Variable frequency drives

Variable-frequency drives, or VFDs, are a means of controlling the speed of an ac motor, typically an ac induction motor. It does so by varying the frequency of the power source.

VFDs operate by switching their output devices, which can be transistors, IGBTs, or thyristors, on and off. VFDs can be either constant voltage or constant current. Constant voltage types are the most common type of VFD. It uses pulse width modulation (or PWM) to control both the frequency and the voltage applied to the load.

Why use VFDs? They are a powerful way to control the speed of ac induction motors and are fairly simple and easy to use. Among the benefits of using a VFD for motor speed control is the actual energy savings. Controlling the amount of current drawn by the motor can save a lot on energy costs because the motor will not run at full current all of the time.

Especially since Congress passed the Energy Independence and Security Act of 2007 (EISA), motor efficiency has become a top priority. For instance, single-phase induction machines (specifically, permanent split-capacitor motors) and universal motors, widely used in industrial washers, are managed with simple voltage-control techniques. Contrast this with high-end, high-performance machines where three-phase motors are more common and which use VFDs.

Switch reluctance motors (SRMs) are not yet an appropriate alternative because their control schemes are still evolving, but three-phase motors are readily available and may be a smart choice because their VFD control techniques have improved significantly. More important, VFD electronics costs have been dropping as well, making them more cost-effective.

In the same way, an OEM using a universal motor with simple triac control may now find that a three-phase VFD control will provide better energy efficiency, while OEMs using three-phase/VFD configurations may make the move to technologies like brushless dc motors.

Another advantage of VFDs is seen on motor start-up. Without a VFD, an induction motor on start-up has to handle a high initial in-rush current. As the motor speeds up and approaches a constant speed, the current levels off from the peak in-rush values. So with a VFD, the motor’s input starts off with low voltage and a low frequency, avoiding the problem of high in-rush currents.

Of course, the main reason any kind of speed control is used on motors is to gain greater and more precise control over motor speed and therefore adjust the motor speed to meet the requirements of the load and also to reduce energy costs.

Another benefit of using a VFD for motor speed control is the reduction of mechanical wear on the motor components. Eliminating the in-rush currents upon start-up gets rid of the excessive torque on the components, and thus increases the life of the motor and reduces maintenance costs and the need for repair.

There are some drawbacks to using VFDs, however. The main one is the possibility of harmonic distortion which can effect the power quality as well as the operation of other machinery. However, VFD manufacturers have been developing solutions that mostly eliminate this problem.

Bison Gear’s TightDrive™ Motor-Mounted Speed Control

St. Charles, IL — Bison Gear’s new TightDrive™ speed control for permanent magnet DC (PMDC) motors was designed for applications that require a convenient location for the control … on the motor. The majority of PMDC gearmotors and motors used in industry today require speed controls in order to operate. Typically, the control is wired to the motor through a costly, complicated cable system and the motor and control can be many steps away from each other. Bison’s voice of the customer research brought forth the need for a convenient “point of use” motor speed control that was robust for industrial environments, while offering practical features and exceptional value.

Bison Gear & Engineering’s new TightDrive™ motor-mounted speed control can be easily field mounted on Bison PMDC gearmotors up to 1/6 horsepower (124 watts) and offers a 20:1 speed range with maximum output of 90 Volts. The TightDrive™ is housed in a durable aluminum extrusion which offers NEMA 1 (IP 30) protection and superior heat dissipation. Speed is controlled with a combination on/off switch and speed potentiometer. In addition, three easily accessible adjustable potentiometers provide settings for minimum RPM, maximum RPM, and current limiting. The simple, yet innovative, SCR control architecture also provides much tighter speed regulation than alternative controls.

The TightDrive™ is designed for 115 volt 50/60 Hz operation and comes complete with three foot power cord and NEMA 5-15P plug. Users can mount the drive directly to the motor in 90° increments to optimize position of cord exit and motor leads. The new TightDrive™ speed controls are available for immediate shipment from Bison’s distribution network or direct from the Bison Gear web site.

www.BisonGear.com

Bison Gear’s TightDrive™ Motor-Mounted Speed Control

St. Charles, IL — Bison Gear’s new TightDrive™ speed control for permanent magnet DC (PMDC) motors was designed for applications that require a convenient location for the control … on the motor. The majority of PMDC gearmotors and motors used in industry today require speed controls in order to operate. Typically, the control is wired to the motor through a costly, complicated cable system and the motor and control can be many steps away from each other. Bison’s voice of the customer research brought forth the need for a convenient “point of use” motor speed control that was robust for industrial environments, while offering practical features and exceptional value.

Bison Gear & Engineering’s new TightDrive™ motor-mounted speed control can be easily field mounted on Bison PMDC gearmotors up to 1/6 horsepower (124 watts) and offers a 20:1 speed range with maximum output of 90 Volts. The TightDrive™ is housed in a durable aluminum extrusion which offers NEMA 1 (IP 30) protection and superior heat dissipation. Speed is controlled with a combination on/off switch and speed potentiometer. In addition, three easily accessible adjustable potentiometers provide settings for minimum RPM, maximum RPM, and current limiting. The simple, yet innovative, SCR control architecture also provides much tighter speed regulation than alternative controls.

The TightDrive™ is designed for 115 volt 50/60 Hz operation and comes complete with three foot power cord and NEMA 5-15P plug. Users can mount the drive directly to the motor in 90° increments to optimize position of cord exit and motor leads. The new TightDrive™ speed controls are available for immediate shipment from Bison’s distribution network or direct from the Bison Gear web site.

www.BisonGear.com