Driving greater efficiency through a variable speed future
Adding a variable frequency drive (VFD) is easier than it used to be. Because of the high power factor and efficiency of permanent magnet (PM) motors, it is important to select a VFD based on the motor horsepower, not on motor current.
In the world of energy efficiency, the variable frequency drive (VFD) is king. Thanks to the help of improvements in component quality and miniaturization, adding a VFD to a system is a fairly simple choice. At around 98% efficiency, today's VFDs are easily the most efficient piece of the energy conversion chain, in which a VFD changes electrical to mechanical energy. As a matter of fact, adding a VFD to a system may not only help to reduce energy usage through varying the speed of the motor, but may also improve power factor—when used properly—reduce maintenance costs, and increase the throughput and quality of the end user's product.
These days there is more pressure coming from all sides to increase energy efficiency and reduce total energy expenditures. VFDs have come about as far as they can in terms of energy efficiency, short of a breakthrough in room temperature superconductors. Because of this, VFDs are only one piece of the energy-efficient future. Newer, more efficient, motor technology is one other key piece-premium and super premium efficiency induction motors, permanent magnet synchronous motors, and the relatively new synchronous reluctance motors.
Think HP, not current
Sizing and selecting a VFD for use with induction motors has always been a pretty straightforward task: Provide the motor voltage and motor nameplate HP, or preferably motor nameplate current to the preferred VFD vendor. The latest amendment (2014) to the Energy Policy and Conservation Act of 1975 pushes minimum electric motor efficiency higher, while covering more motor types than ever before. New, higher efficiency motor options, however, don't impact sizing and selecting a VFD for the application. What it means, instead, is that the same rated HP motors use less current. Depending on the current, this can sometimes result in a smaller VFD required to do the same job because most VFD manufacturers specify their horsepower ratings based on the National Electrical Code (NEC) table of full load currents for induction motors. These tables have current values that are often higher than today's induction motors, especially compared to premium and super premium efficiency motors.
Rotor losses in induction motors are difficult to design out without becoming increasingly more expensive, thus limiting this technology's efficiency. To get motors to the next step in efficiency, motor manufacturers created the permanent magnet synchronous motor (PM motor). PM motors, which typically have higher current values than the premium efficiency specifications, have a power factor that is very close to unity. PM motors have historically been used for specialized applications that require either very high operating speeds or high torque at lower speeds. Today, however, it's not unusual to find an increasing number of PM motors being installed in HVAC systems due to the increased efficiency through the speed range. Due to the high power factor and efficiency of the PM motors, it is important to select a VFD based on the motor horsepower instead of motor current.
When selecting a PM motor for an application, it is important to understand that they require VFDs to operate. Some motor manufacturers are investigating across-the-line starting of PM motors, but they are, as of yet, unavailable in the marketplace. In addition, PM motors operate differently from induction motors so the selected VFD requires a specially designed motor control algorithm. This algorithm requires a bit more processing power so only a few VFD manufacturers offer PM motor control in compact/micro class VFDs. Some VFD manufacturers also require specifying PM motor type under control (surface magnet or interior magnet); some have found it difficult to control all PM motors with only one motor control algorithm while other VFD manufacturers can handle the challenges of PM motor control without any issues whatsoever.
Lastly, to get the most out of a PM motor, the VFD should have a motor identification feature (sometimes referred to as motor ID or motor autotune) that physically runs the motor through the speed ranges and measures key points along the speed curve. A motor identification feature that only takes a standstill measurement will not optimize the VFD properly, which will result in sub-par motor control, especially at low speeds. A manual motor tuning process is possible with PM motors, but it is mostly trial and error where the built-in motor identification function in the best VFDs requires no further tuning.
In the past, PM motor control was only a closed loop/encoder feedback affair, but the open loop, motor control algorithms have come a long way, so more basic applications don't require encoder feedback for accurate and efficient control. This allows for wider application of PM motors and VFDs in HVAC systems, where the motors are often run at lower speeds; for induction motors, the efficiency tends to fall off. Combined with the affinity laws already in play with variable torque pump and fan applications, the payback time on these installations is reduced even further from the already impressive 6- to 12-month span. If your application requires a wider speed range from the motor, such as cranes or hoists, pairing the PM motor with a premium class VFD capable of encoder feedback is required.
Synchronous reluctance motor
The last motor type to consider in an application is the newcomer to the market. Developed as an attempt to bridge the efficiency and low-cost worlds that the PM motors and induction motors play in, respectively, the synchronous reluctance motor (SynRM) is a motor that uses a special rotor design to improve motor efficiency while eliminating the potentially costly and accurately named rare earth materials which are so integral to PM motors. The rotor is designed to take advantage of the reluctance—or magnetic resistance—property by creating air slots in the rotor to help the rotor tend toward aligning with the stator fields. By adding more air slots in a given quadrant of the rotor, the magnetic field of the stator has a stronger pull allowing for more torque in a smaller footprint. Because of the rotor's air slot design, there's no current flow, which allows for a reduction of rotor losses and allows these types of motors to reach, and even exceed, Super Premium Efficiency guidelines when paired with a VFD.
As with PM Motors, SynRM machines require a VFD for operation, but the algorithm developed for PM motors doesn't just work out of the box for SynRM machines. Only a small handful of VFD manufacturers so far have developed the motor control algorithms needed to use the SynRM machines in the most effective way. Because of this, it's important that when you contact your preferred VFD manufacturer to select a VFD for your SynRM machine you inform them which type of motor you're running. Also, since SynRM machines have a lower power factor, the selected VFD should be based on motor voltage and motor current and not motor HP, otherwise the VFD may not have the current capacity to run the motor properly. SynRM machines are ideal for applications below 300 hp for variable torque pump and fan applications or for open loop, constant torque applications.
- Sean Gaffney is senior product manager, Vacon. Edited by Eric R. Eissler, editor-in-chief, Oil & Gas Engineering, firstname.lastname@example.org.
- When selecting a PM motor for an application, it is important to understand that it requires VFDs to operate.
- VFDs should have a motor identification feature that physically runs the motor through the speed ranges and measures key points along the speed curve.
When selecting a VFD, make a decision based on horsepower and not current.
- See related stories about variable frequency drives (VFDs) below.