A new approach to improving energy efficiency in electric motor design

As energy conservation becomes a global concern, the energy efficiency of motor design has also become a concern. As governments around the world have introduced various regulations to require improved energy efficiency, motor drive circuits have become increasingly complex to meet the energy efficiency challenge. This article discusses product and industry trends in motor drive circuits and provides solutions that help designers reduce energy consumption, improve reliability, reduce component count, and achieve environmental protection.

Types of motors in the industrial sector

For nearly 100 years, electrical appliances have mainly used induction motors, even variable frequency drive appliances. Now some new appliances are beginning to use motors with higher efficiency, more compact size and lighter weight. These new motors can be divided into two categories, namely brushless DC motors and switched reluctance motors.

Many common household appliances use brushless DC motors with variable frequency drives. These efficient, general-purpose motors have high torque density. As energy prices soar, there has been a renewed interest in brushless DC motors. However, cost and the overall complexity of the drive design have historically hindered widespread adoption.

Switched reluctance motors are mostly used in appliances such as vacuum cleaners and handheld power tools, where motor noise and torque fluctuations are not a concern. Switched reluctance motors are characterized by high torque and high speed, but at a very competitive price.

Both brushless DC motors and switched reluctance motors use a microcontroller or DSP to perform waveform conditioning and then use power switches (power MOSFETs or IGBTs) to amplify these conditioned waveforms.

Drive circuit

There are many different ways to design variable frequency drive circuits. In a typical three-phase motor, the most popular low-frequency drive scheme is the trapezoidal wave drive circuit, as shown in Figure 1. Figure 2 shows the actual test waveform of the trapezoidal wave drive circuit.

Figure 1: The most popular low-frequency drive solution - trapezoidal wave drive circuit.

Figure 2: Trapezoidal wave control method and actual test waveform.

If higher frequency and performance are required, PWM method can be used to generate sine wave. If you want to further improve efficiency, you can use space vector modulation (Space Vector Modulation) method.

There are two popular types of permanent magnet three-phase synchronous motors, namely sinusoidal permanent magnet synchronous motors and trapezoidal wave brushless DC motors. Sinusoidal permanent magnet synchronous motors are very similar to trapezoidal wave brushless DC motors (electrical performance), and their differences are mainly reflected in the following two aspects: (1) The motor structure or the waveform of the back electromotive force (BEMF) is different. One is an induced voltage sinusoidal permanent magnet synchronous motor, and the other is a trapezoidal wave brushless DC motor; (2) The waveform of the control voltage is different. One uses a three-phase sine (current flows through all three phases at the same time), and the other uses a rectangular six-step commutation.

Among new drive devices, sinusoidal permanent magnet synchronous motors are becoming more and more popular, and are beginning to replace brushed and brushless DC motors, universal (AC asynchronous) motors, and other motors (such as household variable frequency air conditioners, industrial sewing machines, etc.) in many applications. The reason is that it is more reliable (brushless), more efficient, less noisy, and has very high voltage utilization and low-frequency torque in terms of electrical control. Figures 3 and 4 respectively show a system block diagram of a position sensorless vector controller based on field oriented control (FOC) and its experimental waveform.

Figure 3: Block diagram of an interior permanent magnet synchronous motor (IPMSM) vector control system.

Figure 4: Electromagnetic torque equation and experimental waveforms of phase current and estimated angle.

Innovative solution: Intelligent power module

Smart power modules (SPMs) are power interfaces between microcontrollers or DSPs and motors, which can reduce motor size and simplify design. The advantages of this module over discrete solutions are lower parasitic inductance and higher reliability, because all power devices in the module use chips from the same batch and have consistent test performance. This smart power module can directly interface with the low-voltage TTL or CMOS output of the microcontroller and has protection circuits. The module has built-in thermistors to monitor junction temperature, logic protection circuits to prevent direct conduction of upper and lower bridge arms, dead time control, and drive waveform shaping circuits to minimize EMI, etc. In the module, each driver IC can be optimized to complete the switching action of the power device with minimal EMI and drive loss. Three-phase drive modules will continue to be widely used in electrical products. Figure 5 shows a typical application circuit for motor control using SPM.

Figure 5: Typical application circuit.

Figure 6: Schematic diagram of the appearance and internal structure of Motion-SPM.

Figure 6 shows the appearance and internal structure of the Motion-SPM smart power module in Mini-DIP package. Motion-SPM is an ultra-small power module that integrates power components, upper and lower bridge gate drivers and protection circuits in a dual in-line transfer mold package for AC 100~220V low-power motor drive variable frequency control.

