Author's design represents a new word in the development of electric motors
In the first decade of the XX century, 38% of all Machines in the US worked on electricity – and this percentage fell to almost zero with the rise in the dominance of ICE in the 1920s. Today's desire to save energy and reduce harmful emissions has fueled a new life in electric vehicles, but their high cost and limited mileage hold back sales.
Most of the attempts to solve these problems are related to the improvement of batteries. Of course, improving energy storage systems, be it batteries or fuel cells, should remain part of any strategy for improving electric vehicles, but there is room for improvement in another fundamental component of the machines: the motor. The last four years we have been working on a new concept of traction electric motor used in electric cars and trucks. Our latest development greatly improves efficiency compared to conventional models – enough to make electric vehicles more practical and affordable.
Last year, we proved the performance of our engine in comprehensive laboratory tests, and although it is still far from being placed in the car, we have every reason to believe that there he will show himself as well. Our motor can increase the mileage of modern electric vehicles even if we do not make any progress in battery technology.
In order to understand the complexity of our task, it is necessary to recall the basis of the electric motor circuit ( EM). In comparison with ICE, EM is simpler, they have only a few critical components. Mechanics require a housing. It is called a stator, since it does not move. Requires a rotor, a rotating shaft and creates a torque. In order for the motor to work, the stator and the rotor must interact by magnetism, turning electrical energy into a mechanical one.
The concepts of motors differ precisely in the field of magnetic interfaces. In DC collector motors, current flows through brushes sliding along the collector node. The current flows through the collector and transfers energy to the winding on the rotor. The winding is repelled by permanent magnets or stator electromagnets. Brushes, sliding on the collector, periodically change the direction of the current, and the magnets of the rotor and stator repel each other again and again, as a result of which the rotor rotates. In other words, rotational motion is provided by a varying magnetic field produced by a collector connecting the coils to a current source and cyclically changing the direction of the current when the rotor rotates. However, this technology limits the torque and suffers from wear; It is no longer used in traction EM.
In modern electric cars an alternating current is used from the inverter. Here, a dynamic rotating magnetic field is created in the stator, and not in the rotor. This makes it possible to simplify the rotor circuit, which is usually more complex than the stator, which facilitates all tasks related to the development of EM.
AC motors are of two types: asynchronous and synchronous. We focus on synchronous ones, since they usually work better and more efficiently.
The advanced cooling system conducts the liquid directly through the coil (on the left) and not through the motor casing Right)
Synchronous motors are also of two types. More popular is a synchronous machine with permanent magnets [permanent-magnet synchronous machine, PMSM]using permanent magnets built into the rotor. To make it rotate, a rotating magnetic field is organized in the stator. This field is obtained due to the winding of the stator connected to an alternating current source. During operation, the poles of permanent magnets of the rotor are grasped by the rotating magnetic field of the stator, which causes the rotor to rotate.
Such a scheme used in the Chevrolet Volt and Bolt, in the BMW i3, in the Nissan Leaf and many other machines, Achieve efficiency of 97%. Permanent magnets are usually made of rare earth elements; Vivid examples are very powerful neodymium magnets developed in 1982 by General Motors and Sumitomo.
Pole synchronous motors [Salient-pole synchronous machines, SPSM)] use not permanent but electromagnets inside the rotor. The poles are coils in the form of pipes directed outward, like spokes of a wheel. These electromagnets in the rotor are powered by a direct current source connected to them via contact rings. The contact rings, unlike the collector, do not change the direction of the current. The north and south poles of the rotor are static, and the brushes do not wear out so quickly. As in the case of PMSM, rotation of the rotor is due to the rotation of the stator magnetic field.
Due to the need to feed the electromagnets of the rotor through the contact rings, these motors usually have a slightly lower peak efficiency in the range of 94 to 96 %. The advantage over PMSM lies in the adaptability of the rotor field, which allows the rotor to produce torque more efficiently at higher speeds. The total efficiency at use for overclocking the machine increases. The only manufacturer of such motors in the production cars is Renault with its models Zoe, Fluence and Kangoo.
