Motor Control

Around 45% of our global energy consumption is from the operation of motors (World Energy Council: 2013 Survey Summary). They are a power-hungry but essential part of our daily lives and are found everywhere from small residential uses, such as household appliances and tools, through electric cars and trains in the transportation sector, right up to the largest industrial motors for offshore oil rigs and damns.

Since the industrial revolution we have been reducing time and labour by powering anything that we possibly could with a motor. A mass of electrically-powered devices have made our home lives easier and more convenient and our workplaces more efficient and profitable. But at what cost?

Energy-eating motors and the environment

In our modern world of diminishing fossil fuels, amidst fears for our ecology and environment, manufacturers of any goods driven by motors face challenging times ahead. With no wide-scale alternatives yet derived from renewable energy sources, the problem is exacerbated by an increase in demand coming from growing economic development in Africa, Asia and South America, coupled with our rapidly expanding global population.

Around the world, governments have been putting increasing legislative measures into place to reduce energy consumption for some years and these are only set to increase further. In addition, consumers in a retail sense are far smarter and look for low energy-consuming products routinely; the same is true for industrial customers looking to invest in more efficient equipment.

Miniaturising motors

In addition to minimising power usage, engineers are also challenged with squeezing motors, drives and their controllers into increasingly smaller dimensions. A washing machine that features a larger drum capacity offers a value-adding selling point to the customer, but it still needs to fit within standard dimensions. Reducing space for electronic components creates thermal management issues which pose further design challenges for engineers. Adding cooling mechanisms will only increase power consumption, so the motors themselves need to be engineered with improved efficiency levels so less heat is generated in the first place.

Open and Closed Loop Motors

To explain this in its most basic and simple context, an open loop system does not incorporate any feedback. The speed of the motor is controlled to a set point which can vary under different load conditions.

A closed loop system incorporates feedback by returning information to the input stage to adjust itself. So when speed is controlled to a set point and the load changes, the controller will adjust the speed back to the set point; a good example of this is a positional motor on a telescope which will constantly readjust itself to track the required co-ordinates.

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Motor Types

Electric motors basically use magnetism to create motion. There are two main categories of motors: AC (alternating current) and DC (direct current).

DC motors were the first to be invented and are still the simplest form of motors. DC motors are driven by passing a flow of current through a conductor inside a magnetic field to produce rotating torque. Main types of DC motors are Brushed DC and Brushless DC, with brushed motors generating power by connecting opposite poles of a power source to deliver negative and positive charges to the commutator when it makes physical contact with the brushes.

As the name suggests, brushless motors have no brushes, instead, permanent magnets are mounted around the perimeter of the motor. This eliminates the need for commutators and connections, as well as brushes. Brushed motors are simple and lower cost, but require more maintenance as the brushes need regular cleaning and replacement. Brushless motors on the other hand tend to be more accurate for applications that require controlled positioning, and have the added benefit of requiring little or no maintenance. This comes at a price however, as brushless motors cost more to manufacture and require a motor controller which could cost as much as the motor itself.

AC motors can also be categorised into two main types: Induction and Synchronous; with a third, less common type – Linear AC motors.

At a very basic level, AC motors consist of two primary parts: the outside part of the motor is known as the stator – the stationary part of the motor – which has coils that are supplied with alternating current to produce a rotating magnetic field; inside the rotor conects to a shaft which produces another rotating magnetic field. Linear motors are similar in principle to rotating motors but are configured with moving and stationary parts in a straight line – ultimately, instead of rotation, this produces linear motion.

Induction motors are named as such because torque is produced using electromagnetic induction. These are commonly known as squirrel cage or wound rotor motors.

Synchronous motors differ from induction motors in that they operate in precise synchronisation with line frequency. In contrast, induction motors rely on inducting current to produce magnetic field and require some ‘slip’ (slightly slower rotations) to actually induce current.

What to look for when choosing a motor

When selecting a motor there are a number of key attributes to look out for: