VFD, motor strategies for energy efficiency
VFDs aren’t black boxes
Compared to a VFD, motors are big, dumb hunks of iron, copper wire, and insulation. It should never be forgotten that VFDs are fairly sophisticated power electronics consisting of numerous discrete components. While the service life of a motor is dependent primarily on the condition of the rotor shaft bearings and insulation system for the windings, the service life of a VFD is directly related to the reliability of the individual VFD components—any of which can cause that VFD to fail. Depending on the size and age of a VFD, it is often more cost-effective to replace the entire VFD rather than attempt to replace individual failed components.
Historically, the focus has been on minimizing motor failures when designing and specifying equipment with VFDs. This is not unusual, given the numerous horror stories of motor failure after short periods of time due to a lack of consideration to possible harmful interaction between the motor and VFD. These issues of reduced motor life caused by VFD operation can be partially mitigated through several methods: by use of true inverter duty motors that conform to NEMA MG1-2006 Part 31, shaft grounding rings, conductive bearing lubricants, and keeping motor feeder lengths short. However, a fact that is often overlooked in this emphasis on ensuring the reliability of the motor is that the cost of the VFD can approach the cost of the motor. What do we do to ensure the reliability of that VFD and protect that investment?
Just like motors, VFDs are not 100% efficient. The rectifier-dc bus-inverter design of a typical VFD is not totally loss-less. Most VFD efficiency ratings are in the mid-90% range. The losses in a VFD are primarily attributed to conduction (electrical current flowing through the device) and switching losses (the power transistors on the rectifier input and inverter output). While higher carrier frequencies in a pulsewidth modulation scheme generally result in better VFD output waveform shaping, there is a downside. VFD conductive losses are fairly consistent, but switching losses are directly proportional to the carrier frequency at which the transistors operate.
A significant portion of the input power to the VFD is wasted as heat. That same heat and the ability to properly manage it has a direct effect on the aging of the VFD's electronic components. But even with proper thermal management, the useful service life for a VFD can still be significantly less than that for the motor it serves. Many critical VFD components may require replacement in a time span as short as 5 to 10 years. These components include cooling fans, control boards, power capacitors, and more. Replacement of some components may represent a disproportionally large percentage of the cost of the VFD.
The key to reliable VFD operation is to maximize the life expectancy of the individual component by controlling temperature, humidity, and dirt/dust in the location where the VFD is installed. VFD failure modes caused by poor operating conditions are numerous. Fans and filters can clog with dust and dirt. Components can corrode due to high humidity. Poor power quality issues can fry control boards. The drive can overheat with no air circulation in high ambient temperatures. The list goes on, but the primary environmental consideration is always heat (see Figure 3).
While VFDs are designed to operate in up to a 104 F ambient temperature, the optimum ambient temperature for power electronics is 59 to 86 F. The typical rule of thumb is that for every 17 F reduction in operating temperature, the life of the device doubles. This serves to emphasize that the performance and overall reliability of the system can be dramatically impacted by something as simple as ensuring that there is adequate airflow around the VFD (see Figure 4).
We often put motors and VFDs in locations that are poorly suited to ensuring their reliable operation. Unfortunately, due to the nature of the loads that we apply VFDs to, there are often limited options as to where the VFD can be located. This may mean putting a VFD in an unconditioned pump house, on the roof in a packaged air hander. While these types of situations may represent a compromised design, it should be understood what effect this has on the reliability of the design and what additional levels of redundancy in the design are appropriate to account for this reduced level of reliability. Is a wrap-around bypass across-the-line contactor, or a separate standby VFD for emergency starting appropriate for the application?
Scalar and vector control
Assuming that we properly characterized our load type, found the right motor for the load, and determined that the load profile of the load makes a VFD useful, how do we control that VFD? There are two primary types of VFD control schemes: "Volts per Hertz" (also known as V/Hz, or scalar control) and vector control.
Scalar is the simpler (and cheaper) control methodology. The pulse width modulation used by modern VFDs to synthesize a sinusoidal output can be used to change not only frequency but also voltage. If the ratio of input voltage to frequency is known (e.g., 460 V/60 Hz = 7.67), maintaining the VFD output at a fixed voltage to frequency ratio theoretically can allow consistent torque over the entire operation speed range of a motor. At very low motor speeds/frequencies, if the voltage approached zero, the torque output of the motor would also approach zero. To address this, a fixed voltage (voltage boost) is added to the prescribed V/Hz ratio to maintain torque. As the speed is increased, the voltage boost is removed. In general, the primary benefit of this control methodology is that it is very simple and does not require direct motor feedback. If multiple motors are connected to a single VFD, this would be the likely control method. However, this lack of feedback to the drive is also one of its primary disadvantages. Using a set V/Hz ratio and no direct motor feedback, speed and torque regulation for a load becomes “best guess.” Scalar drives are generally recommended for turndown ratios of no more than 6:1. Other disadvantages include reduced ability to overspeed a motor, poor breakaway torque characteristics, and poor low speed torque—even with voltage boost features. With variable speed/constant torque load types, these downsides can often be deal breakers.
There are two forms of vector control, direct and indirect. There are two subsets of indirect, closed loop (feedback) and open loop (sensorless). All use the same basic concept. By shaping the VFD’s output voltage and frequency, we can separately address the magnetizing current (referred to as the “d” vector component) and torque producing current (referred to as the “q” vector component) in the motor's stator. Whereas scalar controls use a fixed V/Hz ratio, the ability to decouple these components and independently address them opens a greater possible range of torque and speed control. However, as the motor interacts with a dynamic load, the VFD’s microprocessor needs some feedback reference signal to ensure that the motor is providing stable speed and torque to the load. When feedback is provided to the microprocessor, it can calculate changes on the fly to the VFD output to better regulate the motor speed and torque.
In direct vector control, you would want to directly measure the motor air gap flux within the motor. However, this is not very common due to the level of accuracy and cost associated with the additional sensors needed. In closed loop indirect vector control, a shaft encoder is added to the motor to tell the VFD exactly what the position and speed of the motor shaft is. Shaft encoders are typically mounted on the non-drive end of the motor. In open loop indirect vector control, there is no sensor on the motor. Rather, the VFD compares its output current to the motor and compares this to a mathematical model of the motor to determine if adjustments to the output are required. Since this is only an approximation of speed based on a model and not a direct measurement, speed regulation, and overall operational speed range, is not as good as with closed loop control.
Vector control methodologies are typically used to accommodate high turndown ratios (very wide operational speed range) or where precise torque control is required. Where a motor is required to produce rated torque at zero speed and “hold” a load for a period of time, there is typically no choice but to use a vector drive.
John Yoon is the senior staff electrical engineer for McGuire Engineers. He has nearly 20 years of experience in the design of electrical distribution systems. His project experience covers a broad spectrum, including high-rise building infrastructure renewal programs, tenant build-outs, mission critical data centers, good manufacturing practice (GMP) cleanroom facilities, and industrial facilities.