What Is A Variable Frequency Drive

What is a variable frequency drive? Variable frequencydrives are also known as variable speed drives, VFD’s, adjustable speed drives, and inverters.

Variable frequency drives are solid state motor control systems designed to control the speed of an AC (alternating current) electric motor. Variable frequency drives operate as load controls within AC electric motor applications; and variable frequency drives can reduce energy costsup to 50% by speed reduction on electric motorswhere the full speed (RPM) of the electric motor is not required.Variable frequency drives are used in AC servo systems, air compressors, conveyor systems, lathes, mills, plastic extrusion, slitter lines, food processing, waste water treatment systems, submersible pumps, HVAC fans and blowers, and many more electric motor applications.

Many manufacturers apply AC variable frequency drives to rotating equipment because variable frequency drives reduce amperage spikes upon start up of large electric motors.Choosing the right AC variable frequency drive for an application will benefit rotating equipment by providing less wear on the electric motors where applied.Adjusting the acceleration and deceleration time of electric motors can extend the lifespan of an electric motor. AC variable frequency drives provide the ability to control the frequency of starting and stopping of an AC electric motor.This ability allowsan AC electric motor to only operate when needed for the equipment it’s rotating, and electric motors have a longer lifespan if they are not continuously operating when they don’t need to be.

Approximately one third of the world’s electrical energy is supplied by electric motors in fixed-speed centrifugal pump, fan, and air compressor applications. These fixed-speed applications hardly ever require the full load speed (RPM) of the electric motor in which they’re operating. By installing AC variable frequency drives to these applications, electric motor speeds are reduced, and power costs can be reduced by 50% or more. Technology has allowed cost and physical size reduction of AC variable frequency drives, and has improved performance through advances in semiconductor switching devices, simulation, control techniques, and control hardware and software.

Power Savings: What Is A Variable Frequency Drive

The majority of AC variable frequency drives in the market today contain electronic circuitry that converts 60 Hertz Line power into direct current. The variable frequency drive converts this line power into a pulsed output voltage that duplicates varying alternating current to a desired frequency (speed).A properly applied AC variable frequency drive when paired with an AC electric motor will significantly reduce operating costs for manufacturers. This is particularly true for variable torque loads such as fans, blowers, and pumps.Blowers are often used with dampers to control air flow; these dampers may operate either manually or automatically. When dampers are closed, 50% of the electric motor current will drop to approximately 60% of full load nameplate current. By utilizing an AC variable frequency drive in blower applications, the current draw of the motor will be reduced 30% for every 10% drop in speed. The same electric motor operating froman AC variable frequency drive at 50% speed, will draw approximately 20% of the full load current.

Example Application:

A 10 horsepower AC electric motor, rated 90% efficient, operating across the line without an AC variable frequency drive, with the dampers operating between 50 70%, for 2000 hours per year will require 11,996 KWH. If the KWH charge is $.08 per KWH, the cost to run this motor will be: $1,248.00 annually.The same 10 horsepower electric motor operating from an AC variable frequency drive, between 50 70% speed for 2000 hours per year will require 4,676 KWH. Operating cost at the same KWH rate will be: $432.00 per year. This represents a savings of $816.00 per year and should be enough to pay for the AC variable frequency drives investment and installation costs, within the first 12 months of operation.If any electric motor application operates more hours than in the above example, and/orthe KWH charge is higher, the savings will quickly compound.The energy saved on a utility bill from using a variable frequencydrive is often significant enough to pay for the variable speed frequencywithin a couple of months from installation date.Increasing and/or decreasing the start up time on an AC current electric motor via a variable frequencydrive can add years to the motor’s overall lifespan. Using a variable frequencydrive can also improve efficiency on production demands. Variable frequencydrives provide the ability to control the frequency of starting and stopping an AC electric motor.This ability provides a means by which an AC electric motor is only operating when needed. AC electric motors have a longer lifespan if they are not continuously operating when they do not need to be.

Types Of Drives: What Is A Variable Frequency Drive

Volts Per Hertz drives are the most common type of variable frequency drive and areknown as a V/Hz drives, or volts by hertz drives. V/Hz variable frequency drives are used inapplications such as fans, pumps, air compressors, and other related applications wherehigh starting torque is not required. V/Hz variable frequencydrive applications typically do not require full torque when the AC motor is operating at less than the base speed (RPM) of the electric motor. V/Hz variable frequencydrives are the most inexpensive type of variable frequencydrive. V/Hz variable frequencydrives do not provide full motor torque at low RPM.

Open-Loop vector drives are also known as “sensorless vector” variable frequencydrives. Open loop vector drives adapted the name “sensorless vector” because they do not use an external encoder for speed feedback to the motor.Open loop vector drives are used in applications where high starting torque and full torque at low speed (RPM) is required. Open-Loop vector drives operating a motor a zero RPM should not be used on crane or hoist applications. Most open-loop vector drives are used on CNC machines, mixers, mills, lathes, and other applications where high starting torque or full torque at low RPM is needed.Open loop vector drives are usually more expensive than a V/Hz variable speed drives.

Closed-Loop vector drives are used in applications where precise speed control (0.01%) is needed, or in applications where extensive programming is needed. Closed-Loop vector drives use an encoder on the motor to provide constant shaft position indication to the drive’s microprocessor. The encoder feedback allows the drive microprocessor to constantly control torque no matter how many RPM the motor is operating at. Closed-Loop vector drives are used to provide the motor to operate at full torque even at zero RPM. Closed-Loop vector drives are commonly used on hoist and crane applications because crane and hoist motors must produce full torque prior to it’s brake being released, or the load will drop and it will not be able to stop.

To learn more about variable frequency drives or for repair and replacement quotes, contact Precision Electric, Inc.