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2.3. Electric motors and manufacturers.
Picture 2.3-1: motor of the Renault Formula E (circled in red). Dimension are similar to that of the 18 inches rims (https://www.pinterest.it/).
Electric traction in automotive and motorsport is still an innovative field. For this reason it’s not easy to find devoted products. An usual Formula E car, which is one of the rare electric vehicles in motorsport, has a mass of 900kg and a maximum power of about 200 [kW].
Performance of that kind of vehicle are too far from Formula Student regulations, depicted by Chapter 2.2. Therefore, It’ easier to find more suitable products in the hybrid vehicle field. Power-units of Formula 1 cars are provided by traction electric motors. Such motors, utilized in the Kinetic Energy Recovery System, K.E.R.S., features 60 [kW] with a very limited weight. However this technology is very expensive and concealed, not suitable for a F.S.A.E. project.
Another field in which it’s possible to find many good solutions to apply in F.S.A.E. power-trains is the industry. Modern machine tools employ very interesting revolution speed varying motors, these motors are utilized by many F.S.A.E. teams. That’s an important occasion for manufacturers to test their products on a different field of employ. For this reason, different solution have been studied and evaluated at the start of the project, such YASA 400 and Evo Electric AFM140.
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Picture 2.3-2: Main dimensions of YASA 400 motor (https://www.yasa.com/).
Picture 2.3-3: Main dimensions of Evo Electric AFM140 (https://avidtp.com/).
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Some other motors that has been evaluated, in example Wittenstein MRSF049: this unit features high power and low weight for this reason it would represent a competitive choice. One further motor that has been evaluated is the AMK DT7-72-20-RxW-5000, which is similar to the Magneti Marelli one, and quite cheap respect to the others. By the way, as of previous explanation, Squadra Corse chose Magneti Marelli motors, specific for hybrid cars. Italian motors, for Squadra Corse, represent the better trade-off between packaging, performances and business themes.
2.4. Different types of electric motors.
All the units that have been evaluated are three-phase AC synchronous motors. The reasons of this boundary are explained here below. Available direct current motors don’t feature power ratings suitable for a racing car. Moreover, need to be used under steady load condition, in order to obtain the best performance level. This lack of flexibility led many car manufacturers to discard this kind of motors and prefer the alternating current ones.
Picture 2.4-1: AGNI DC which is employed on a motor bike(https://www.groen7.nl/).
Among the DC motors, AGNI and Lynch units have been evaluated but, as previously explained, didn’t match requirements proper of a race car. Lacks are displayed in terms of power, which is often below 30 [kW], and in terms of energy demand. This last issue is related to the presence of brushes in almost all DC motors. Some positive aspects, though, are the easy fitting and power supplying connection.
Moreover, DC motors are usually smaller and less noisy, even if exploit some lacks for what concerns cooling and isolation. One more positive aspect of DC motors is the need of less cables and connectors, in spite of their homologues.
Usually, AC motors feature bigger size, and this forces developers to manage with more attention the available room into the chassis. Usually, DC motors tend to stress the transmission more than AC motors, whose torque delivery can be more progressive and manageable. This difference is mainly due
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to the presence of easily programmable inverters linked to AC motors, instead of DC controllers.
Benefits are evident for both the battery pack and the drive-train system.
2.5. Permanent magnet motors.
AC motors and DC motors can both feature permanent magnets instead of windings of electrical wires.
This is a further distinction that leads to different options. Permanent magnet motors feature high torque, high efficiency and low power consumption. On the negative side, magnets can be demagnetized in case of high temperature inside the stator. In this case, more electromagnetic interference is generated by permanent magnets. S.C.12e motors belong to the permanent magnet category and this choice led to an accurate analysis on cooling system. Design of cooling has been assisted by track test gathered data, in order to prevent damages on magnets. Anyway, It’s important to remember that temperature is not important for reliability only, temperature strongly affects efficiency.