Electric cars need to be built with not only efficient but also light components. The most obvious way to improve the ratio of power to weight is to reduce the size of the motor. However, such a machine will give a smaller torque for the same rotation speed. Therefore, in order to get more power, it is necessary to rotate the motor at higher speeds. Today's electric vehicles run at 12,000 rpm; In the next generation there will be motors working at 20,000 rpm; Already work on the motors, running at a speed of 30,000 rpm. The problem is that the higher the speed, the harder the reducer turns out – the speed of rotation of the motor is too much higher than the speed of rotation of the wheels.
Perfect storm: in the author's version (above), the Lorentz force and the shifted inductance (gray) are summed to the maximum total force (blue) ) Equal to 2. In the usual motor (bottom) the sum of two forces – the Lorentz force and the magnetic resistance (gray) – give the total force (blue), reaching a peak of only 1.76, with the rotor angle of 0.94 rad. The difference in this example is 14%
The second approach to improving the ratio of power to weight is to increase the strength of the magnetic field, which increases the torque. This is the meaning of adding an iron core to the coil – although this increases the weight, but at the same time enhances the magnetic flux density by two orders of magnitude. Consequently, practically all modern EMs use iron cores in the stator and rotor.
However, there is also a minus. When the field strength increases to a certain limit, the iron loses the possibility of increasing the flux density. This saturation can be slightly affected by adding additives and changing the process of making iron, but also the most effective materials are limited to 1.5 V * s / m 2 (volts per second per square meter, or Tesla, T). Only very expensive and rare vacuum iron-cobalt materials can reach magnetic flux densities of 2 T or more.
Finally, the third standard way to increase the torque is to amplify the field through the amplification of the current passing through the coils. Again, there are limitations. Increase the current, and increase the loss of resistance, reduce efficiency and there will be heat, which can damage the motor. For wires, metal, which conducts current better than copper, can be used. Silver wires also exist, but their use in such a device would be absurdly costly.
The only practical way to increase current is to control heat. Advanced cooling solutions conduct the liquid right next to the coils, and not farther away from them, outside the stator.
All these steps help to improve the weight-to-power ratio. In racing electric vehicles where the price does not matter, the motors can reach 0.15 kg per kilowatt, which is comparable to the best ICE from Formula 1.
We with students developed and created such high-performance electric motors for the car participating in the student Formula three years ago. We created motors in our laboratory at the Electrotechnical Institute of the Karlsruhe Institute of Technology. Each year the team created a new machine with an improved motor, reducer and power electronics. There are four motors in the car, one per wheel. Each has only 8 cm in diameter, 12 cm in length and 4.1 kg in weight, and produces 30 kW on a permanent basis and 50 kW at its peak. In 2016, our team won the world championship.
So this can really be done if the cost does not excite you. The main question is whether it is possible to use such efficiency improving technologies in mass production, in a car that you could buy? We created such a motor, so the answer to the question is positive.
We started with a simple idea. Electromotors work well both in the role of motors and in the role of generators, although for electromobiles such symmetry is not particularly needed. For a car you need a motor that works better as a motor than as a generator – the latter is only used to charge the batteries during regenerative braking.
To understand this idea, let's consider the operation of the PMSM motor. In such an engine, the motion is created by two forces. First, the force due to permanent magnets in the rotor. When the current passes through the copper coils of the stator, they create a magnetic field. Over time, the current passes from one coil to another and causes the magnetic field to rotate. The rotating field of the stator attracts the permanent magnets of the rotor, and it begins to move. This principle is based on the Lorentz force affecting the motion of a charged particle in a magnetic field.
But modern EMs receive some of the energy from magnetic resistance-a force that attracts the iron block to the magnet. The rotating stator field attracts both permanent magnets and rotor iron. The Lorentz force and magnetic resistance work side by side, and – depending on the motor circuit – are approximately equal to each other. Both forces are approximately equal to zero when the magnetic fields of the rotor and stator are equalized. As the angle between them increases, the motor generates mechanical energy.
