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Posted on 01 November 2019

Sensorless Motor Control Algorithm

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Delivery of Improved Appliance Efficiency

Conservation of energy is one of the most important challenges facing us in the 21st century. Recent advances in sensorless permanent magnet motor control technology can help to address this challenge by significantly reducing the energy used in appliances such as refrigerators, washers, fans, and air conditioners.

By Aengus Murray, International Rectifier

 

Electric motors are the primary power users in home appliances such as refrigerators, fans and air conditioners. Unfortunately, the single-phase shaded pole or permanent capacitor induction motors typically used in many fans or pumps have efficiencies as low as 25%. And while the larger single-phase induction motors used in air conditioner and refrigerator compressors are better with typical efficiencies greater than 65%, a great deal of energy is still lost during start-up of fixed speed compressors with low duty cycles.

In recent years, manufacturers have improved appliance efficiencies with variable speed permanent magnet motors featuring control systems that use electrical power inverters to vary motor speed by changing the motor winding voltage frequency. In some appliances, such as fans, hall sensors detect rotor position to synchronize it with winding switching in order to maximize efficiency and simplify start up. The advantage of this approach is in that the control requires very simple electronic circuits. However, a sealed compressor cannot accommodate hall sensors and so a sensorless algorithm is required.

One popular sensorless algorithm used in six step permanent magnet motor drive systems detects the zero crossing of the winding back emf. The control algorithm typically uses an 8-bit microcontroller to manage the phase advance and start up sequence. One disadvantage of the six-step system is that there is a torque glitch as the motor current switches (commutates) between windings. In many fan and pump applications these glitches produce an annoying acoustic noise especially at low speeds when the fan blade noise is almost zero. The ideal solution is to drive the motor with sinusoidal currents, which completely eliminates the glitch. This type of control also enables the use of an alternative motor design with interior permanent magnets (IPM). IPM motors can produce 15% more torque than permanent magnet motors, which allows for further efficiency improvement.

Sensorless Control

Advances in hardware and control technology have made it possible to build cost effective drives for IPM motors based on a field oriented control (FOC) algorithm that drives the motor with sinusoidal currents for maximum efficiency and low acoustic noise. Now, International Rectifier is taking these advances further with a set of integrated design platforms for appliance motor control, in which a mixed signal control IC implements a sensorless FOC algorithm in hardware eliminating the need for software development. The sensorless algorithm detects the rotor position based on the motor current measurement only, while the control hardware allows motor winding currents to be measured without expensive isolation circuits. These technologies provide designers with a route to using IPM motors for driving compressors, as well as eliminating the need for hall sensors in fans with surface magnet or IPM motors.

Figure 1 illustrates the algorithm functions used to drive either an IPM or surface magnet motor with sinusoidal currents and without sensors. A key element of the algorithm is the FOC structure that uses a vector rotation block (e-jè) to transform ac motor winding currents into two dc current components one producing torque and the other controlling the flux. The current inputs to the rotation block are first transformed from threephase to two-phase equivalent values using the Clarke transformation block. The rotor flux angle enables the splitting of the current into the D component, which aligns with the flux, and the Q component that produces torque. The tuning of the two current control PI compensators matches to the motor winding RL time constant and does not have to change with the frequency of the ac winding currents. The forward vector rotation block (e) transforms the dc voltage outputs of the PI compensators into ac voltages matched to the rotor frequency. The Space Vector PWM unit calculates the power inverter transistor switching times needed to apply the calculated ac voltages. Space Vector modulation produces sinusoidal voltage modulation with automatic third harmonic injection to maximize the use of the dc bus voltage. It also includes a two phase modulation function that will minimize the power inverter switching losses.

Sensorless PM control algorithm

Maximizing the motor torque output per ampere achieves maximum motor efficiency. In the case of a surface magnet motor, the controller maintains the flux component of current (ID) at zero to maximize efficiency. However, the construction of an IPM motor produces an additional torque component known as reluctance torque. When driving an IPM motor, the IPM control block increases the ID current from zero as a function of the IQ target in order to operate at maximum efficiency. In either case, the speed loop compensator calculates the required torque current needed to maintain the speed at the target value. There are some applications, such as washing machines, that require an expanded speed range. Here, the field weakening control function inserts negative flux current (ID) to reduce the effective back emf of the motor and allow the motor to run at a higher speed before the back emf reaches the dc bus voltage limit.

A unique feature of this algorithm is its ability to derive rotor position and velocity from the motor winding currents without a physical sensor. The sensorless algorithm derives the rotor flux functions from the motor circuit model based on the following equations:

Equations

The controller drives the stator voltages and the current reconstruction circuits measure the resultant motor currents. A simple reordering of the equation terms and mathematical integration yields the sine and cosine terms. A phase locked loop tracking algorithm, derives both angle and velocity.

The second unique feature of the algorithm is the phase current reconstruction unit that derives the motor phase currents from the current flowing in the inverter dc link. The principle, illustrated in Figure 2, is that for any active inverter state there will always be one winding connected to one bus rail and two windings connected to the other bus rail. This means that in every PWM cycle there are two motor winding current values available. The reconstruction unit includes a sample timing generator based on the SVPWM inputs, a sampling A/D converter and the mathematical unit to calculate the third phase current. The significant advantage of the approach is that it eliminates the requirement for isolated current sensing and so makes the current sensorless algorithm cost effective in appliance applications.

Three phase current reconstruction

Additional Functionality

In addition to the algorithm implementation, as the illustration in Figure 3 shows, the IR control IC integrates the A/D converter and buffer amplifiers needed for current measurement, and additional hardware functions for error handling and start up sequencing. The IC also integrates an 8-bit microcontroller core with its own memory to allow the appliance engineer to implement additional, application-specific functionality.

Block diagram or similar of controller IC functions

 

 

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