Tips for Achieving Efficient AC Motor Drive System Design

Tips for Achieving Efficient AC Motor Drive System Design

For an engineer tasked with making an efficient motor drive system, there's some bad news and some good news. The bad is, you have a lot of options to look at, and the good is exactly the same thing, plus the knowledge that you can make a really efficient motor/drive combination if you put just a little time and money into it.

Permanent magnet synchronous AC motors (PMSM) are the talk of the town, mostly because that's what electric vehicles (EVs) are using most of the time and they are working really well. Lower power versions of these same types of motors are getting less expensive and more available.

Speaking of availability, there are many, many motor manufacturers. Hundreds. And there are a few motor types to select from. Be aware that there is an even larger-than-normal batch of hyperbole out there on this subject. This is most likely because it's a really big business area. Globally, electric motor sales are projected to reach US$214.5 billion by 2025, a CAGR of 7.9% during the forecast period, according to a report by Grand View Research. This includes electric motors for applications as varied as heating, ventilation, and cooling (HVAC) equipment, vehicles, home appliances, and industrial machinery. You can find most any specification you want. They are all over the place. So, you must be diligent, check exact numbers, and if you can't find 'em, move on to another motor brand.

Three Main Motor Types

Designing products with motors 25 years ago 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 went to a brushless DC (BLDC) motor (servo motor) and an analog input controller.

All of those AC induction motors are still in use, and are sometimes still being designed in, because they are cheap and they work. But they are very inefficient. There are a few varieties of this motor type. There's split phase and the capacitor start, along with the permanent split capacitor (PSC) variations. They are similar, single-speed devices with around 20 to 30% efficiency. The PSC types are a bit better (and a bit more expensive) at 35 to 45% efficiency.

Figure 1. A standard AC induction motor. Photo courtesy of S.J. de Waard/Wikipedia Commons.

Next in the hierarchy comes electronically commutated motors (ECM) that are basically brushless DC motors with AC-to-DC conversion built-in. These will cost 60% more, be 30% smaller and 30% lighter, and have an efficiency of 70 to 85%. The old induction motor may be designed to run at decent efficiencies at a single speed/load, while ECM motors maintain high efficiencies across wide speed and torque ranges.

ECM motors provide a soft start that reduces the common "clunk" start-up noise, and motor noises in general are greatly reduced. An ECM replacement for a furnace PSC motor, as an example, provides noise reduction benefits to the homeowner in addition to greatly improved efficiency.

Last on the list is the PMSM motor mentioned at the start. It can reach a 95% efficiency over a broad speed range and has another 30% price increase over an ECM type. One worry is that the motor's magnetic materials, including high-permeability steel, neodymium-iron-boron, and cobalt-iron alloys may be subject to commodity price and/or availability pressures at some point.

The PMSM motor can generate torque at zero speed, provides smooth low- and high-speed performance, has low audible noise, and has low electromagnetic interference (EMI). Using a (quite complicated) field-oriented control scheme extends smoothness and efficiency across a very broad speed/load range. It is important to understand that various PMSM motors can have significant performance differences due to variation of materials and design. You cannot lump all these together.

The PMSM control system, with or without a rotor position sensor, can be pretty complicated. A more simple, trapezoidal type control usually works with three Hall sensors built into the motor. It may be affected by torque ripple and may not be suitable for low-speed operation.

Research Offers Some Real Numbers

Research reports from (U.S. Department of Energy, Office of Scientific and Technical Information) provide some good, hard data. The September 2019 report titled Permanent Magnet Synchronous Motors for Commercial Refrigeration (by Brian A. Fricke and Bryan R. Becker) is very application-specific—it's about refrigeration evaporator fan motors. It has some excellent, measured data on three types of motors garnered from both dynamometer and field testing.

According to the report, shaded-pole induction motors, which are the lowest cost motors, have an efficiency of approximately 25%. You'll find them in applications such as display cases, walk-in coolers, and in other commercial refrigeration uses.

Their data shows that state-of-the-art ECM motors are approximately 66% efficient. The use of these premium-priced ECM motors for commercial refrigeration fan applications began in earnest 10 to 15 years ago. The PSC motor offers a mid-point between shaded-pole and ECM motor price and efficiency. PSC motors are typically about 29% efficient.