In a synchronous motor, the stator and rotor fields work together, without the delays that exist in asynchronous machines. The stator field is at a certain angle to the rotor field, which can be adjusted during operation to achieve maximum efficiency. The optimum angle for creating a torque at a given current can be calculated in advance. Then it adjusts, as the current changes, to the power electronic system giving alternating current to the stator winding.
But here is the problem: when the stator field moves relative to the position of the rotor, the Lorentz force and the magnetic resistance increase , Then decrease. The force of the Lorentz increases by a sinusoid, reaching a peak 90 degrees from the reference point (from the point at which the stator and rotor fields are aligned). The force of the magnetic resistance varies cyclically twice as fast, therefore reaches a peak at 45 degrees.
As the forces reach a maximum at different points, the maximum motor strength is less than the sum of its parts. Let's admit, at any certain motor at the certain moment of work it appears, that the optimum angle for a maximum of total force will be 54 degrees. In this case, this peak will be 14% less than the total peaks of the two forces. This is the best possible compromise of this scheme.
If we could remake this motor so that the two forces reached a maximum at one point in the cycle, the motor's power would increase 14% completely free. You would lose only the effectiveness of working as a generator. But we, as will be shown below, have found a way to restore this ability, too, so that the motor can better restore energy during braking.
Developing an ideal leveling motor field is not an easy task. The problem is the combination of PMSM and SPSM in a new hybrid scheme. The result is a hybrid synchronous motor with a displaced axis of magnetic resistance. In fact, this motor uses both wires and permanent magnets to create a magnetic field in the rotor.
Others tried to work in this direction and then abandoned this idea – but they wanted to use permanent magnets only to strengthen the electromagnetic Field. Our innovation consists in using magnets only to give an accurate shape to the field in order to optimally align the two forces – the Lorentz force and the strength of the magnetic resistance.
The main development problem was the search for a rotor design that could change the shape of the field, While remaining strong enough to rotate at high speeds, without breaking at the same time. At the center of our scheme is a multi-layered structure of the rotor, which carries the copper winding on the iron core. We glued the permanent magnets to the poles of the core; Additional spikes prevent their departure. To keep everything in place, we used strong and light titanium pins, passed through the electromagnetic poles of the rotor, pulled by nuts to the rings of stainless steel.
We also found a way to get around the lack of the original motor, reducing the torque while running the generator . Now we can change the direction of the field in the rotor so that generation during regenerative braking works just as efficiently as the motor mode.
We did this by changing the direction of the current in the rotor winding while working in the generator mode. This works as follows. Imagine the initial view of the rotor. If you walk along its perimeter, you will find a certain sequence of northern and southern poles of electromagnetic (E) and permanent magnetic (P) sources: NE, NP, SE, SP. This sequence is repeated as many times as there are pole pairs in the motor. Changing the current direction in the winding, we change the orientation of the electromagnetic poles, and only them, as a result, the sequence turns into SE, NP, NE, SP.
Having studied these two sequences, you will see that the second is similar to the first backwards. This means that the rotor can be used in the motor mode (first sequence) or in the generator mode (second), when the current in the rotor reverses direction. Thus, our machine works more efficiently than conventional engines, both in the role of a motor and in the role of a generator. On our prototype, the change in current direction takes no more than 70 ms, which is fast enough for cars.
Last year we built a prototype motor on a bench and subjected it to thorough inspections. The results are clear: with the same power electronics, stator parameters and other limitations of the conventional motor, the machine is capable of delivering almost 6% more torque and 2% more efficiency in peak. In the cycle of driving, the results are even better: it requires 4.4% less energy. This means that a car traveling on one charge of 100 km would drive 104.4 km with this motor. Additional kilometers get to us almost by set, because in our scheme there are only a few additional parts, much less expensive than additional batteries.
We contacted several equipment manufacturers and they found our concept interesting, although it will take a long time Before you see one of these asymmetrical motors in a production car. But having appeared, as a result, it will become a new standard, as extracting all possible benefits from your available energy is a priority for both automakers and the whole of our society.