giovedì, gennaio 24, 2019

Electric motors and their impact on mechanics

Electric Motor

In the past, when motors were the conversation subject, it was almost unavoidable to think of a standard explosion engine, maybe of some old frames of some black and white movies. Today instead, speaking of motors from the industrial point of view certainly means referring to a constant process of technological innovation as well as to an increasingly green production and consumption methodology of goods. We take care of our health and of the one of our planet and we more and more tend to design and build a healthy environment. The atmospheric pollution is one of the main causes of mortality and this phenomenon can be more surveyed in cities because in narrow spaces a higher concentration of CO2 persists.

Therefore, we design fully “green” houses and entire cities; an ecologic culture grows and, obviously, the entire automotive industry cannot get away from this phenomenon. As a matter of fact, Toyota, Nissan, Citroen, BMW, Mercedes, Kia, Opel, Fiat, Hyundai and Volkswagen, all of them have a selection of electric models in their portfolio. Volvo, for instance, has announced it is going to modify, starting from 2019, the entire production of electric cars. A phenomenon – the implementation of electric motors in the various industrial fields – that no longer concerns just cars. Today it is possible to drive electric bikes with more and more professional characteristics; motorbikes with exceptional performances, such as CRP or Harley Davidson; to navigate with Riva boats; to compete with an electric single-seater. But not only, the industrial world is gradually getting in touch with it, packaging, food processing machines, machine tools, all those fields where there is the need of doing actions through a motion inside the machine itself. Rising matter, then, and with noteworthy numbers. Let us consider, for instance, once more the car world. Today 2 millions of electric cars are circulating whereas about 600 millions are expected in 2014, against, however, 1 billion of cars that will circulate still using the green petrol and diesel. We will witness a cohabitation among the various propulsion modalities – thermal, electric and hybrid – and an adaptation by the industrial system to what nowadays still seems a not mature system, however already steeply rising. What about small and medium engineering companies that produce mechanical components for the big production field of motors and of motion in general? What will be the repercussions on this manufacturing area? Certainly, we will no longer need certain components in the same quantities as today. Companies and subcontractors must and will have to seize the signs that will increasingly come from the market and from their customers, modify the supplied sectors and sometimes give up activities that have stopped being remunerative and demanded by the various manufacturing fields. New competence and specialization areas, specific training and new figures to be employed in companies will be necessary. Some entrepreneurial realities will look at electric motors with a critical attitude because they will have probably failed their own transformation but any novelty or change always generates new opportunities and it up to us reaping them. The future will be greener than today, complex, competitive and even more variegated, issuing new challenges for our engineering companies.
Stefano Colletta, Technical Director of Subfornitura News-Tecniche Nuove

Alternate or direct current motors?

ac or dc motors

Obviously, we are not referring to AC-DC, the group of “Highway to Hell” and “Back in Black”. We are instead asking the question whose answer determines the first engineering choice, in other words “alternate current or direct current motor”?
It is worth noticing that this choice simply concerns the motor power supply type, even before opting for a precise technological solution. The power supply type, in fact, determines some important structural characteristics of the actuator and, consequently, influences its use type and the relative performances.
Let us try, then, to give an answer to the question: AC or DC?

Direct current motors

Big, massive and powerful, they are the legacy of an age when the regulation, waiting for the future PWM techniques, was possible only on direct current systems.
In particular, the most performing structural typologies of DC motors provided for the possibility of regulating independently field voltage and current (i.e. of the stator winding when present as replacement of permanent magnets) and armature voltage and current (that is to say of the rotor winding).With configurations like the above-described one (called with independent regulation), it was possible to obtain specific operation curves for each type of application.ac or dc motors
Opportunely regulating, for instance, the voltage and current magnitudes, it was possible to obtain situations in which the torque delivery was the highest in standing starts and then decreased in almost linear manner with the speed rise. These were (and still are) typically the drive requirements. However, beyond applicative situations, we are analysing what are the advantages and the disadvantages connected with the use of this type of motor.

Brushes and sparks

Structurally, all DC motors have the wound rotor; it is clear that, to keep the rotation direction constant, it is necessary to supply armature current so that the magnetic field generated always interacts in the same sense with the magnetic stator field; however, since the rotor rotates on its own shaft, the magnetic interaction between rotor magnetic field (mobile rotary) and stator magnetic field (fixed) inverts the direction every 180°; a DC motor powered in this way, instead of rotating, actually would oscillate between a 0° position and 180° one. Extending the reasoning to each angle fraction, we deduce that the part of rotor winding to be powered at each fraction of angle is different from the one of the previous fraction and from the one of the successive fraction.
In DC motors, therefore, the rotor winding is actually composed by many sections and each of them is powered for a determinate fraction of the round angle.
The rotor shaft of direct current motors is then always equipped with a ring subdivided into longitudinal sectors, insulated one another, each pair of them acts as a contact terminal for a section of the armature winding.
Since the rotor turns, the rotor power supply, which is provided by sliding contacts (brushes), powers then in succession the different sections of the rotor winding, keeping the interaction between the magnetic fields constant and maximum.
Therefore, DC motors imply several circuit switchovers during their rotation; we can even state that the higher is the switchover number (i.e. the more is fractioned the rotor winding), the more the motor offers a constant torque corresponding to the highest that can be supplied. Unfortunately, each switchover requires that brushes open a circuit and immediately close the successive one and this means spark production that, as such, is source of radio-electric disturbances; such disturbances, depending of the motor power and rotation speed, can also be of notable entity and prevent or affect the operation of other contiguous electronic parts. This problematic aspect is then joined by the costs of machine downtimes determined by the necessary periodical maintenance owing to the wear of sliding contacts.

