Selecting and applying VFDs
A wide variety of variable frequency drives (VFDs) are available, and are sometimes referred to as inverters, ac drives, variable speed drives, or adjustable speed drives. Despite the differences in terminology, these components are all VFDs and control an ac motor’s speed and torque by adjusting the input frequency and voltage to 3-phase ac induction or synchronous motors.
Modern VFDs have made changing the speed of a 3-phase ac motor simple, but it has not always been that way. Even though 3-phase ac induction motors were invented in the late 1880s, for almost 100 years, operating an ac motor at more than one or two speeds was difficult. The number of magnetic poles and an ac motor’s physical construction made a variable speed ac motor drive cost prohibitive, so instead, dc motors were used in variable speed applications.
In the 1980s, VFD technology started becoming less expensive and more reliable. Today, the VFD competes well with traditional dc motor control, but when specifying VFDs, a clear understanding of the application, installation methods, and configuration is critical. Common issues with VFD application, operation, and configuration include:
- Drive selection
- Drive overload
- Drive overvoltage
- Sources of noise
- Electromagnetic interference (EMI)/electromagnetic field (EMF) problems
- Grounding issues
- Incorrect configuration and/or parameter settings.
The main function of a VFD is varying the speed of a 3-phase ac induction motor. VFDs also provide overload protection, start and stop control, and adjustable acceleration and deceleration. Programmable acceleration and processor-controlled current limiting can reduce motor inrush current at start-up, an important feature for controlling a factory’s maximum instantaneous power load and corresponding peak demand, which is often used by the utility company to set rates or surcharges.
When specifying a VFD, it’s important to understand the application and select the drive accordingly (see Figure 1). The operating profile of the load must first be considered. With both constant torque applications, such as conveyors (see Figure 2), mixers, and compressors-and variable torque applications, such as pumps, fans, and blowers-careful attention must be paid to overload ratings.
For example, attempting to drive a fan motor faster than its base speed can significantly impact the amount of power required as the fan horsepower varies with the cube of the speed. Running a fan too fast can thus consume excess power and may overload the VFD, while running it at half speed can reduce horsepower requirements by 75% or more, per the affinity laws, which apply to pumps and fans.
Many applications can take advantage of this reduced power consumption at lower speeds to save energy. An example is using a VFD to vary fan speed to match the load, instead of using dampers to reduce airflow from a fan running at full speed.
To avoid the possibility of drive overload, the VFD should be sized based on its maximum current requirements and peak torque demand, as sizing by horsepower alone may not satisfy the maximum demands placed on the motor. Although most VFDs can handle a wide range of horsepower, oversizing is advised when limits are approached.
An oversized motor is less efficient than a properly sized motor, but a VFD helps to minimize this inefficiency, reducing the oversizing penalty to little more than the initial cost for oversizing the drive and motor.
See more on next page about over hauling loads, VFD noise, and drive configuration.
Another application that can cause issues is an overhauling load—a high inertia load that must be slowed faster than what would occur when coasting, or a load that back-drives the motor during normal operation. When overhauling loads are present, the motor becomes a generator and the energy produced must be dissipated. There are multiple options for handling this type of load.
In some instances, an oversized drive will help, but this works only in marginal cases. A more common solution is to use dynamic braking units with large resistors that convert the excess energy into heat. While some VFDs can produce up to 20% braking torque with their built-in resistors, adding an external braking resistor can greatly increase a VFD’s braking torque. Larger VFDs typically require external braking units to accommodate overhauling situations.
A common issue in overhauling situations is an overvoltage drive fault during deceleration. However, a properly sized braking resistor can eliminate these overvoltage faults as the excess energy generated by the motor is simply dissipated as heat through the resistor.
Higher end, more expensive solutions include regenerative drives that feed excess energy back to the line side of the drive, and common-bus drives. In common-bus systems, each of several VFDs has its dc bus connected to a common bus so other drives can use the excess power generated by the overhauling drive. These two types of drive systems can be very cost-effective when the amount of excess power generated from overhauling is high.
Where’s the noise?
