Process variables, including the pressure and flow of gases and liquids, long have been regulated using mechanical clutches, throttles, and adjustable-inlet guide vanes. These components waste energy, require frequent maintenance, and provide inaccurate control. Adjustable-frequency drives provide more efficient maintenance-free performance and more accurate control, having become the preferred method of control for variable-speed applications. Adjustable-frequency drives provide more benefits than traditional control methods, including:

  • Reduced costs and easier application via the use of rugged squirrel-cage induction motors.

  • Advanced performance via digital microprocessor control and serial communications.

  • Competitive first costs through the use of standard off-the-shelf components.

CHARACTERISTICS

Most adjustable-frequency drives consist of four basic sections (Figure 1):

  • The converter section, which rectifies alternating-current- (AC-) line input power into a direct-current (DC) bus/filter.

  • The DC bus/filter section, which smooths the DC ripple.

  • The driver-regulator section, which consists of the control, measurement, logic, and command circuits necessary to integrate drive elements into a system.

  • The inverter section, which converts the filtered DC bus into an AC output.

The voltage level, power level, and type of adjustable-frequency-drive technology determine the size and type of power semiconductors in the converter and inverter sections. Most inverter sections consist of solid-state switching devices, such as insulated-gate bipolar transistors. These transistors represent state-of-the-art technology in power semiconductor devices and have fast turnoff times that allow them to switch at rates of up to 15 to 20 khz. With insulated-gate bipolar transistors, the current waveform is nearly sinusoidal, reducing peak currents by as much as 42 percent, compared with older bipolar transistors. This results in higher available torque throughout the speed range. Insulated-gate bipolar transistors also eliminate motor noise and reduce motor losses and heating.

The components of the driver-regulator section may use analog or digital techniques. However, most adjustable-frequency-drive manufacturers take advantage of the flexibility of digital microprocessor-driven regulators.

Microprocessors allow manufacturers to include optional control schemes via software modifications. They allow additional functions to be supplied at minimal incremental cost to the user. Furthermore, microprocessors provide enhanced fault diagnostics because most fault data can be stored and viewed at a later time. These features add to a single drive design's performance and its ability to meet a wide variety of application requirements. As a result of microprocessor-based digital control, adjustable-frequency drives are resistant to damage and easy to start, operate, and troubleshoot.

Another characteristic of microprocessors that contributes to advanced system integration is a serial communications protocol for control and monitoring. Many adjustable-frequency drives use programmable-logic controllers or building-automation systems to manage data collection from peripheral controls. This allows an operator to customize drives with software-executed features and programmable parameters.

An adjustable-frequency drive with pulse-width modulation precisely controls the width of and space between DC pulses. This allows an adjustable-frequency drive to simulate a sinusoidal-shaped output pattern. In turn, the output voltage has lower harmonic content. Pulse-width modulations have a wide speed range, smooth low-speed operation, multimotor operation ability, and a high input power factor. Microprocessors provide improved modulation techniques. Higher-speed switching devices, such as insulated-gate bipolar transistors, are making adjustable-frequency drives with pulse-width modulation the standard in the ⅛- to 500-hp range.

A drive and motor must be thought of as a system in the application of adjustable-frequency drives. The duty cycle of an adjustable-frequency-drive/motor combination must be checked at all load conditions to ensure the combination is suitable for an application. Also, it is important to understand load requirements. Depending on the application, a load can be classified as a single load profile or a combination of three basic load profiles (Figure 2).

CONSTANT TORQUE

“Constant torque” implies that any speed in an operating range requires the same amount of driving torque. Conveyors are an example of a constant-torque application.

Adjustable-frequency drives with many control features are ideal for conveyor applications. With advanced factory automation, energy savings during transportation are ensured. Adjustable-frequency drives start with a low frequency and voltage that increase. Much less current and torque is needed during acceleration than during a cross-line start utilizing commercial power supplies. Therefore, an adjustable-frequency drive can eliminate a reduced-voltage soft starter.

Because less current is used during acceleration, motor heating is reduced, allowing frequent run/stop operations. For example, feed conveyors running at a constant speed consume more energy than a conveyor running only as fast as necessary. This happens even when there is no material to be fed. If a conveyor changes its speed rapidly, workpieces may be damaged. Such problems can be avoided and product quality can be stabilized because adjustable-frequency drives can change speed slowly with a soft start/stop time adjustment.

