Resurgence for two-speed motors

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 alternatives 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.

Probably 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 well suited for 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 5 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.

Operating principles

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. It is also notable that the two rotors don't have to be the same size, and in fact they can be significantly different. Thus the motor can be designed and built to meet the set-speed needs of the particular application for which it is destined.

A pole-changing motor could also be mistaken for a single-speed one, but a second look reveals it to have twice as many magnets or poles as usual. The poles are wired in two alternating sets; one set is permanently energised, the second set can be switched in and out. This arrangement is simple to build and robust in operation, but there is a limiting factor in that the high speed is always twice that of the low speed, so they can only be used in applications where this is suitable.

Theoretically it is possible to combine both dual-rotor and pole-changing approaches into a single motor to get a four-speed machine. Also, it would be possible to configure a dual-rotor motor so that it could run with just the bigger rotor, just the smaller rotor or both rotors to give three set speeds. However, in both cases the extra engineering required means that they are unlikely to be price competitive with an off-the-shelf motor and inverter set-up (which most users today are comfortable with and naturally opt for).

Unlike standard motors, both forms of two-speed motor come with one of three types of output - constant torque, variable torque, or constant power - which must be specified before installation.

Constant torque is common in relatively complex machines like conveyors, extruders and hydraulic systems and provides a steady output from the machine. Variable torque is required for simpler pumps and fans (the most common use for motors), where low speed requires low torque and high speed needs more torque. Power is the product of torque and speed, so in a constant power motor if the speed doubles the power halves and, vice-versa, if the speed halves the torque doubles.

Thus when specifying a two-speed motor, it is necessary to consider the duty and state the type of output required.

A few years ago, two-speed motors were probably seen as old fashioned and destined to die out over time. However, their simplicity and robustness are now recognised as important attributes that give them advantages in many applications, particularly in the developing world, demanding operational environments, and where an inverter is an unnecessary complication.

To learn more about two-speed motors go to www.exico.co.uk.