequency. For example, to get a 50% of the speed of a motor, the equivalent frequency is 30 Hz.
A VFD has four major components: Rectifier, DC-Link, Inverter, and Controller. The incoming power lines go to a Rectifier that converts the sinusoidal voltage into a Steady DC output with the use of diodes. The DC-Link tries to keep a steady and free of spikes voltage with capacitors and inductors. The inverter takes the voltage from the DC-Link and converts it to a sinusoidal wave again but not necessarily back to 60 Hz using IGBTs. The Controller monitors and direct every single step of this process and interacts with the control interfaces (communications, HMI, etc.).
Maybe all this seems a little bit confusing, let’s get back to the real world. Have you ever seen a Car Inverter? They feed by a cigarette lighter plug and it converts the DC Voltage from the car to 120 VAC. In a Car Inverter, you can plug any kind of low power electric component (i.e. laptop) as in a regular plug. The difference with a VFD is that they have a previous rectifier step and they can control the output frequency.
We talked previously about diodes, capacitors, inductors and IGBTs, all of them are electronic components and some are semiconductors. One of the biggest reasons the VFDs are getting popular is because of the increase of the reliability and the price drop of power electronic components.
What a VFD can actually do? It can control the start acceleration; run a motor from a different range of speeds and slow down when the process pushes the motor to accelerate.
All the power that the motor is going to demand will pass through the power electronic elements of the VFD. If the load has peaks, then the VFD will need to be rated to a higher load. This is necessary because the internal elements have current and power limits. The life of the power electronics elements is reduced if they are demanded to their maximum capacity causing the VFDs to go bad.
Understanding how a VFD operates is the first step to find applications where they can be useful. A VFD is a tool that can help to make processes more efficient as they become more versatile and accessible to the general industry.
