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Motor Drive & Control

For various motor types; AC Induction (ACIM), Brushed DC, Brushless DC (BLDC), Permanent Magnet Synchronous and Stepper find the right devices, software and support to precisely control the position, velocity and torque.


AC Induction Motors (ACIM) - Overview

Motor Control - AC Induction Motors (ACIM) Overview

The AC induction motor (ACIM) is the most popular motor used in consumer and industrial applications, and represented the "muscle" behind the industrial revolution. The concept of this "sparkless" motor was first conceived by Nicola Tesla in the late nineteenth century as a polyphase structure consisting of two stator phases in an orthogonal relationship. It has since been modified to the more common three phase structure, which results in balanced operation of the motor voltages and currents.

The motor does not have a brush/commutator structure like a brush DC motor has, which eliminates all the problems associated with sparking; such as electrical noise, brush wear, high friction, and poor reliability. The absence of magnets in the rotor and stator structures further enhances reliability, and also makes it very economical to manufacture. In high horsepower applications (such as 500 HP and higher), the AC induction motor is one of the most efficient motors in existence, where efficiency ratings of 97% or higher are possible. However, under light load conditions, the quadrature magnetizing current required to produce the rotor flux represents a large portion of the stator current, which results in reduced efficiency and poor Power Factor operation.

ACIMs perform best when they are driven with sinusoidal voltages and currents. One of the advantages of ACIMs is the incredibly smooth operation they can provide as a result of low torque ripple. To achieve this, most ACIMs consist of a slotted stator structure where the windings are placed in the slots with a sinusoidal winding distribution, resulting in a sinusoidal flux distribution in the airgap. This flux also links the rotor circuit, which consists of copper or aluminum bars shorted at each end, and mounted on a stacked laminate structure comprised of soft iron, or other ferrous material. In most cases, motor efficiency can be increased by decreasing the rotor bar resistance. As the flux cuts across these conductors, a d-flux/dt voltage is impressed across the rotor bars, which results in current flow in the rotor. In other words, current is induced in the rotor circuit from the stator circuit; much the same way that secondary current is induced from the primary coil in a standard transformer. This rotor current produces its own flux, which interacts with the stator mmF to produce torque. However, in order to achieve this d-flux/dt effect on the rotor bars, the rotor cannot rotate at the same speed as the rotating stator field. As a result, induction motors are classified as asynchronous motors. The difference in rotational speed between the stator flux vector and the rotor is called slip. As more torque is required from the motor shaft, the slip frequency increases. In conclusion, the motor speed is a function of the number of stator poles, the motor torque (and consequently motor slip), and the frequency of the AC input voltage.

The three phase topology represents an ideal choice for variable-speed applications. Three phase inverters are commonly used as shown in the diagram, where motor speed can be controlled by simply varying the voltage and frequency of the applied waveform (open-loop V/Hz or scalar control). Alternately, speed can be controlled by wrapping a speed loop around a torque loop incorporating Field Oriented Control (FOC). The former can be easily achieved with an economical device such as an MSP430, but FOC is more suitable to a powerful 32-bit processor such as TI's C2000 processors.

AC induction motors are also available in single-phase versions. Most single phase versions actually have two phases, where one phase is used to help get the motor started. Once the motor reaches a certain speed, this phase can be disconnected, resulting in the motor operating on just one phase.

Motor Control: AC Induction