Sensorless Vector Control of AC Drives

Key Takeaways

• As an initiative to reduce the capital investment on drives, sensorless vector control is used in motors to eliminate the sensors from the drive system. Since sensorless vector control performs comparably to the drives with sensors, it is often used in projects involving independent speed and torque control. 

• Sensorless vector control is implemented either as constant Volts per Hertz control or flux vector control.

• Sensorless vector control is a flux vector control where the amplitude, frequency, and phase of the AC voltage supply to the motor is varied to keep the motor speed as desired. The three-phase input AC voltage is modulated to control the three-phase stator current, which in turn controls the rotor flux vector and rotor current independently.

Motors are employed with electric drives 

Motors are the most common rotating electrical machines installed in industries. Generally, in industrial applications, the torque and speed profile of the motor needs to be modified according to the load demands. 

Electric drives employ motors to control the torque and speed as per the requirements. The drives utilize converters, feedback loops, and sensors to trace the rotor position or velocity of the motor to output the desired speed or torque. 

As an initiative to reduce the capital investment on drives, sensorless vector control is used in motors to eliminate the sensors from the drive system. Since sensorless vector control performs comparably to the drives with sensors, it is often used in projects involving independent speed and torque control.

Sensorless Vector Control of Industrial AC Motor Drives

Industries need high-performance motor drives enabling variable speeds in induction motors. With speed variation, the torque may be kept constant or be modified according to the load. The stator voltage control is used to change the speed and torque of AC motors by controlling the applied voltage and frequency.

In induction motors, the voltage induced in the stator is directly proportional to the product of flux and frequency. Sensorless vector control is based on the relationship between supply voltage, flux, and frequency in a motor.  Sensorless vector control has many advantages, including:

• Excellent performance with low power consumption and high energy efficiency

• The ability to identify motor dynamics and rapidly adapt and settle the system to the desired profile.

• Be implemented either as constant Volts per Hertz control or flux vector control. 

The constant Volts per Hertz control method applies to induction motors, preferably with known loads that are varying slowly. The speed of the induction motor is controlled by varying the magnitude and frequency of the input three-phase AC voltage, keeping the magnetic flux in the motor constant. The ratio of voltage to frequency (V/f) is kept constant to keep the magnetic flux the same throughout the motor operation. 

The applied voltage is kept constant above the rated speed. This operation is called constant voltage operation. Below the rated speed, the V/f is maintained constant, except at lower frequencies. At lower frequencies, the V/f ratio is increased to keep the maximum torque constant. 

Flux vector control is another technique that is suitable for constant torque profile, and the drive is designed to supply constant flux. 

Flux Vector Control

In this type of drive, the magnitude, as well as the orientation or vector of the AC excitation, is controlled. When controlled relative to the rotor flux vector, the current vector allows flux vector magnitude and torque to be controlled independently. 

The flux control method is also known as field-oriented control. The open-loop flux vector control incorporates a flux control block which produces voltage and frequency commands. This is followed by the variable frequency variable voltage source to maintain the V/f ratio constant.  

The sensorless vector control is a flux vector control method where the amplitude, frequency, and phase of the AC voltage supply to the motor is varied to keep the motor speed and torque as desired. The three-phase input AC voltage is modulated to control the three-phase stator current, which in turn controls the rotor flux vector and rotor current independently. The flux vector control is best suitable for induction motors and brushless DC motors. 

Sensorless Vector Control of Permanent Magnet Synchronous Motor Drives

Sensorless vector control utilizing the rotor-flux estimation method based on fundamental components of voltage and currents is effective in Permanent Magnet Synchronous Motors (PMSM) for high and medium speed operations. 

At low speeds, the rotor-flux estimation is affected by parameter variations. The technique of rotor position estimation is based on change rates of currents and is used in PMSM. The applied voltage causes current variations, and measuring these change rates helps to find the rotor position and rotor angle. 

The applied voltage supplied from a three-phase inverter can be modulated using different techniques to shape the currents. Since there are only currents and voltages involved in the rotor position estimation, it is not affected by PMSM parameters or measurement errors. 

Ripples in torque and harmonics in the phase voltage are significantly reduced by modulating the three-phase inverter using an appropriate modulation technique. Space Vector Modulation (SVM) is a promising technique for this type of application. 

Improved Performance Using Sensorless Vector Control

Sensorless vector control is a promising approach to ensure the transient performance of AC motor drives while reducing power consumption and enabling high energy-efficiency. For a wide range of speed applications requiring varying torque profiles, sensorless vector control is the best AC drive for induction motors and PMSMs.