Alternate current: no disturbances

Alternate current motors do not need, in the vast majority of cases, sliding contacts because the rotor is not wound; in this type of actuators, the magnetic field of the mobile part is generated by induction directly by the one of the part fixed on a sort of “virtual” rotor winding, existing in virtue of its structural shape said “squirrel cage”.ac or dc motors
Just for information, in the technical literature these motors are called in several ways, including “induction motors “, “asynchronous motors”, “motors with rotor in short circuit “, as well as, naturally, with explicit references to squirrels.
The absence of brushes and of consequent sparks annuls all maintenance requirements imposed by DC motors, limiting reset interventions to the mere replacement of bearings when worn out. Moreover, being structurally much simpler that DC motors, AC motors provide the not secondary benefit of low investment costs.
On the other hand, this type of motors strongly suffers standing starts, requiring even ten-time higher pickup current than the nominal one. If not managed, this phenomenon induces violent overheating that, in many cases, can be even lethal for the motor. AC motors, used in applicative situations where no frequent stops or slowdowns occur, need, then, opportune cooling extra-ventilation and/or opportune oversizing.
They are not linear elements, and this is an even more important characteristic of AC motors: the torque they provide is not linear function of any significant magnitude (voltage, current, rotation speed and so on.), featuring instead a fluctuating trend in the nominal operating range, with a single peak at a rotation speed approaching the highest permitted. This means that induction motors are affected by serious regulation troubles of the rotation speed and, apart from simple ON-OFF applications (for instance a pump or a conveyor belt), all AC induction motors today find wide use in motion applications only if controlled by opportune electronics (inverter) able to linearize their operation curve, i.e. to make the delivered torque constant.

Brushless is better

Some years ago, somebody had the idea of combining the advantages of DC motors with those of AC motors: capability of maintaining the maximum torque in the entire speed range, unwound rotor and, therefore, absence of sliding contacts, possibility of starts and restarts without overheating damages, user-friendly speed regulation.
The new class of motors, with much pragmatism, has been called “brushless”, that is to say “without brushes”, i.e. without sliding contacts.
The technical and functional characteristics of this class of actuators are notable indeed: almost constant torque in the entire speed range, unwound rotor, possibility of constant speed variation without appreciable torque loss, possibility of frequent stops and restarts.
Since in any motor the rotation occurs due to the interaction between the stator and rotor magnetic fields, if in brushless types the rotor is not wound, it means that it must be in some ways magnetic; the rotor, in fact, is composed by powerful permanent magnets whereas in the stator (powered) it is generated a rotary magnetic field that “drags” the rotor.ac or dc motors
To deliver high torque, the magnetic fields must be very intense; the stator one can be made such by opportune current values whereas for the rotor one is important the quality of the permanent magnets that, to be up to the situation, are made with special materials. This explains one of the reasons for the higher cost of a brushless motor than a standard induction motor.
To achieve a uniform rotation and always the highest possible torque, inside brushless motors is always housed an angular rotor position sensor that provides a feedback to the controller on how to generate the stator magnetic field.
Such sensor can be of discrete type, i.e. able to recognize only a finite number of angular positions, or analogue, able to provide a different piece of information for each recognizable angle according to its resolution.
As for a discrete sensor nothing changes in the entire angle portion included between two distinguishable positions, this type of brushless motor is powered in DC; the motors equipped with analogue sensors are instead powered by sinusoidal alternate current where the progression of the angular position corresponds to an equivalent progression in the supply voltage.
It is evident that the higher resolution allows a better torque supply homogeneity.

Energy efficiency

The motor is an object consisting of two parts: the stator that, precisely “stays”, integral with the fixing surface, and the rotor that, exactly, “rotates” inside the stator.
It is clear for all that accelerating or slowing down an object means to win its inertia; it is then as clear for all that the inertia depends on the mass (to have a confirmation, it is sufficient to try to push first a bicycle and then a truck). Well, in motors with unwound rotor the rotor mass is limited and then with low inertia. All that results in a noteworthy energy saving because all the energy supplied is used to produce torque and not to win mechanical inertias, as it happens instead in DC motors. Not only: low inertia means also high dynamics, i.e. performance in quick speed changes and this, in modern industrial machines, is irremissible.
First in the energy-efficiency ranking are then AC motors with unwound rotor, that is to say asynchronous and brushless. However, if the energy density is at stake, then brushless are the real winners because, in virtue of the most efficient interaction between magnetic fields, with the same power as asynchronous competitors, they have much smaller physical sizes; this, in forefront industrial machines, is almost as irremissible.
Last, but not for this reason negligible, DC motors that, even if energy-eaters, grant high performances in critical sectors like drive and lifting. (Alberto Pivari)