The paradigm of energy efficiency increase, branded industry 4.0, is accomplished also through the definition of spindle solutions oriented to energy saving: new scenarios at the horizon for machine tool manufacturers, which behold opportunities connected with the eco-design of the spindle subgroup and the advent of innovative direct-drive solutions.
Manufacturing represents one of the industrial sectors characterized by a high energy demand: for this reason, national and European institutions, universities, industrial associations, regulatory bodies and companies have started – at different levels and each with its own role – introducing policies, directives and solutions oriented to increase the sector efficiency. Various studies and works carried out on stock removal machine tools have highlighted how a potential energy saving can be achieved through opportune design choices aimed at the use of more efficient technologies and components and at a better machine use in terms of definition of operating parameters and/or machining strategy. In the specific case, the present article focuses on the aspects of efficiency and energy consumptions of the spindle systems that equip CNC machining centres. The current developments referred to the spindle/electrospindle ambit are mainly aimed at the enhancement of static, dynamic (stability, vibrations, chatter, removal capacity and so on) and productive performances of such machine elements, however paying growing attention to “eco” and energy consumption aspects. It is in fact worth underlining that the spindle, together with the relative auxiliary systems, constitutes one of the main energy-eater machine components in use phase, during the execution of machining processes for the manufacturing of finished products.
Spindle system and operating conditions
Spindles are rotary elements generally designed and manufactured for a broad array of machine tools used in several application sectors (aerospace, automotive, etc.) with the general target of maximising the stock removal rate and accuracy, consistently with the existing constraints. Since NC machining centres generally need to execute several machining operations on the same workpiece, the spindle system must be suitably designed for assuring the demanded flexibility level and the desired performances (speed, torque, power and accuracy) within the wide range of admissible operational conditions.
Energy saving and direct-drive spindles
Speaking of eco-oriented design of machine tool electrospindles, it is useful to highlight also the potentialities offered by the adoption of specific direct-drive spindles. Relevant example is represented by the result of the pre-competitive research and development activity carried out in the ambit of Erod project, funded by MiSE, with the participation of Milan Polytechnics, the manufacturer of HSD spindles and the manufacturer of machine tools Jobs Spa. Such activity has led to the development of an innovative direct-drive spindle and to the comparison of the latter’s energy performances with those provided by a standard mechanical spindle. The examination is specifically focused on Jobs Jomax 265 machining centre, analysed in terms of energy consumption in two different configurations: with a mechanical spindle typically used in a broad range of machine families by Jobs Spa and with innovative solution of direct-drive spindle opportunely designed. The traditional solution consists of an asynchronous electric motor, Redex Andantex transmission with two speed ranges (direct one and one providing for the spindle speed reduction to 1/5 of the motor rotation one), one chiller for the electric motor cooling and one unit constituted by an oil circulation pump and a second chiller for the relative conditioning. Such mechanical spindle can deliver torque exceeding 1000Nm and perform machining up to 4000rpm. The “innovative” solution is instead composed by a high-efficiency electrospindle intended for multitasking machines, i.e. to be used for performing both high torque/low speed machining and high-speed machining operations. This provides for the replacement of the above-mentioned asynchronous motor with a synchronous motor purposely designed and equipped with electronic speed gear. This solution allows neatly simplifying the system and, compared with the traditional solution adopted as benchmark, only the electric motor chiller is conserved. More in detail, the direct-drive spindle can cover an analogue speed range (max. speed 6000rpm) in virtue of a double configuration of stator windings that can be opportunely selected in relation to the technological requirements of the machining to be carried out. From the point of view of the maximum available torque as well, the performances of the new spindle motor are comparable to the conventional solution’s. From the energy point of view, the direct-drive solution, characterized by a drastic system simplification, aims at decreasing the consumptions connected with the presence of the lubrication system, the conditioning of the speed gear oil and, obviously, at eliminating its losses. On the other hand, however, the synchronous electric motor, having to operate on such a broad speed range, will hardly exhibit high efficiency performance, comparable to the asynchronous solution’s. The interesting design aspect is then the analysis of energy advantages and drawbacks of the two spindle solutions in comparison, targeted to quantifying the energy saving associated to the replacement of the standard spindle with the direct-drive one in a typical day of use of the considered machining centre.
