Field-oriented control can be easy for electric motors, says TI

TI

Design kit for electric motor control

The field-oriented control (FOC) approach offers many advantages compared to other control strategies used with electric motors. Among the most significant benefits is the fact that FOC offers precise control features based on torque and speed.

However, the sensor required by conventional field-oriented control schemes leads to cost increases and limited freedom for designers. InstaSpin-FOC combined with the FAST software encoder algorithm from Texas Instruments provides a solution enabling a straightforward entry into designing FOC applications.

The efficiency of electric motors is becoming more and more important for designers due to rising electricity costs or tightening energy efficiency regulations imposed by legislation. For instance, the European Union’s 640/2009 directive states that the majority of electric motors be equipped with inverter-based controls by 2017. This applies to all asynchronous motors between 0.75 and 375kW.

According to calculations published by the International Energy Agency (IEA), worldwide electricity consumption could be reduced by between nine and 14 per cent by increasing the efficiency of systems using electric motors. This includes pumps, compressors, fans and drives used in a variety of applications including cooling systems, industrial pumps and e-mobility systems.

Efficiency Improvements Achieved with Field-Oriented Control

Field-oriented control (FOC) is a proven option for increasing the efficiency of electric motors. With FOC, it is possible to operate motors at their optimum torque and speed at any time. In addition, the precise and fast speed regulation features offered by the FOC approach is a benefit for applications subjected to many dynamic load changes, including washing machines, fans or compressors. Furthermore, reduced torque fluctuations let the motor rotate more smoothly.

On the other hand, FOC has some drawbacks including the fact that a sensor is needed for determining the exact rotor position. This information is required to generate the magnetic field resulting in maximum torque. In most cases, an electro-mechanical sensor or a complex software algorithm (observer) is used.

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To maintain efficiency of induction motor under low-load conditions the control system is stabilised by dynamically adapting the dynamic torque and the speed profile.

Sensors and observers, however, increase the cost, size and error rate of an application. Traditional sensorless FOC solutions including the Luenberger observer or the Sliding Mode Observer (SMO) require a lot of designer expertise.

In addition, controls based on these techniques are lacking stability in the lower speed range and during transitions between motor and generator operation. This is a problem especially for motors operating at low speed as well as during the acceleration phase and under low-load conditions.

InstaSPIN-FOC from Texas Instruments significantly mitigates the aforementioned disadvantages of sensorless controls. It combines the benefits of two key technologies:

• The FAST software encoder algorithm (Flux – Angle – Speed – Torque) and

• The C2000 Piccolo microcontroller (MCU).

Using this solution, designers only need minutes to tune and control any 3-phase asynchronous or synchronous motor across different speed and load levels.

The software is integrated into the MCU’s ROM (Read-Only Memory). Software costs are included in the price of the MCU. For design and evaluation purposes, TI provides motor-control libraries (modules, drivers, reference systems and documentation) within the MotorWare package, representing the latest trends in the world of object-oriented C programming and API-based coding. The free GUI Composer tool provided by TI is available for lab testing.

The free InstaSPIN Simulation Tool (http://www.ti.com/tool/instaspinsim#2) from Texas Instruments can be used for interactive on-line simulations of the InstaSPIN-FOC technology. Designers can use this tool for simulating the functionality of InstaSPIN-FOC and the FAST algorithm under real-world conditions based on different motor parameters and load scenarios.

Optimise Any Kind of Electric Motors

InstaSPIN-FOC is currently available for the 90MHz/32-bit floating-point Piccolo F2806xF MCUs and the ultra-low-cost Piccolo F2802xF series. The technology will soon be provided with additional Piccolo MCUs as well.

Motors supported by InstaSPIN-FOC include:

• Brushless DC motors (BLDC)

• Permanent-magnet synchronous motors (PMSM)

• Brushless interior permanent magnet motors (IPM)

• AC induction motors (ACIM)

InstaSPIN-FOC offers many advantages compared to traditional techniques. For instance, it enables the design of robust, low-speed/high-torque motors for compact, low-noise direct drives suitable for e-bikes as well as electric or hybrid vehicles (passenger cars or busses). Due to the FAST algorithm’s ability to make precise estimations of magnetic flux, rotor angle, speed and torque at any time, many applications can be implemented without rotary encoders.

Increasing Energy Efficiency

The energy consumption of Texas Instruments‘ solution is positively affected by the fact that the angle information is preserved even at rotor speeds significantly below 1Hz (typ.) at full torque, when the direction of rotation is reversed or when the motor is stalled (with straightforward removal of the stall condition).

Furthermore, motor start-up problems are resolved using integrated start-up modes provided by InstaSPIN-FOC. Less than one electrical cycle is required to determine the rotor angle. Overall, optimised efficiency can be achieved for all kinds of drives using the FAST encoder.

Improving the Efficiency of AC Induction Motors with PowerWarp

PowerWarp can be used to further increase the efficiency of AC induction motors, which represent about 80% of the energy consumption of industrial motors. Thanks to PowerWarp, manufacturers do not have to eliminate these motors even if energy costs will continue to rise.

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Block diagram of the InstaSPIN-FOC design

Motor efficiency can be improved dramatically using PowerWarp especially at low loads. To keep the control system stable, PowerWarp enables balancing of the dynamic torque and speed profile based on reducing the copper losses in the motor’s stator and rotor.

These savings particularly apply to the partial-load range, while the improvements are less pronounced at full load. However, most motors rarely operate at full-load.

Reduced Material Requirement

Environmentally friendly electric motors and drives not only boast optimum efficiency and low energy consumption. Traditional sensor-based FOC motor controllers using mechanical sensors instead of FAST software sensors rely on more complex cabling, especially if the control unit is located farther away from the motor.

In these situations, complex shielded wiring is required in order to eliminate interference (noise).

The environmental performance also suffers from the fact that sensors often need separate power supplies in conventional sensor-based FOC implementations, leading to higher costs and failure susceptibility.

Writer is Michael Seidl, Business Development Manager, Texas Instruments

 

Tags: motor control, Texas Instruments

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