Intelligent power modules are designed to achieve maximum design flexibility and can be used in different output voltage and power ranges.

High voltage (600V) bridge drive technology

(600V) High-voltage bridge drive technology enables small, low-cost modules to drive changes in motor drive circuits. Modern high-voltage bridge gate drive circuits are carefully designed to reduce the parasitic drain capacitance inherent in high-voltage IC chip processes. As a result, the drive circuit is very robust and can withstand negative voltages exceeding -9V. Positive and negative spikes on the power supply voltage will not cause the drive circuit to latch up and gate control failure, which is a major change in gate drive circuits in the last 10 years. Matching propagation delays of less than 50ns allows switching frequencies to reach 100kHz or 150kHz. The addition of common-mode dV/dt noise elimination circuits within the IC also helps reduce the possibility of false turn-on, which also helps make the power circuit more robust and more compact by eliminating additional filtering components. Modern ICs (such as FAN7382 and FAN7384) have lower quiescent currents and lower operating temperatures, making them more reliable. The reduction in system power board space and cost reflects a major advantage of modular technology, which eliminates the four power supplies common in the previous generation of motor drive circuits and the optocoupler circuit between the microcontroller PC B and the power switch PCB.

Comparison between NPT type and PT type IGBT

For 20 years, IGBTs have been the power switching device of choice for motor drives. IGBTs are designed to minimize losses for a certain switching frequency. For the motor drive industry, this means that IGBT series are required for different frequency ranges, from 5kHz switching for some consumer electronics motors to 20kHz switching for many industrial motors, and even higher frequency switching for applications other than motor drives.

Improvements in IGBT technology (such as on-state voltage and off-state power consumption per switching cycle) have also further improved reliability and reduced module costs. In the past five years, conventional IGBTs have achieved great improvements in functionality, and new non-punch-through (NPT) IGBTs have also been widely used.

Although NPT IGBTs look similar to conventional punch-through (PT) IGBTs, they are manufactured in a very different way. Unlike MOSFETs or conventional IGBTs, NPT IGBTs use a P-type region and a back metal region during the silicon wafer manufacturing process.

NPT IGBTs often have a lower on-state voltage (VCE(SAT)) than conventional IGBTs, or they turn on more slowly, but they are usually more robust and can withstand short circuits or overcurrents for longer periods of time. This makes them popular in motor control applications. In addition, if you look at the switching waveforms of the two IGBTs, you will find that the EMI generated by NPT IGBTs is much lower than that of PT IGBTs. The trailing edge of the NPT IGBT switching pulse is basically a simple ramp, while the conventional IGBT has a large dI/dt region followed by a long tail with a very slow current drop rate and high device losses. In the high dI/dt region, the EMI generated by conventional IGBTs is large, which generally affects the drive circuit, and it is often necessary to isolate the power switch from the drive circuit. Another advantage of NPT IGBTs is that they can form a positive temperature coefficient relationship with VCE(SAT), which is very useful for parallel applications of IGBTs.

Fairchild Semiconductor's low on-resistance 600V SuperFET MOSFET series products are specially packaged in DPAK (TO-252) to meet the latest ultra-small and thin device requirements for motion control applications. To minimize switching and conduction losses to meet the system efficiency requirements of certain high switching frequency motor control designs, the on-resistance of these products is reduced to 1/3 (0.6~1.2 ohms) of traditional planar MOSFETs. In addition, these products can withstand fast voltage transients (dv/dt) and current transients (di/dt), allowing the system to operate reliably at high switching frequencies.

Conclusion

Recently, the market demand for energy-efficient home appliances has been strong. Among home appliances, a refrigerator will consume more than 10% of the total household electricity consumption. Since refrigerator compressors mainly operate at low speeds, there is a huge potential for energy saving by improving the motor drive efficiency at low speeds. To achieve this goal, Fairchild Semiconductor has developed a corresponding solution for brushless DC motors based on sinusoidal inverters for refrigerators and air conditioners. The new motor drive technology targets high and low speed compressor motor applications, which can further improve the overall drive efficiency.

With an estimated 65% of industrial electricity consumed by motors, it’s no wonder that major players are increasingly focusing on energy conservation as the key to improving profits and competitiveness. One answer is to save energy, especially in motors. There are two main ways to do this: use variable speed drive solutions to efficiently control the motor’s operating speed, provide real-time feedback on parameters such as the motor’s operating status, and improve the efficiency and performance of the motor itself, as variable speed drives can improve performance while saving energy and increasing productivity.