As part of a VFD application and installation, proper accessories must often be specified with the drive to deal with noise issues. Electrical noise can be present at the line side and/or the load side of the drive from external sources, and it can also be created by the drive. Existing noise on the factory or line side of the drive generally does not affect modern VFDs. However, the drive itself can create harmonic noise on the line side that may affect other devices in the facility.
For most applications, the installation of a line filter upstream of the VFD is worth the expense. At a minimum, one should consider leaving room in the control enclosure for filters, reactors, or drive (isolation) transformers—just in case they are found to be needed after installation (Figure 3).
A VFD and the motor it controls can create EMI that can affect sensitive nearby devices, particularly analog wiring and circuits. Using proper grounding techniques goes a long way toward reducing EMI. Using a common power circuit and single-point grounding can eliminate ground loops that occur when pieces of equipment are connected to more than one grounding path. Power filters, line filters, line/load reactors, or even isolation transformers may also be necessary to minimize EMI/EMF from the drive.
Although not typically considered electrical noise, VFDs can also create disturbances on the motor cabling, the most notable being harmonics and reflected waves. Harmonics are caused by the high switching frequencies of the insulated-gate bipolar transistor that produce the pulse width modulated output from the VFD to the motor. Load reactors may be necessary to minimize harmonics on the output side of the drive, as these harmonics can reduce motor efficiency.
Reflected waves on the drive-to-motor cabling can effectively double the voltage that reaches the motor at a given point in time. This can produce potentially damaging voltage stress on the motor insulation. Installing load reactors on the drive output cables is recommended, particularly if cable distance is greater than 125 ft. Specifying motors with proper insulation ratings also helps prevent reflected wave problems. For example, inverter duty motors with 1,000-V insulation rating or higher should be used if running at a high line voltage such as 480 Vac (or 575 Vac commonly found in Canada).
There’s no excuse for failing to enter the correct motor nameplate date, but it often happens. Considering that the VFD also provides overload protection for the motor, improperly entered nameplate data can cause a variety of faults or even lead to motor damage.
If there is a problem due to bad nameplate data or other issues, it can often be located by checking the fault code on the drive’s display. Older drives may display somewhat cryptic codes, so quick access to the operating manual is a must for translating codes into actionable information. Newer VFDs usually display fault information as text in English (or other languages if available and if so configured) instead of alphanumeric codes, greatly simplifying troubleshooting (Figure 4).
There are dozens of VFD configuration parameters that must be understood. Although many parameters work fine at their default settings, it’s a good idea to read the manual and adjust parameters to optimize drive operation. Typically, suppliers can assist in this area because they are familiar with the nuances of their products.
At a minimum, a new VFD should be programmed with the motor nameplate data (full load current, rated voltage, and speed), desired control mode (keypad control, 2-wire, 3-wire, or network communications), and desired speed reference (0 to 10 V, 4-20 mA, keypad, network communications, etc.).
A common configuration parameter setting is activation of the auto-tune algorithm in a vector drive, a feature that often increases efficiency and improves control. Understanding these configuration parameters and settings, and adjusting as necessary from default values can ensure proper operation, maximum efficiency, and optimal control.
Performing a few simple checks can help ensure effective application of VFDs. For any application, specifying the correct input voltage and understanding the nature of the load are critical. By understanding the load, most overcurrent conditions can be easily eliminated. For most applications, effective operation can be achieved by performing the following checks:
- Ensure the input voltage is correct
- Understand the nature of the load
- Eliminate overcurrent conditions
- Stretch out acceleration and deceleration time if possible
- Get/keep the noise out of the VFD-and the rest of the plant.
Ease up on aggressive acceleration and deceleration ramps to reduce overload or overvoltage faults and save energy. Check the drive display and address any recurring faults. Follow good noise reduction techniques.
Careful attention to drive selection, parameter settings, noise sources, grounding issues, overload conditions, and overvoltage conditions will ensure successful drive operation. With proper understanding of common drive issues, the application can withstand changes to the motion profile and motor speed. By following these recommendations, modern VFDs will offer years of trouble-free service.
Chip McDaniel works in technical marketing for AutomationDirect and is a graduate of Georgia Tech. His 30 years of experience in the industrial automation field includes designing, building, and commissioning control systems of all types.
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