Variable Frequency Drives in Electric Actuators

BASIC DESIGN

There are many different ways to run an electric motor with variable output speeds. Variable-frequency drives (VFDs) require 3-phase voltage at the motor. This voltage must be changeable both in terms of frequency (speed) and amplitude (voltage level determining torque). However, the main supply voltage provided by a plant is constant in terms of frequency and amplitude and can be either 1-phase or 3-phase. This voltage conversion takes place in two steps—rectifying/smoothing and inversion.

During the first of these steps, the incoming plant voltage is converted into DC voltage using rectifier diodes. This voltage is smoothed further through capacitors, which are used as buffers for the energy. During the inversion stage, fast “electronic” switches connect the DC voltage in a precisely defined rhythm to the three motor terminals. By switching very quickly and selecting the proper voltage very quickly (pulse-width modulation), a 3-phase voltage system internal to the actuator is established.

WHY USE VFDs?

There are many reasons why VFDs are used, beginning with the fact they can start up slowly and consistently.

In certain applications, valves sometimes have to overcome a large differential pressure. When these particular valves are operated quickly over the entire stroke, turbulences and vortices may be created within the piping. Ultimately, this can result in large stresses and represents a hazard for valves, piping and connected components. In addition to these large stresses, other potentially undesirable or damaging side-effects include possible cavitation and water-hammer, which can spread throughout the entire piping system. These unwanted and unnecessary effects can be prevented by controlling the operating speed of the valve. By opening the valve slowly at first, pressure compensation can occur. Once the valve reaches a predefined position, the actuator can be operated at increased speeds and still meet the overall required valve stroke operation time.

VFDs also are used because they can help close tightly without over torque. Moving the valve at full-speed into the valve seat fulfills a “tight shut-off” requirement. However, this can come at the expense of a high dynamic torque for both the valve and the actuator. Once the actuator motor is shut off electronically, kinetic energy continues to apply torque to the valve until the actuator and valve ultimately come to a stop. To prevent this kinetic energy torque, brakes at the motor can be used. Still, this can create added cost and maintenance on the actuator. Actuators with a VFD can control the output speed, and therefore slow down before reaching the end position. By doing this, the kinetic energy is significantly reduced, which likely will extend the life expectancy of the valve and actuator.

VFDs also can vary speeds throughout the stroke or direction. Depending on the application, there may be advantages associated with varying the operation speed of the valve and actuator at different positions throughout the valve stroke. For example, it may be desirable to keep process flow variables such as pressure, temperature or flow rate, at a fixed level, which is sometimes known as linearizing the valve characteristics. With fixed-speed actuators, this can be accomplished through switching the actuator on and off, or pulsing the signal. Unfortunately, this can result in many startup current spikes that can cause increased heat and ultimately premature failure of the actuator motor.

A VFD can also be programmed so that the speed in the closing direction can be different from the speed in the opening direction (Figure 1). This might be desirable, for example, in slowly lowering a decanter. Such an operation would be used in sewage treatment plants to avoid swirls in the activate sludge basin followed by a faster operation to raise the filled decant arm and allow fast filling of the basin for the next process operation.

Another benefit to using a variable frequency drive is the output speed is slowed down as a set point is approached (Figure 2). By doing this, overshooting and hunting for the set point will be reduced, which equates to reduced load on the system components.

VFDs also offer different speeds for normal versus emergency operation.

Plant engineers must design systems for normal operation as well as for emergency conditions. The two situations often have two totally different scenarios for valve and actuator operations. During normal conditions, it may be desirable for the actuator to precisely and slowly modulate a valve to maximize a predefined operating parameter. This slow operation may not be desirable during an emergency condition, however. VFDs allow both operations to be accomplished easily by predefined intelligent programming. When the actuator receives an emergency input signal (or loss of control signal) different speeds can be preprogrammed for single or both directions of travel (Figure 3). It also is possible to predefine a specific speed versus a position curve for the actuator to prevent the valve from closing or opening too quickly.

VFDs can be used with various available power supplies. Occasionally, actuators need to have the ability for operating during a power failure. In the case of a conventional 3-phase actuator, it typically would not be cost effective to have a 3-phase backup power supply. Similarly, conventional 1-phase AC motors are equipped with starting capacitors that have a large starting current draw. Because of this, a big, expensive, 1-phase backup power supply would be necessary. By design, a VFD has a very small starting current. Therefore, it can convert a 1-phase backup power supply into a variable 3-phase voltage to operate the actuator motor from a small, cost-effective backup power supply.

CONCLUSION

Although they may not be needed for all applications, VFDs provide solutions that a single-speed actuator may not be able to offer. Actuators with an integral, variable frequency drive may have a slightly higher initial cost as compared to a conventional single speed actuator, but given the benefits associated with this technology, the higher cost may end up as an overall cost savings in the long run. Ultimately, understanding the specific application requirements will help determine if these drives are suitable solutions for the challenges of that application.

Justin Ledger is the marketing and business development manager at AUMA Actuators, Inc. (www.auma-usa.com). Reach him at justin.ledger@auma-usa.com or 724.743.2862.