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Resurgence of the two-speed motor

Resurgence of the two-speed motor

Two-speed motors can bring flexibility to many machines, and more engineers are coming to appreciate the technology. Lucie Hodkova of Exico explains why they can be the best solution.

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. 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 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. 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. And of course, many of the pumps and fans in industry may benefit from for a two-speed drive capability.  

While two-speed motors look very like standard fixed speed motors from the outside, internally they are quite different. In fact there are two principles upon which they can work: dual winding and pole changing. In a dual-wound motor, there are two windings, each separately connected to the power. 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 has twice as many magnets or poles as a standard motor. 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 solutions 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.

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, 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.

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