A PMSM AC motor operating off of grid-supplied AC exhibits an efficiency of 75%, according to this study, and has the potential to significantly reduce the energy consumption of evaporator fan motors in commercial refrigeration equipment. The study also highlights the much better power factor of PMSM motors. Table 1 from the report provides a summary of evaporator fan motor efficiency. Figures 2A and 2B, also from the report, provide graphical representations of motor efficiency.

Table 1. Summary of Measured Evaporator Fan Motor Efficiency and Power Factor. Courtesy of Oak Ridge National Laboratory.

Motor Type Efficiency (%) Power Factor
6–12 W fan motors
Shaded Pole 26 0.64
ECM 63 0.61
PMSM 75 0.91
38–50 W fan motors
Shaded Pole 24 0.61
PSC 50 0.97
ECM 69 0.61
PMSM 82 0.92

Figure 2A. Fan motor efficiency and power factor for 38–50W shaded-pole motor and PMSM motors. Image 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 retrofit of one store's evaporator fan motors (262 of them), where fan power usage was reduced by 52% and the power factor greatly improved.

Two Other, Less Common Motor Types

Switched reluctance motors provide excellent starting torque and high reliability, good efficiency, and very simple construction. They can run at stall indefinitely without overheating—a feature that has enamored them with the nuclear power and safety crowd. Torque production is unaffected by motor speed. In general, however, they have had a low adoption rate because of excessive torque ripple problems that have labeled them as unacceptable for consumer applications.

Then there is one last highly efficient motor type: the axial flux motor. Its design places the permanent magnets on the face of two rotors which are on either side of the stator. The flux loop starts at a magnet on the rotor and passes through the air gap to the stator and then through the first stator tooth and to a second magnet on the other rotor. Unlike the radial flux motor, the flux path is one-dimensional, allowing the use of grain-oriented magnetic steels for greater efficiency. Efficiency is said to be 10% better than the PMSM. The short, pancake-shaped motors are envisioned for high power loads, especially EVs, and are being prototyped by a number of manufacturers.

Power Transistors and Gate Driver ICs

The high-frequency switching used with the PMSM increases power density, leading to smaller motors. Current ripple is also reduced, meaning smaller and less expensive passive components for filtering. The high-frequency operation also reduces torque ripple that can cause motor vibration and premature wear.

Wide bandgap (WBG) semiconductors like gallium nitride (GaN) and silicon carbide (SiC) can operate efficiently at higher frequencies because of lower output capacitance. They exhibit a higher breakdown voltage (above 600V) than silicon. They feature high electron saturation velocity, often called electron mobility. The higher mobility allows the devices to handle twice the current density (A/cm2) of silicon. WBG semiconductors can operate safely at higher temperatures—up to around 300°C. SiC has a thermal conductivity of 4 versus 1.5 for silicon and has become the favored power semiconductor for driving PMSM motors. Discreet and integrated SiC FET bridge modules are worth consideration.

Figure 3. An 11.5A, 900V silicon carbide power MOSFET from Wolfspeed. Image courtesy of Wolfspeed, a Cree company.

Driving these great FETs has been made pretty easy. For example, the Maxim MAX22701E isolated gate driver is a single-channel device that features ultra-high common-mode transient immunity (CMTI) of 300kV/μs (typ) and will withstand 3kVRMS for 60s. It is designed to drive SiC or GaN transistors in various inverter or motor control applications.

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

The MAX22700 and the MAX22702 have a maximum RDSON of 1.25Ω for the low-side driver; the MAX22701 is 2.5Ω. All three devices support a minimum pulse width of 20ns with a maximum pulse width distortion of 2ns.

Take A Careful Look at the Motor Landscape

Take a careful look around the motor landscape and you'll find it filled with glittering generalities and just plain un-truths. For example, many tout their miraculous motors as having 60% better efficiency than older designs—which means almost nothing. They are really comparing their very standard ECM or PMSM motor to an ancient induction split phase motor, the worst of the energy users, and 60% of their 25% efficiency is 15, so we have 40% efficiency. You can do much better.

Even the specification sheets are very often misleading. Look carefully. Call and ask about specific numbers—not general terms.