New horizons for the electric household motor

The electric motor in household applications is evolving towards solutions with more user-friendly implementation and less challenging in terms of materials used.

The household market is going to be greener and greener. Not only because the legislation urges towards increasingly sustainable solutions; not only because “green” is a label that brings a good “sentiment” of consumers towards the brand that can boast it but also because consumers are gradually understanding the importance of a starting investment that is perhaps slightly higher against lower management costs, especially today when the electric energy has high prices. That is why we start giving importance to the energy label that, among the other features, also indicates energy consumptions. A good part of them in household appliances can be ascribed to the electric motor, therefore manufacturers are boosted to research increasingly efficient solutions for this component.

Once upon a time it was asynchronous
The first motors used in household appliance ambit were asynchronous and “universal” with brushes. They were and they still are motors that involve some positive aspects: they are simple to manufacture and they have a good starting torque. Prof. Marco Villani, Dep. of Industrial and Information Engineering and of Economics, University of L’Aquila and Technical Director of Factory, in fact states: «The double-winding asynchronous motor was used in the first washing machines and enabled only two operation speeds: one for washing and one for spin-drying. There was no other possibility. Such solution was then given up for the impossible fine adjustment of speed».
One of the most significant evolutions that have led to an important improvement in terms of electric motor’s efficiency was the shift from these asynchronous double-winding motors to motors with inverter and brushless motors.

Inverter and variable speed, revolution of the electric motor
The electronics development has allowed the establishment of the inverter. The inverter is a device that allows modulating the frequency and the supply voltage of an electric motor (and therefore its speed) according to the real load requirements.
Let us take the domestic refrigerator as example. In the refrigerator the electric motor is installed in the compressor. An inverter-free compressor supplies its power constantly and the temperature is regulated by the motor switching on and off (motor on/off). With the introduction of compressors with inverter, the power delivered (and then the cold generated) depends on the real temporary needs. Such motors are also called at variable speed precisely because their speed changes according to delivery demands. A motor with inverter needs much more technology and electronics on board than an on/off motor but, according to some manufacturing companies, it enables energy savings by even 40%. In the household sector, this transition is going on quite quickly: in the case of refrigerators, for instance, about 70% of the domestic world has already adopted variable-speed compressors and the migration from a fixed-speed compressor to a variable-speed one has an average payback period of less than two years.
Generally, inverters are mounted on brushless motors, that is to say motors without brushes, more reliable than models with brushes because they miss parts subjected to mechanical deterioration. «In the case of washing machines, the use of variable-speed motors, that is to say with inverter, enables a fine regulation of the spin-drying speed that, in fact, can be adjusted from 0 to 1500 revolutions» Villani explains.
The transition from the asynchronous motor to the motor with inverter was also accompanied by a further transformation that is to say from single-phase to three-phase motors. «The three-phase asynchronous motor coupled with the inverter allows achieving further advantages in terms of efficiency, speed regulation and performances» Villani affirms.

“Direct drive” systems
An evolution that we are living today, for instance in the world of washing machines and of suction hoods, is the use of “direct drive” systems. Villani explains: «In these systems, the electric motor consists of an internal stator and of an external rotor directly coupled with the drum of the washing machine. Therefore, the need of a motion transmission belt from the motor to the drum, and then of an element subjected to wear, is eliminated. This notably simplifies the mechanics, improves the stability and allows decreasing noise» Villani explains. «The same evolution occurs also in suction hoods: the use of “direct drive” solutions and of suitable software enables more versatility, efficiency and saving. Today, certain typed of hoods adjust their speed according to the quality and the intensity of the fumes to be sucked, through a system of sensors that allows adjusting the speed of the electric motor».

Permanent magnets? We can do without them
The latest generation of electric motors – in the household as well as in automotive – is the permanent magnet one. «Permanent-magnet motors are the most performing and feature a high efficiency and power density. However, they imply some drawbacks and presently the most evident one is that they contain “rare earths”. They are minerals for whose provisioning Europe strongly depends on third States, mainly China. This creates a dependence from which it would be certainly advisable to free ourselves and many companies – in both the household and in the automotive – are already doing that» Prof Villani affirms. Moreover: mining the minerals at the base of permanent magnets causes not few environmental damages in the Countries of origin, which would annul the energy efficiency benefits of these motors. «It would be convenient to use a bit less efficient motors but neatly more “ecologic” from the point of view of the supply chain, which do not create excessive dependence in terms of provisioning» prof Villani believes.

Synchronous reluctance motors: a possible alternative
Just in the wake of this idea, some years ago the University of L’Aquila started studying a new motor typology for the household: the synchronous reluctance motor. A synchronous reluctance motor is characterized by a ferromagnetic rotor without windings or brushes or permanent magnets and then structurally simple and cheap. The produced torque is connected with the anisotropy of the rotor within the “flux barriers”. The evident advantage of this motor is mainly the material making it up – only ferromagnetic recyclable material, at disposal in a broad part of the globe; then, the user-friendly structure that essentially has neither windings nor brushes. «The University of L’Aquila has designed various prototypes of reluctance motors for industrial applications but also for the domestic sector» Villani explains and adds: «Considering the results achieved with our prototypes, we daresay that for the household sector it is possible to propose products without permanent magnets, with a power of some hundreds of Watts. We are remarkably staking on these solutions because we have an inkling that precisely companies are going to search for new typologies of as ecologic motors as possible. This means also without permanent magnets, that is to say with easily recyclable materials. The reluctance motor might be the next generation of motors, even if today we are still in the prototyping stage: there are no companies that have them in their catalogue, there are not large-scale applications, yet». Besides the user-friendly manufacturing and the absence of permanent magnets, reluctance motors have also a lower cost than all others, even maintaining an acceptable performance. For this reason, the University of L’Aquila is proposing them as possible next generation of electric household motor.