Most engineers practising today will have grown up with inverters or electronic variable speed drives, and are specifying them more and more often as a way to meet increasingly stringent energy management requirements. In fact you could say that inverters really are a technology for today's world.
They first emerged in the 1980s, were perfected in the 1990s and really came into their own after the Millennium as a way to reduce energy usage, thus helping manage global warming and climate change. An inverter lets you turn down the speed of a motor and thus reduce its energy consumption, and very significant savings can be made. Given that about half of the world's electricity is used to power motors, the inverter unlocks huge potential energy savings.
It is perhaps not surprising that people took to inverters and, for a while, tended to forget alternative solutions for speed control. However in recent years there has been a renewal of interest in two speed motors, leading to an increase in their sales and installation. For Exico, one of the few suppliers of this technology in the UK, the greatest advantage of two speed motors is their simplicity and reliability.
While they do not provide the infinitely variable speed control of an inverter, they can be ideal in applications that require 'high' and 'low' speed settings rather than total adjustability. They are simple to install, have a long working life, are bombproof in terms of reliability and do not require the extra space of an inverter.
As well as these technical advantages, two-speed motors can be good for plant and machines that are going to be located in areas where support services are less than ideal, in remote locations and where environmental or operational conditions such as extremes of heat, cold and humidity, vibration, water ingress and impacts could strain electronic circuitry.
Typical applications where a two-speed motor may be preferred over an inverter set-up include the many pump and fan applications that need two set speeds. There are many such applications in the water distribution industry where there is basically a daytime demand level and a lower night time one. Another potential application area is HVAC (heating, ventilation and air conditioning) pumps and fans in office buildings that are occupied 8-10 hours a day for five days and virtually empty the rest of the time.
In the industrial world, some conveyors have predictable light and heavy load requirements so would benefit from twin-speed capability. Cranes and winches often need a fast setting for long lifts and a slow setting for other duties. Of course, many of the pumps and fans in industry may benefit from for a two-speed drive capability.
One of the great practical advantages of two speed motors is that they look very like standard fixed speed motors, at least from the outside. This means maintenance technicians are not usually wary of them, that they generally fit into the same spaces as regular motors and are mounted in the same way.
However, internally they are quite different. In fact there are two principles upon which they can work, dual winding and pole changing. At first glance a dual-wound motor rotor looks remarkably like a conventional one, but closer inspection reveals that there are in fact two windings, each separately connected to the power. Put simply: switch on one rotor and you have the first speed; switch on both rotors and the second speed comes into play. A closer look reveals that the rotors, while in the same lamination pack, are configured to have opposite polarities.