Types of Variable Speed Drives
Variable speed drives (VFD’s) are solid state motor control systems that are mainly used for industrial manufacturing. As the name suggests, their responsibility is to control the speed of an electric motor. Variable speed drives can reduce energy costs by up to 50% through speed reductions where an application is not required to operate at full speed.
Variable speed drives are generally used for pump, fan and air compressor applications and are frequently used to increase or decrease the start up time for alternating current (AC) electric motors. This will add years to the lifespan of an AC motor and can also improve efficiency on production demands. The ability to control the frequency of starting and stopping an AC motor means that the motor is only operating when needed, saving on energy and wear.
Not all VFD’s are the same and it is important to know which type of variable speed drive will be best suited to your needs. Varying variable speed drives have differing capabilities and it is important that you ensure that your drive isn’t over or under specified.
Types of VFDs
The V/Hz (volts per hertz) is the most common type of variable speed drive. They are applied to applications such as fans, pumps, air compressors and other applications where a high starting torque is not required. V/Hz VFDs applications generally do not call for full torque when the AC motor is operating at less than the RPM of the electric motor. The V/HZ variable speed drive is the most inexpensive type of variable speed drive.
The Open-Loop vector drives, which are also known as ‘sensorless vector’ variable speed drives, are used in application where high starting torque and full torque at low speeds is required. The name ‘sensorless vector’ was adopted because the drive does not use an external encoder for speed feedback to the motor. The Open-Loop vector drives are generally used for mixers, mills, lathes and other applications that require high starting torque or full torque at low RPM. Open-Loop vector drives are more expensive than V/Hz variable speed drives.
The Closed-Loop vector drives are used in applications where precise control speed is needed. Through the use of an encoder on the motor the drive can provide constant shaft position indication to the drive’s microprocessor. This allows the drive microprocessor to constantly control torque no matter how many RPM the motor is operating at. Closed-Loop vector drives are normally used for hoist and crane applications.
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