Iftitah Imawati

This month's cover shows a view of a shot tower in Baltimore, MD, looking up from its base. The paper beginning on page 218 of this issue discusses the physics behind how lead shot was made in the 18th and 19th centuries. (Photo taken by Carl Mungan.)

The territory of high-performance motor control has been dominated by synchronous DC motors. This group of motors includes brushed, brushless, wound-field and permanent-magnet varieties. The simple reason for this domination is that DC motors are easier to control. This is especially true if the application requires good control of motor torque, velocity, or position. The electromechanical model of a DC motor shows that motor torque, within limits, is an approximately linear function of the input current. So, it is a relatively easy task to derive solid performance out of a DC motor with proportional-integral-derivative (PID) controllers. In the " real design world " , the selection process for a type of motor to use in an application can be complex. A particular motor can't be chosen based solely on how easy it is to control. There are many other system-related variables to juggle, such as: • How easy is it to maintain the motor? • What happens to the system when the motor fails? (i.e. a shorted winding) • What will be the operating environment? • How will the motor be cooled? • What is the cost of the motor? The list of considerations can go on and on…. AC induction motors (ACIM) have distinct advantages over other types of motors, and have typically been used when a robust, fixed-speed solution is desired. The evolution of microcontroller (MCU) and power electronic devices has made inexpensive variable-speed control of an ACIM possible. However, the performance of a DC motor cannot be matched using basic control methods. This article will explore the topic of field-oriented control (FOC) and how it can be used to improve the control of an ACIM using a Digital Signal Controller (DSC). FOC lets you use DC control techniques for an AC motor, and can remove one of the variables in the motor-selection process for your next design. How a Motor Works An electric motor produces a mechanical force, when current flows in proximity to a magnetic field. A synchronous motor has a source of magnetic field. This field can be provided by permanent magnets or by windings that are energized with a source of current. Within limits, the torque response of the motor is a linear function of the current and the magnetic field strength. The linear response makes these motors easy to control in high-performance applications. A PID controller can be used to control the motor current and resulting motor torque. If needed, secondary PID controllers can be used to control position or velocity.