2.6. Comparative chart.
As previously explained at Chapter 2.3, basic technical information about performances have been collected from different manufacturers and organized in the following comparison. A range of different motors have been evaluated and compared to Magneti Marelli TMG Small, MGU 012. It’s necessary to clarify that performances refers to an input tension of 350 [V].
Plot 2.6-1: Comparative chart of electric motors.
The comparative Plot 2.6-1 shows that Evo Electric motor features good performances. However, weight and dimensions displayed in Picture 2.3-3 make this motor difficult to fit into a small
single-30 18
31 75
94 60
55 52 27
250 260 80
25 25 32 27
42,5 25
0 50 100 150 200 250 300
Magneti Marelli MGU012 AMK
DT7-72-20-RxW-5000 Bosch MAD 130
YASA 400 Evo Electric AFM140 Wittenstein MRSF049
Weight [kg]
Nominal torque [Nm]
Nominal Power [kW]
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seater vehicle. In order to overcome these drawbacks, Yasa motor has been evaluated. That features similar performances by the point of view of power and torque. However Yasa is much more lighter and compact, as it’s shown in Picture 2.3-2. Anyway, taking into account chapter 2.2, it’s clear that these two kinds of motor are oversized for a car exploiting a two independent motors layout.
A valiant solution to take into account may be the Wittenstein motor. Weight and torque are very interesting but the power makes this motor too weak for a single-motor layout and oversized for the case of two.
It’s clear that it’s necessary to evaluate smaller motors. One opportunity may be the motor from Bosch, but the weight it’s too high if compared to performances which are suitable for the desired layout.
By virtue of that, final challenge regards AMK versus Magneti Marelli. First of all It’s necessary underline that biggest issue of AMK motor regards tension. If AMK is operated around 350 [V] tension displays a dramatic drop of power. For this reason, AMK motors are a good solution by point of view of torque output and weight but are much more less competitive than Magneti Marelli units by point of view of exploited.
2.7. Magneti Marelli TMG small size (MGU012).
Plot 2.7-1: Performance of Magneti Marelli MGU 012.
Magneti Marelli automotive traction motors take their origins from the experience acquired in Formula 1. Indeed M.M. is the exclusive supplier of K.E.R.S. systems for all the teams of F1 world championship from 2009 season. That makes the company able to fit electric power-train solutions in tight volumes, with the maximum save of room and weight. That feature is especially useful when it’ necessary to fit an hybrid system on a vehicle designed to house the internal combustion engine only. Motors supplied
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to Squadra Corse are the result of decades of experience in the electric field, combined to the modern techniques of FEM calculation and simulation. M.M. MGU 012 small size is a motor specific for hybrid power-trains of road cars. Design of such motor, that is still to prototypal stage, is oriented to fulfil requirements of the mass production by virtue of low investments needed. Another important characteristic, required by the automotive field, is the modularity. M.M. motors are designed to be easily fitted on different platforms, featuring short times required for the validation of test results.
Most relevant features about chosen M.M. MGU 012 small size motors are:
High power density: up to 5 [kW/kg].
Reduced size: up to 15 [kW/l].
Large rpm range: 0 to 16.000 [rpm].
Good peak torque: 90 [Nm].
By mechanical point of view, design of the motor is quite conventional. In the large aluminium casted core, a smart system of cooling ducts is housed. By the exterior, it’s possible remark the two brass fittings that feature the inlet and the outlet of the liquid coolant. In the front, a large flange houses the threaded holes necessary to fasten motor on its specific housing. The mechanical output is represented by a steel grooved shaft that hangs out from the front flange. Output shaft is bounded by two slim radial spherical bearings located externally to the rotor. On the back side of the motor, an insulated plastic box contains the connections of the three phases and the plug of the rotational speed sensor.
Differently from Picture 2.7-1, contact boxes have been modified in order to receive one only three-phase cable. Such a cable is much easier to route and fit into a tight race car layout.
Picture 2.7-1: One of the two MGU 012 electric motors delivered to Squadra Corse. Data in the late refers to 350V input (https://www.facebook.com/SCPolito/).
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