Additional benefits of using adjustable-frequency drives in conveyor applications include:

  • The use of smaller and lighter drives because motors do not require space for speed changes, such as mechanical converters.

  • The use of totally enclosed fan-cooled motors under adverse conditions, such as constant feeders with excessive or sieve dust and paint lines with adhesive compositions.

  • The elimination of maintenance from brushes and commutators not being part of induction-motor design.

  • Use with existing motors under some conditions. By applying an adjustable-frequency drive to existing motors, conveyors that operated at a constant speed can be run at stepless variable speeds.

  • Remote management of a wide range of speed control (up to a ratio of 40-to-1) by changing the frequency reference for optimum speed.

  • The elimination of conventional main-circuit contactors because phase-rotation switching of adjustable-frequency-drive transistors performs the forward/reverse operation.

VARIABLE TORQUE

Variable-torque loads often require driving torque to vary proportionally with a load's speed. HVAC systems are an example of a variable-torque application.

Centrifugal fans and pumps are sized to meet the maximum flow rate required by a system. However, in most applications, maximum demand volume is required for only a small percentage of the total operating time. Most of the operating time is spent providing 40- to 70-percent capacity. For centrifugal devices, torque varies by speed squared, while horsepower varies by speed cubed. Reducing fan speed reduces air volume and motor power consumption.

Most pumps are centrifugal. Their operation is defined by two independent curves (Figure 3):

  • The pump curve, which is a function of pump geometry and motor characteristics.

  • The system curve, which depends on the geometry of the piping and valves connected to a pump.

The intersection of these curves determines the natural operating point. If a system is part of a process that requires adjustable flow rates, then some method is needed to alter either the system characteristics or pump parameters. These methods include utilizing valves or throttling to change a system curve or utilizing an adjustable-frequency drive on a pump to modify a pump curve.

CONSTANT HORSEPOWER

Constant-horsepower loads require high torque at low speeds and low torque at high speeds. Machine-tool applications are perfect examples of such loads.

In gear-type speed changers, spindle speed can be selected only in steps so that delicate peripheral-speed constant control is not available. Although DC spindles can make stepless speed changes, they are expensive and inefficient and require regular brush maintenance. Using adjustable-frequency drives instead of gear-type or DC spindles eliminates those problems. Because a standard motor can achieve stepless spindle-drive speed changes, the clutch mechanism can be eliminated. Utilizing an adjustable-frequency drive in place of a gear-type or DC spindle provides many benefits, including:

  • Higher accuracy in cutting soft workpieces.

  • The elimination of brush maintenance.

  • The elimination of the need for improved efficiency field winding.

  • Increased machine output.

Automated grinding is another constant-horsepower application. Grinding speed requirements range from 20,000 to 180,000 rpm. At the beginning of a typical grinding cycle, a wheel moves toward a workpiece at a fast feed rate. As it approaches the surface of the workpiece, the feed rate slows to a normal grinding rate. Because of variations in part dimensions, the distance traveled to reach a workpiece at a slow rate can be significant. To overcome this hurdle, some adjustable-frequency drives offer a load-sensing circuit to produce a signal that is a function of the current drawn by the grinding-wheel drive motor. With this feature, the grinding wheel advances at a fast rate toward the workpiece until it makes contact. Within 20 to 40 milliseconds after the wheel contacts the workpiece, the adjustable-frequency drive produces a signal that adjusts the feed rate to the normal lower value.

CONCLUSION

The use of adjustable-frequency drives makes an important contribution to the efficient operation of industrial and commercial equipment. Advances in adjustable-frequency drives have led to high-tech cost-effective solutions offering substantial advantages over mechanical systems and DC drives.

The most effective adjustable-frequency drives use state-of-the-art technology. Pulse-width modulation combined with microprocessor control provides optimum induction motor control and superior flexibility. The combination of insulated-gate bipolar transistors and surface-mounted-device technology has allowed a design that is more compact, less complex, and less costly than other variable-speed-drive technologies.


Currently marketing communications manager for Yaskawa Electric America Inc., Neil Koepke has contributed to the company's technical-training, field-sales, and application-engineering initiatives for more than 12 years. He holds a bachelor's degree in technical communication from the Milwaukee School of Engineering.