While controlling flow or output in systems involving fans or pumps may seem straightforward, it involves carefully considering various factors such as flow rate, pressure, timing, and energy efficiency. Proper control methods are vital to achieving optimal system performance.
For example, flow and velocity must be synced in pump applications and fans to meet the specific requirements of any custom system. Some standard methods for controlling flow include adjusting the position of a valve, altering the angle of impellers or blades, and using variable frequency drives (VFDs) to adjust the motor speed and, consequently, the flow rate.
Variable Frequency Drives (VFDs) control motor speed in pumps and fans. It is a highly efficient method with several advantages over traditional throttling with valves. Nick Pittaoulis, Territory Manager for Wistex, says, “VFDs provide precise control, which allows you to match the system’s exact flow or output requirements. With VFDs, it’s possible to change motor speed to meet the demand.”
Wistex Builds Custom VFDs and Control Panels
Wistex is an electrical distributor servicing the Philadelphia, New Jersey, Delaware, Maryland, and New York area. “We’re an industry-leading panel shop, and we build a lot of controls in-house,” Pittaoulis notes. “We also service motor control systems and offer protection components, including switches, fuses, and breakers. Finally, we create specialized batteries and battery backup systems. If you need an electrical control system, such as a cabinet to control a system for lighting systems, conveying systems, or almost any kind of industrial, commercial, life science, or municipal market, Wistex can create a customized solution.”
Wistex products feature parts from various manufacturers like SOLA ABB , Toshiba, Eaton, WEG, but they also build custom solutions designed around a customer’s specifications. “For example,” Pittaoulis adds, “a customer might need a special kind of enclosure or specialized functions or features on a device that a manufacturer doesn’t offer. In these cases, we can engineer it in-house and build it as part of any package solution. Wistex builds and tests everything before it leaves our door so that customers can just hook it up and be up and running.”
The Importance of Selecting the Right VFD
When selecting a motor and drive, it’s essential to factor in the application’s requirements, including torque, speed, and current draw. “At Wistex, we choose the right equipment for your needs rather than relying solely on horsepower ratings,” Pittaoulis explains. “Some people size a motor and drive based on horsepower. That’s a common mistake because motors have different speeds, resulting in a torque difference. A higher number of poles produces pulls requires higher torque, which in turn requires higher current. For example, a pull motor with 3600 base RPM has a low torque with a fast speed. So, it also requires a low amount of current. Consequently, Wistex sizes drives based on the Full Load Current (FLC) or Full Load Amp (FLA) of the motor.”
The most important consideration at Wistex is selecting a correctly sized VFD for the motor. The second consideration is the application. “The application can be variable or constant torque,” says Pittaoulis. “With variable torque, the load will vary as the speed increases. With constant torque, the torque and the load stay constant throughout the speed range, factoring in the overload duty cycle.”
He continues, “The drive should be able to handle the load based on the overload duty cycle in a variable or constant torque application. To enable VFDs in variable torque to perform properly, they should be able to handle 100% of the current continuously, but they should also be able to handle 110% of the full load current intermittently. For example, it’s not uncommon at Wistex to work with an HVAC variable torque application that requires 110% overload for 1 minute, every 10 minutes of operation.”
This kind of overload consideration also applies to industrial applications. “For example, with a mixer or crane conveyor, constant torque would require overload,” Pittaoulis notes. “Wistex may want to size the drive for 150%, or even 200% overload for one minute, based on the duty cycle. How you incorporate overload amps and how you size the drive for the application is critical. That’s why we often do the torque calculations, first considering the speed or fan curve and designing the motor based on that.”
V/f Controls
Volts per Hertz (V/f) control is a standard method used in Variable Frequency Drives (VFDs) for motor control. V/f control is a widely used approach for controlling the speed of AC induction motors. “VFDs have multiple control modes. Volts per hertz is a linear form of control,” says Pittaoulis. “The drive is putting out a linear voltage at a fixed frequency or wavelength. Both voltage and frequency control are based on zero to 60 hertz, although you can run faster in some applications. You’re just controlling and putting out linear volts per hertz ratio. It’s typical in variable torque applications.”
Sensorless Vector Control
Sensorless vector control, or “vector control without an encoder,” is a sophisticated control technique used in Variable Frequency Drives (VFDs) to precisely control motor speed and torque without needing an encoder or other external feedback devices. “It uses feedback based on the calculation of the current and what the current is doing, which is kind of a direct correlation of the load,” Pittaoulis explains. “When we build these at Wistex, we make sure they adjust the frequency and the voltage to maintain a set speed or set current value in terms of torque. This means that torque, or speed, is controlled without an encoder, telling the VFD driver exactly what speed to run. The drive uses an algorithm to measure feedback based on the speed it’s running and adjusting, slowing up and speeding down as needed.
Closed Loop Vectors
The closed loop vector is an encoder feedback to the drive, enabling more precise feedback and adjustment. Because it can operate at levels as fine as 0.1% or even better on accuracy, a closed loop vector allows for extremely tight control over motor performance. “In a Wistex closed-loop control, an encoder is used to provide continuous feedback to the drive about the motor’s actual speed and position,” Pittaoulis adds. “It’s always precisely monitoring RPM because the encoder is counting revolutions, feeding back to the drive. That means operators have precise speed control and precise torque control. That might be important when there’s a conveyor that always needs to run at the same speed, such as a mixer that needs to maintain a constant viscosity.”
Want to Learn More About VFDs and Control Panels from Wistex?
Coward Environmental Systems, Inc. works with Wistex to create custom solutions for a variety of projects nationwide. Contact us today to discover how Wistex can be a part of your next project.
