A Comprehensive Guide to Variable Frequency Drives

Variable frequency drives (VFD) are used in appliances and compressors of all sizes. They can help make electrical motors more efficient. They work by managing the frequency and the voltage so that they use energy in the most efficient manner possible. They are being used more and more as the development of the technologies has made the drives smaller and improved their performance. Here are some things to know about variable frequency drives.

AC Motor
Usually the motor for a variable frequency drive is a three phase induction. This is because such motors are generally the most economical. Less commonly used are wound rotors, permanent magnets, and synchronous motors.

A VFD has a controller that runs because of three sub-systems. Those sub-systems are the converter, the DC capacitor, and the inverter assembly. The converter stage is where the AC voltage is converted to DC voltage. The DC capacitor assembly and the inverter assembly work in tandem to initiate the three phase voltage. In 1983, the insulated gate bipolar transistor made it so that a VFD cost less and improved its amperage ratings.

VFDs have an extensive operator interface. It can start and stop the motor, as well as adjust the speed. There are types that allow for reversing the motor and even switching from manual control to automatic. VFDs are operated by either a computer network, digital controls, or the operator interface controls on the unit.

Drive Operations
A VFD will have three options for driver operations. Single quadrant operations are most common, and involve using constant or variable torque. Two-quadrant loads are needed when the torque alters its polarity but the speed is constant. For the highest performance, a four-quadrant load can handle speeds and torques that change direction and force.

Types of Variable Frequency Drives

Controlled Source Inverter
A controlled source inverter (CSI) works by using a SCR converter to convert all the incoming power to DC. CSI have a large internal inductor so that they can work efficiently and properly to handle the incoming power load. The advantages of this type of VFR are that the circuitry is relatively simple, they are very reliable, and they are capable of regeneration. Some of the drawbacks are that it relies on that large conductor, which is expensive, and that it is not compatible with multi-motor applications.

Voltage Source Inverter
Voltage Source Inverters (VSI) are similar to CSI in that the prime function is the covert power to DC by using a SCR converter section. However, VSI drives cannot regulate current, which the CSI drive can do. The VSI can create an adjustable frequency output for motors.

Pulse Width Modulation
The pulse width modulation (PWM) design is the most common drive controller. It can work with the largest variety of motors, and is very reliable. It is the most sold by manufacturers as well, so there are many brands from which to choose. As opposed the the VSI and CSI drives, PWM converts AC voltage to DC voltage. It does this by using an inverter section that also has gate bipolar transistor that are insulated. This creates a near-sine wave output. These drives are a low cost upfront, are compatible with multiple motors, and can operate through a power loss. The only issues are that it cannot regenerate, and can break down because of the switching transistors.

Benefits of Variable Frequency Drives
VFDs come with a host of benefits. For one, they are very energy efficient, in particular with regards to centrifugal load service. Estimates show that well over 60% of all used electrical energy is consumed by power motors. VFDs and other efficient technologies help to reduce that energy footprint.

Performance Control
VFDs allow you to regulate the amount of power a motor delivers. With fixed speed drive, a motor might be delivering more power than is needed. To have better process control, VFDs will regulate that power and only use as much energy as is required.

Now you know about the different types and benefits of using VFDs. Make sure to choose the right one for the job you have to do.

If you want your HVAC system to run more efficiently, the experts at Corporate Mechanical can help. Reach out online or call 763-533-3070.

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