Electrospindles and efficiency maps
Let us consider, as an example, a machining centre for milling operations: a relevant percentage of the energy consumption is connected with the spindle, depending on the torque and rotation speed demanded by the process. If an electrospindle is used, it is fundamental to consider also the consumption of the chiller that cools motor windings: the coolant circulation pump determines an important contribution to the “basal consumption” (independent from the various activities executed by the machine). In this case, the efficiency of the spindle + chiller subsystem can be defined as ratio between the output mechanical power from the spindle (towards the process) and the overall electric power absorbed by spindle and chiller, which is function of the demanded load. Such efficiency can be assessed experimentally through power measurements carried out by subjecting the spindle to different combinations of torque and speed, thus achieving a mapping. Then, it is possible to distinguish:
acceleration/deceleration phases related to the spindle start and stop (that actually use the whole available power)
parting off/facing machining (at growing speed, with the decreasing radius)
The brushless motor used would reach 95% of efficiency close to the high-torque region and lower speeds than defluxion one. The chiller action damages low-torque “zones”: since the majority of the test cycle examined occurs in regions with efficiency in the 20-70% range, the overall efficiency produced is much lower than the theoretical maximum. Hence, we therefore infer an indication for machine tool manufacturers: in designing a machine, it is important to evaluate the spindle system efficiency (with the eventual addition of the chiller and other auxiliaries) compared to the real requirements of the user regarding the machining cycles to be executed, in terms of speed and torque to be delivered. Moreover, it is desirable that the regulation activity concerning the evaluation of the environmental impact of machine tools – promoted by ISO WG12 and, on a national scale, by the relative UNI work group –can lead to standards for the definition of tests of “machine-based” energy measurements, aimed at a characterization, even simplified, of such efficiency maps: the goal is prescribing the execution of measurements that “sample” the efficiency in some reference points.
The use of virtual prototyping techniques allows competing and standing out in a market turning out to be increasingly dynamic and fast. The analysis of the stamping process through the finite element simulation allows assessing the relations among the various geometrical parameters of tool equipment and the necessary magnitudes to estimate the project validity; without implementing the numerous physical prototypes that would be necessary to succeed in defining correctly a method assuring the geometrical and dimensional tolerances demanded by customers.
This characteristic allows Bora, company headquartered at Moie di Maiolati Spontini (AN) that performs the activities of design, manufacturing and maintenance of tools (besides the sheet metal stamping of components for the automotive and household appliance industries), to offer a co-design service of the component together with final customers, thus reducing critical zones already in a preliminary phase; in addition to a drastic reduction of direct costs connected with the physical prototyping (design and implementation) and of the indirect costs linked with the testing phase.
Shaping the operations for perfect results
The main problems that generally emerge depend on the shape and on the characteristics of the piece to be implemented. In particular, for components characterized by small sizes compared to the thickness and by high deformation gradients, are critical the operations that affect the formability and the thickness reduction due to the deformation.
Using finite-element virtual simulation techniques and relying on the long-term experience gained by the company in the sector, it is possible to model correctly roughing and forming operations, to propose possible modifications of the element shape to the customer. This to improve the feasibility and the attainable thicknesses in short times while providing highly reliable indications. To the ends of a correct simulation of the stamping process, the company’s technical department manages both purely geometrical aspects (such as the shape of tools), and information about the material behaviour (by which are modelled the elastic response, the work hardening, the anisotropy and the formability diagram through the data provided by the supplier of raw materials), and dynamic aspects of the tool (such as the friction between the sheet metal and the various tools, the force and the position of the elastic members controlling the blank holders).
A three-phase, four-pole induction motor is made of two main parts: a stator and a rotor. The stator is made of three parts: a stator core, conducting wire, and frame. The stator core is a group of steel rings that are insulated from one another and then laminated together.
