Tips for Designing Efficient AC Motor Drive Systems

For engineers tasked with making efficient motor drive systems, there is some bad news, and some good news. The bad is that you have a lot of options to look at, and the benefits are exactly the same, plus the knowledge that you can make a truly efficient motor/drive combination if you invest a little time and money.

Permanent magnet synchronous AC motors (PMSM) are the talk of the town, mainly because that's what electric vehicles (EVs) use most of the time, and they work well. Lower-power versions of these same types of motors are getting cheaper and more readily available.

When it comes to availability, there are many, many motor manufacturers. Hundreds. And there are several motor types to choose from. Note that there is also greater exaggeration than normal on this subject. This is most likely because this is a very large business sector. According to a report by Grand View Research, global electric motor sales are expected to reach $214 billion by 5 billion, growing at a compound annual growth rate of 2025.7% during the forecast period. This includes motors used in a variety of applications such as heating, ventilation, and cooling (HVAC) equipment, vehicles, home appliances, and industrial machinery. You can find almost any specification you need. They are everywhere. Therefore, you have to be diligent, check the exact numbers, and if you can't find them, move on to another motor brand.

Three main types of motors

25 years ago, designing a product with an electric motor was a pretty simple proposition. Typically, you chose a single-phase AC induction motor (ACIM) for anything from 1/10HP to 100HP. If you needed to control speed, you used a brushless DC (BLDC) motor (servo motor) and an analog input controller.

All of these AC induction motors are still in use, and sometimes still designed, because they are cheap and effective. But they have very low efficiency. There are several varieties of this motor type. There are split-phase and capacitor start, as well as permanent split capacitor (PSC) variations. They are similar single-speed devices with about 20% to 30% efficiency. The PSC types are better (and a little more expensive) at 35% to 45% efficiency.

Next in the hierarchy are electronically commutated motors (ECMs), which are basically brushless DC motors with AC to DC conversion built in. These will cost 60% more, be 30% smaller, 30% lighter, and have 70% to 85% efficiency. Whereas the old induction motors could be designed to run at decent efficiency at a single speed/load, the ECM motors maintain high efficiency over a wide speed and torque range.

ECM motors offer a soft start, which reduces the common "clunky" starting noise, and motor noise is generally greatly reduced. For example, replacing a furnace PSC motor with an ECM provides the homeowner with the benefit of noise reduction in addition to greatly improving efficiency.

Last on the list is the permanent magnet synchronous motor mentioned at the beginning. It can achieve 95% efficiency over a wide speed range and has a 30% price increase over the ECM type. The concern is that the motor's magnetic materials, including high permeability steel, neodymium iron boron and cobalt iron alloys, may at some point be affected by commodity price and/or availability pressures.

Permanent magnet synchronous motors can produce torque at zero speed, provide smooth low- and high-speed performance, with low audible noise and low electromagnetic interference (EMI). The use of a (fairly complex) field-oriented control scheme extends the smoothness and efficiency over a very wide speed/load range. It is important to understand that the various PMSM motors can have significant performance differences due to variations in materials and design. You can't lump all of them together.

PMSM control systems with or without rotor position sensors can be quite complex. The simpler trapezoidal control usually works with three Hall sensors built into the motor. It can suffer from torque ripple and may not be suitable for low speed operation.

A study from OSTI.gov (U.S. Department of Energy, Office of Scientific and Technical Information) provides some good hard data. According to the report, shaded pole induction motors are the lowest cost motors with an efficiency of about 25%. You can find them in applications such as display cases, walk-in coolers, and other commercial refrigeration uses.

Their data shows that the efficiency of the most advanced ECM motors is about 66%. The use of these high-priced ECM motors in commercial refrigeration fan applications began 10 to 15 years ago. PSC motors are between the shaded pole and ECM motors in price and efficiency. PSC motors are typically around 29% efficient.

According to the study, PMSM AC motors operating on grid-supplied AC have an efficiency of 75% and have the potential to significantly reduce the energy consumption of evaporator fan motors in commercial refrigeration equipment. The study also highlighted the better power factor of permanent magnet synchronous motors. Table 1 in the report provides a summary of evaporator fan motor efficiencies. Figures 2A and 2B, also from the report, provide a graphical representation of motor efficiency.

Table 1. Summary of measured evaporator fan motor efficiency and power factor. Courtesy of Oak Ridge National Laboratory.

Figure 2B. Fan motor efficiency and power factor for 38–50W ECM and PMSM motors. Image courtesy of Oak Ridge National Laboratory.

The study included a complete rebuild of one store's evaporator fan motors (262 of them), which resulted in a 52% reduction in fan power usage and a significant improvement in power factor.

Two other less common motor types

Switched reluctance motors offer excellent starting torque and high reliability, good efficiency and very simple construction. They can run indefinitely at stall without overheating - a feature that has made them enamored with the nuclear power and safety crowds. Torque production is unaffected by motor speed. However, overall, their adoption has been low due to issues with excessive torque ripple, which labels them unacceptable for consumer applications.

Then there is the last high-efficiency motor type: the axial flux motor. Its design places permanent magnets on the surface of two rotors on either side of the stator. The flux loop starts with the magnet on the rotor, through the air gap to the stator, then through the first stator tooth and the second magnet on the other rotor. Unlike radial flux motors, the flux path is one-dimensional, allowing the use of grain-oriented magnetic steel to improve efficiency. Efficiency is said to be 10% higher than PMSM. The short pancake-shaped motor is envisioned for high-power loads, especially electric vehicles, and is being prototyped by many manufacturers.

Power Transistors and Gate Driver ICs

The high-frequency switching used with PMSM increases power density, thereby reducing motor size. Current ripple is also reduced, which means that passive components used for filtering are smaller and less expensive. High-frequency operation also reduces torque ripple that can cause motor vibration and premature wear.

Wide-bandgap (WBG) semiconductors such as Gallium Nitride (GaN) and Silicon Carbide (SiC) can operate efficiently at higher frequencies due to their lower output capacitance. They exhibit higher breakdown voltages than silicon (above 600V). They have high electron saturation velocities, often referred to as electron mobility. The higher mobility enables devices to handle twice the current density (A/cm2) of silicon. WBG semiconductors can operate safely at higher temperatures - up to around 300°C. With a thermal conductivity of 4 compared to 1.5 for silicon, SiC has become the power semiconductor of choice for driving PMSM motors. Concealed and integrated SiC FET bridge modules are worth considering.

Driving these outstanding FETs has become very easy. For example, the Maxim MAX22701E isolated gate driver is a single-channel device with ultra-high common-mode transient immunity (CMTI) of 300kV/μs (typical) and can withstand 3kV rms for 60s. It is designed to drive SiC or GaN transistors in a variety of inverter or motor control applications.

Figure 4. Functional block diagram of the MAX22701E gate-driver IC.

The MAX22700 and MAX22702 feature a maximum R low-side driver of 1.25Ω; the MAX22701 has a maximum R of 2.5Ω. All three devices support a minimum pulse width of 20ns and a maximum pulse-width distortion of 2ns.

Take a closer look at the motor layout

Take a close look at the automotive landscape and you'll see it's full of glittering generalizations and pure non-truth. For example, many people tout their miraculous motor as being 60% more efficient than older designs, which means almost nothing. They're actually comparing their very standard ECM or PMSM motor to the ancient induction split-phase motor, which is the worst user of energy, and 25% of their 60% efficiency is 15, so we're at 40%. You can do better.