Smart Motor Controllers Take Charge of Expanding Applications

Mar 1, 2011 12:00 PM
Roger Allan, Contributing Editor

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Take a look at a modern motor controller and one of the first things you'll notice is that the control circuit is about the same as the motor it controls. That ís a far cry from the 10X or size bigger sizes of just a few years ago. It's also less expensive and smarter. Credit IC chip advances in the form of more powerful and lower-cost DSPs, microcontrollers, amplifiers, encoders, power ICs, etc., for this.

This comes against a background of increasing demand for motor controls in many more sectors like the automotive, computer, industrial, medical, toys, as well as emerging white goods, power tools, and vacuum cleaner consumer appliances.

Motor control is a complex task involving not only understanding for the designer of complex operations, but also advanced hardware and software algorithms. Thus, it is not surprising to see field-programmable gate array (FPGA) vendors and vendors of intellectual property (IP) algorithms entire this field more recently. IC microcontroller unit (MCU) and DSP vendors are actively working with these aforementioned suppliers to present complete system development kits and platforms, particularly for users of embedded MCU and DSP users who are more software knowledgeable than hardware oriented.

In using stepper motors, engineers are challenged to maximize control algorithms for greater efficiency. This means boundary conditions of the complete electro-mechanical system must be mapped. This can be daunting since all system variables such as temperature, mechanical degradation, acceleration, velocity, supply voltage, vibrations, etc. must be accounted for.

The types of motor drives are widespread in both ac and dc varieties. These include universal ac and dc motors, high-frequency pulse-width modulated (PWM) universal types, brushed and brushless motors, induction motors, scalar variable frequency drives (VFD) motors, vector drive motors, as well as stepper motors.

Intelligent motor control ICs provide advanced control capabilities for multi-phase motors, most commonly brushless dc motors and three-phase induction motors. Microprocessors and DSPs are providing relatively inexpensive intelligent field-oriented control (FOC), or vector control, a math-intensive technique for controlling brushless dc and ac induction motors more efficiently. It reduces motor size, cost and power consumption. It achieves this by directly measuring the field position within the motor.

Another variant technique being used is indirect FOC. Here, the motor's field position is measured indirectly via slip calculations using a mathematical model of the motor.

A big push is on for more energy efficient motor control and the key to this is smart control. Smart motor control is indeed helping boost energy savings. This is particularly the case with the use of VFD circuits. These circuits optimize motor acceleration and deceleration ramps, slow down the motor, and turn it off when not in use.

A VFD is a system for controlling the rotational speed of ac electric motor by controlling the frequency of the electrical power supplied to the motor (Fig. 1). VFDs are also known as adjustable-frequency drives (AFD), variable-speed drives (VSDs), ac drives, micro-drives or inverter drives. Since the voltage is varied along with frequency, they are sometimes also called variable-voltage variable-frequency (VVVF) drives.

A VFD can also be used to save regenerated energy. For example, when a motor is trying to stop a high-inertia load, it acts as a generator by dynamically converting the kinetic energy in a motor into useful heat energy using high-wattage braking resistors. Many modern motors feature VFD capabilities.

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