These rings include slots on their inside that the conducting wire will wrap around to form the stator coils. Simply put, in a three-phase induction motor, there are three different wire types. You can call these wire types Phase 1, Phase 2, and Phase 3.
Each wire type is wrapped around the slots on opposite sides of the inside of the stator core. Once the conducting wire is in place within the stator core, the core is placed within the frame.
How does an electric motor work?
Because of the complexity of the topic, the following is a simplified explanation of how a four-pole, three-phase AC induction motor works in a car. It starts with the battery in the car that is connected to the motor. Electrical energy is supplied to the stator via the car’s battery. The coils within the stator (made from the conducting wire) are arranged on opposite sides of the stator core and act as magnets, in a way. Therefore, when the electrical energy from the car battery is supplied to the motor, the coils create rotating, magnetic fields that pull the conducting rods on the outside of the rotor along behind it. The spinning rotor is what creates the mechanical energy needed to turn the gears of the car, which, in turn, rotate the tires. Now in a typical car, i.e., non-electric, there is both an engine and an alternator. The battery powers the engine, which powers the gears and wheels. The rotation of the wheels is what then powers the alternator in the car and the alternator recharges the battery. This is why you are told to drive your car around for a period after being jumped: the battery needs to be recharged in order to function appropriately. There is no alternator in an electric car.
So, how does the battery recharge then? While there is no separate alternator, the motor in an electric car acts as both motor and alternator.
This is due to the alternating nature of the AC signal that allows the voltage to be easily stepped up or stepped down to different values.That’s one of the reasons why electric cars are so unique.
As referenced above, the battery starts the motor, which supplies energy to the gears, which rotates the tires. This process happens when your foot is on the accelerator — the rotor is pulled along by the rotating magnetic field, requiring more torque. But what happens when you let off of the accelerator? When your foot comes off the accelerator the rotating magnetic field stops and the rotor starts spinning faster (as opposed to being pulled along by the magnetic field). When the rotor spins faster than the rotating magnetic field in the stator, this action recharges the battery, acting as an alternator.
Alternating current vs direct current
The conceptual differences behind these two types of currents should be obvious; while one current (DC) is consistent the other (AC) is more intermittent. However, things are a bit more complicated than just that simple explanation, so let’s break these two terms out in a bit more detail.
Direct current (DC)
The continuous current refers to a constant and unidirectional electric flow. Furthermore, the voltage keeps the polarity in time. On batteries, in fact, it is clearly marked which the positive and negative poles is. These use the constant potential difference to generate a current always in the same direction. In addition to batteries, fuel cells and solar ones, also the sliding between specific materials can produce direct current.
Alternating current (AC)
The term alternating current defines a type of electricity characterized by voltage (think water pressure in a hose) and current (think rate of water flow through the hose) that vary with respect to time (fig. 1). As the voltage and current of an AC signal change, they most often follow the pattern of a sine wave. Due to the waveform being a sine wave, the voltage and current alternate between a positive and negative polarity when viewed over time. The sine wave shape of AC signals is due to the way in which the electricity is generated.
Another term you may hear when discussing AC electricity is frequency. The frequency of the signal is the number of complete wave cycles completed during one second of time. Frequency is measured in Hertz (Hz) and in the United States the standard power-line frequency is 60 Hz. This means that the AC signal oscillates at a rate of 60 complete back-forth cycles every second.
Why is this important?
AC electricity is the best way to transfer useable energy from a generation source (i.e., a dam or windmill) over great distances.
This is due to the alternating nature of the AC signal that allows the voltage to be easily stepped up or stepped down to different values.This is why your home’s outlets will say 120 volts AC (safer for human consumption) but the voltage of a distribution transformer which supply power to a neighborhood (those cylindrical grey boxes you see on the power line poles), might have voltage as high as 66 kVA (66,000 volts AC).
