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High-efficiency motor control solutions aim at HVACs

Posted: 19 Nov 2007 ?? ?Print Version ?Bookmark and Share

Keywords:motor control? electric motors? HVACs?

By Luigi Montanaro, Gianfranco Di Marco and Giacomo Timpanaro

Electric motors are an essential component for home and office buildings as they are present in heating, ventilation and air conditioning (HVACs), washing machines and refrigerators. They are responsible for a large portion of the energy consumption in home and office buildings, so savings in consumption and cost using more efficient control techniques is important.

There are different electric motors and control techniques with different efficiencies, costs, reliability and performance features. For example, to have a continuous adjustment of the stator frequency and stator flux as a function of the load in 3-phase motors, a power inverter architecture can be used (the motor windings are connected to the inverter outputs which perform the DC/AC conversion). As described in this article, this system has numerous advantages and will become one of the major motor systems for "green machines."

Over the past years, STMicroelectronics has established partnerships and collaboration agreements with major market players to develop motor control IC components and solutions that help reduce cost and design cycle, and ensure best performance such as reduced acoustic noise levels and efficiency improvement. Improving the performance and the efficiency of motors used in such applications directly impacts energy saving.

Billions of motors are built every year, each of them controlled and driven by systems implementing an MCU as the core. Modern 3-phase motors are driven by pulse width modulators (PWMs) in conjunction with some power transistors. The role of the MCU is very important since its communication capabilities (PWM generators) can determine the speed, torque and power efficiency of the motor. In fact, a carefully-crafted electronic driver system can often make a small motor do the work of a much higher horsepower one and at a significantly lower cost, thus enabling the best motor performance in terms of energy efficiency.

This article describes a simple and low cost motor drive based on an inverter topology and specifically designed for the energy efficient permanent magnet motor.

Practical solution
There are two types of permanent magnet motors: permanent magnet AC (PMAC) motors designed for sinusoidal control, and permanent magnet DC (PMDC) or brushless DC (BLDC) motors designed for trapezoidal control, where the current is switched in six discrete steps.

A BLDC motor consists of a rotor with permanent magnets, and wire wound stator poles. The control electronics energizes the windings in a sequence that results in the generation of a magnetic field which rotates inside the stator. The permanent magnets in the rotor are then attracted by the field of the stator, and the rotor turns. The BLDC motor is synchronous, which means that the rotor rotates at the same frequency as the magnetic field in the stator, thus speed regulation is quite straightforward. The torque generated in the motor is directly proportional to the current applied in the stator windings, therefore torque regulation can be achieved by regulating the current.

Figure 1: General characteristics of permanent magnet BLDC motor drive.

ST's solutions support BLDC motor control in 6-step mode, with either sensor or sensorless feedback and either current or voltage regulation.

The solution presented here is related to PMDC (also called BLDC) motors with back EMF sensing for sensorless control.

Figure 1 shows the system hardware architecture of a BLDC/AC motor drive designed by STMicroelectronics for low end/low cost HVACs used in home and office buildings.

In the figure, the ST7MC 8bit MCU embeds a complete control algorithm and manages the user interface. The ST7MC integrates a dedicated macrocell generating two complementary sets of three sinusoidal PWM outputs, using edged patterns or single/double updated centered patterns, with a 12bit resolution and automatic dead time insertion. Multiple sensor inputs are also supported including tacho-generator, Hall-effect sensors, encoder and dedicated speed acquisition hardware. Alternatively, the ST7MC has also been designed for sensorless applications through an ST proprietary algorithm or via the GE algorithm, reducing system costs without sacrificing efficiency.

Typically, three Hall-effect sensors are used to locate the position of the rotor so that the control algorithm can apply the correct next phase to ensure rotation at the right time. However, Hall-effect sensors are relatively expensive to be used in low-cost control systems, so efficient closed control loops without sensors have been a goal of the industry for a long time.

STMicroelectronics has developed two such control methodologies, which differ in the sampling methodologies. The ST7FMC MCUs include features specifically developed to support the use of both sensor and sensorless control methods. Shown here is the less expensive to implement, involving the lowest component count.

Both methods also avoid the use of RC filters, thus further lowering costs and increasing motor performance and speed range vs. other sensorless techniques. The efficiency of such sensorless solutions equals that of sensor-based implementations.

Figure 2: BLDC system architecture.

The concept is simple (see top left of the diagram). The first part of Figure 3 shows a representation of the motor, with the high-side switch (T1) on. Here, the voltage at the middle of the motor is equal to half of the line voltage. In the second part of the diagram, the high-side switch T1 is off. At this point, because of the freewheeling diode built into the switch, there is a virtual ground created at the mid-point of the motor. This means that if the voltage induced in coil C is sampled, only the voltages induced by the rotora combination of the back EMFsare seen.

To compare these induced voltages to a threshold voltage, dividers or filters are no longer needed. This saves several external components and eliminates the performance-robbing phase delay. The motor voltage is then captured by a comparator embedded in ST7FMC.

This direct sensing of the motor voltage provides a high sensitivity, bringing a number of advantages including wide speed range, effective control even at very low speeds, high starting torque and a high SNR compared to other sensorless techniques.

For each phase, ST7 reads the relevant user inputs and the position feedback from the motor, either using sensors or a sensorless technique. Then, it calculates and generates the required PWM output waveforms, including dead-time generation. The MCU also has an emergency-stop input (which can generate a non-maskable interrupt for the CPU) to take immediate control in case of a fault condition.

Figure 3: Back-EMF sensing for sensorless control.

Going back to the diagram, in the middle it is possible to see three L6386 half-bridge drivers, with the integrated bootstrap diode. On one side, L6386 has CMOS-compatible 5V Schmitt trigger inputs, allowing a direct and easy interface to the ST7. On the other side, each driver features two outputs to directly drive two independent IGBTs, on a voltage rail of up to 600V. Several hardware protection features are included in the L6386, such as undervoltage lock-out on both low- and high-side drivers.

Located at the top of the diagram is the three-phase inverter, consisting of six IGBTs while at the left, you can see the bridge rectifier, delivering rectified mains to the power inverter stage.

The ST7FMC flash-based MCU family is designed for three-phase motor control applications, combining ease-of-use, ruggedness and programmability of one of today's most popular MCU architectures, with motor control performance equal to a 16bit DSP.

The auxiliary power supply for the digital part of the reference design is implemented with an ST VIPer12 smart power switch and a L78L05 as a voltage regulator. The VIPer12 belongs to a series of intelligent integrated switching regulators from STMicroelectronics, combining a PWM power supply controller with a high-voltage power MOSFET. The VIPer is compliant with Blue Angel and Energy star ECO Norms.

The three-phase inverter power stage consists of L6386 600V half-bridge drivers, plus either 60V MOSFETs or 600V short-circuit rugged IGBTs, depending on the power stage selected. A range of rectifier diodes, protection diodes and small-signal transistors complete the design shown above.

The performance of a 3-phase PMDC (BLDC) motor associated with a dedicated digital controller greatly exceeds that of universal brush-type motors traditionally used for such an application. This is achieved by optimizing all the system parameters and by implementing a slip control regulation together with a new gate drive topology. This results in reduced power consumption.

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