Vector Control of Induction Motor Drive

The only motivation for induction motor vector control is to have an electrical drive that performs better than the frequently employed separately excited dc motor in industry. Furthermore, such a drive ought to become a strong, dependable, cost-effective substitute for dc drives. Separately excited dc motors were once thought of as the industry’s main workhorse. This is as a result of the induction motor’s slower dynamic performance. The dc motor’s ability to independently control torque and flux within the motor. And its inherent ability to be doubly fed account for its quicker dynamic response.

Prior to the invention of induction motor vector control, methods for regulating the speed of the cage induction motor drive that were widely accepted included voltage control, frequency control, rotor resistance control, v/f control, flux control, slip control, slip power recovery control, etc. All of these methods of control are referred to as “scalar control of an induction motor,”. And when used with an induction motor, the cage motor performs less dynamically than a separately excited de motor. The researcher sought to realize separately excited dc motor performance and characteristics with an induction motor with cage rotor in order to have a maintenance-free, strong, and high-performance ac drive.

Blaschke introduced the idea of vector control for induction motors in this direction. The field orientation principle was first put forth by Blaschke in 1972. In order to realize dc motor characteristics in an induction motor drive. He used decoupled torque and flux control in the motor for the same purpose, and he called it transvector control.

A high level of dynamic performance is provided by the cage induction motor drive with vector or field oriented control,. And the system’s long-term stability is provided by the drive’s closed-loop control. Scalar and vector controls are similar in many ways,. But the latter has a few characteristics that make it advantageous as a control system with high dynamic performance. The torque and flux current vectors are controlled by the vector control,. Which is also known as an independent or decoupled control.

It is a well-known fact that the cage motor drive is linearized in the vector control mode. And behaves like a fully compensated separately excited dc motor, with the electromagnetic torque produced by the drive directly influenced by the control of armature current. Similar to this, the vector control method used in cage motors allows for independent control of the two quadrature currents. That produce flux and torque production, respectively. Because the torque-producing component of the current responds quickly, the drive has a high level of dynamic performance.

The term “vector” refers to the direction of a cage induction motor’s control that causes it to behave like a fully compensated separately excited dc motor. In this instance, either the stator or the rotor mmf vector rotates in synchronism with the frame of coordinates in which the stator currents are expressed. The flux and torque in the motor are produced by two orthogonal components of the stator currents expressed on these coordinates. These resemble dc motors in that torque and flux are controlled by separately altering the armature and field currents.

It is necessary to control the magnitude and phase of the three stator currents (i_as, i_bs, and i_cs) through a fast inverter in order to independently control the torque and flux (and subsequently speed) in the induction motor. A CC-VSI (Current Controlled Voltage Source Inverter) is typically used for this purpose. A control algorithm of this complexity would be required. A well-known matrix operation called the three to two phase transformation is used. The vector control is used to control the two magnitudes and one phase of the two phase currents, i_ds and i_qs, which are in phase quadrature.

The Vector Control of Induction Motor method is block diagram form is illustrated in Fig. 11.63 certain crucial steps are as follows.

From the motor speed signal (ω_r) and desired speed (ω^*_r) the error ω_e is determined. Speed controller calculates the motor torque (T_o) needed to correct the speed. Which is passed through a Limiter to determine torque signal T^*.

In a parallel Field Weakening block the motor speed ω_r generates another signal.

These two signals are employed to calculate i^*_ds and i^*_qs (ideal quadrature currents) and a speed correction ω^*₂.

ω = ω_r + ω^*_r is integrated which is then used to find the transformation e^jψ. This transformation carried out on ( i^*_ds , i^*_qs ) gives the final ideal set ( i^*_dss , i^*_qss ).

2/3 phase transformation on (i^*_dss , i^*_qss) yields the ideal stator current (i^*_as, i^*_bs, i^*_cs).

The measured stator currents (i_as, i_bs, i_cs) are compared with (i^*_as, i^*_bs, i^*_cs) by the current controller. And the six signals generated control the currents fed to the induction motor.

The above is a simplified explanation of the Vector Control of Induction Motor control.

In the Vector Control of Induction Motor, The stator windings’ current control is used to control the rotor flux. It is also desirable to know its position in order to control the rotor flux. The position of the rotor flux is either sensed or estimated. Therefore, the vector control is either referred to as a direct vector control. Or an indirect vector control depending on the methodologies used for assessing the rotor flux vector position.