AC power allows us to construct generators, motors, and distribution systems from electricity that are far more efficient than direct current, which is why AC is the most popular energy current for powering applications.
How does a three-phase, four-pole induction motor work?
Most large, industrial motors are induction motors and they are used to power diesel trains, dishwashers, fans, and countless other things. However, what exactly does an “induction” motor mean?
In technical terms, it means that the stator windings induce a current to flow into the rotor conductors.
In layman’s terms, this means that the motor is started because electricity is induced into the rotor by magnetic currents instead of a direct connection to electricity, like other motors such as a DC commutator motor.
What does polyphase mean? Whenever you have a stator that houses multiple, unique windings per motor pole, you are dealing with polyphase (fig. 2).
It is most common to expect a polyphase motor to be made up of three phases, but there are motors that utilize two phases. A polyphase system uses multiple voltages to phase-shift apart from each one in order to go intentionally out of line.
What does three phase mean? Based around Nikola Tesla’s basic principles defined in his polyphase induction motor put forth in 1883, “three phase” refers to the electrical energy currents that are supplied to the stator via the car’s battery (fig. 3).
This energy causes the conducting wire coils to start to behave like electromagnets. A simple way to understand three phase is to consider three cylinders, shaped in a Y formation, utilizing energy pointed toward the center point to generate power. As the energy is created, the current flows into the coil pairs inside the engine in such a way that it naturally creates a north and south pole within the coils, allowing them to act like opposite sides of a magnet.
Top performing electric cars
As this technology continues to advance, the performance of electric cars are starting to quickly catch up to, and even outperform, their gas counterparts. While there remains some distance for electric cars to go, the leaps that companies like Tesla and Toyota have made to this point have inspired hope that the future of transportation will no longer be reliant on fossil fuels. At this point, we all know the success that Tesla is experiencing in the field, putting out the Tesla Model S Sedan that is capable of driving up to 288 miles, hitting 155 MPH, and has 687 lb-ft torque.
However, there are dozens of other companies that are seeing massive progress in the field, such as Ford’s Fusion Hybrid, Toyota’s Prius and Camry-Hybrid, Mitsubishi’s iMiEV, Ford’s Focus, BMW’s i3, Chevy’s Spark, and Mercedes’ B-Class Electric (fig. 4).
Electric cars and the environment
Electric engines impact the environment both directly and indirectly at a micro and macro level. It depends on how you want to perceive the situation and how much energy you want. From the individual standpoint, electric cars do not require gasoline to run, which leads to cars with no emissions populating our highways and cities. While this presents a new problem with additional burden of electricity production, it alleviates the strain from millions of cars densely populating cities and suburbs putting toxins into the air (fig. 5).
Note: The MPG (miles per gallon) values listed for each region is the combined city/highway fuel economy rating of a gasoline vehicle that would have global warmings equivalent to driving an EV. Regional global warming emissions ratings are based on 2012 power plant data in the EPA’s eGrid 2015 database. Comparisons include gasoline and electricity fuel production emissions. The 58 MPG U.S. average is a sales-weighted average based on where EV’s were sold in 2014. From a large-scale perspective, there are several benefits to the rise of electric cars.
For starters, there is a reduction in noise pollution as the noise emitted from an electric engine is far more subdued than that of a gas-powered engine. In addition, because electric engines do not require the same type of lubricants and maintenance that a gas engine does, the chemicals and oils used at auto-shops will be reduced due to fewer cars needing check-ups.
The electric engine is changing the course of history in the same way that the steam powered engine and printing press redefined progress. While the electric engine is not paving new grounds in the same vein as these inventions, it is opening up a brand new segment of the transportation industry that is not only focused on style and performance, but also external impact. So, while the electric engine may not be reforming the world due to an introduction of some brand new invention or the creation of a new marketplace, it is redefining how we as a society define progress. If nothing else is to come from the advancements with the electric engine, at the very least we can say that our society has moved forward with our awareness of our environmental impact. This is the new definition of progress, as defined by the electric engine. (Jill Scott)