Formula Student Electric: PTC resistors for formula racing

When not ten, but about 120 teams compete at the Hockenheimring, it is not professional teams with huge budgets that compete against each other. Then teams from different universities are testing themselves against each other in the Formula Student design competition. For some years now, the smell of petrol has been less in the air at the events, and in addition to the howling of the four-stroke engines, the humming of electric motors can be heard more and more often. Since 2010, Formula Student has been supplemented by the Formula Student Electric (FSE) class. As in the other disciplines of Formula Student, the student teams have to construct a new car every year and the requirements for this are continually updated.

"We faced a challenge here in 2021," explains Marius Brölemann. He is "Head of Hardware" and heads one of a total of four sub-teams in the racing team at the Ostwestfalen-Lippe (OWL) University of Applied Sciences in Lemgo. "There were adjusted rules for discharge and its power dissipation in 2021, which we could not fulfil at that time." Until then, the team used a discharge board with three high-load resistors in the aluminium profile. However, this was no longer suitable according to the new requirements and, among other things, was not sufficiently protected against overheating for this application. An enamelled wire resistor was to replace the resistors in the aluminium profile. The team requested a corresponding product from the Krah Group, which supplies electrical resistors and electronic controllers to customers in the industrial, automotive and e-mobility sectors worldwide. Roger Renfordt is Key Account Manager for E-drive technology at KRAH's headquarters in Drolshagen. He handled the enquiry and, after analysing the requirement, advised a PTC resistor.
The new unloading device saves weight and space in the racing car from Lemgo. It is also significantly more robust than the old solution.

More than sport - practical experience for real markets

The Lemgo Racing Team consists of 27 students from different faculties. They are working towards a common goal: to develop an electrically powered single-seater racing car from the idea to the finished vehicle, to take part in the Formula Student design competition with it and, of course, to win.

Some members of the OWL Racing Team at a Formula Student competition in the Czech Republic in 2021

Formula Student is more than a purely sporting event. The aim is to complement theoretical studies with practical experience. The teams work like a small production company. The target group for the final product is the non-professional weekend racer. The speeds are therefore somewhat lower than in professional formula racing: the top speed is reached at about 130 Km/h. The cars in the FSE tend to be driven at a lower speed than in professional formula racing. The cars of the FSE are designed rather for acceleration, 100 Km/h are reached in about 3.5 seconds. By comparison, the best acceleration among electric cars on the road in 2021 is the Tesla Model 3 Performance: it makes it from 0 to 100 km/h in 3.4 seconds.
In addition to planning, design and production, all areas of the production process, from the business plan to marketing, are part of the tasks of the racing teams. So the students also have to find suitable components or design them themselves. In the end, everything has to comply with the rules and regulations of the FSE in order to pass the scrutineering, the technical inspection before the race. And here there was a decisive innovation in 2021.

The OWL 2.1, the latest model in 2022

New rules, new solutions

Like its predecessor, the OWL 1.9, the current vehicle from Lemgo, the OWL 2.1, is driven by two electric motors with 40 kW power each. The two motors are operated by a double inverter to which 600 V DC are applied. According to the rules, this requires a discharge time of 4.75 seconds to achieve a discharge <60VDC, with a repetition of three discharges in a total period of 15 seconds. In addition, the rules stipulate that the discharge device must withstand a permanent voltage of 600 V in the event of a defect in the control unit. It must neither be destroyed nor overheat or exceed a critical temperature. For this purpose, the predecessor model of the OWL 2.1 still used a discharge board with three high-load resistors in aluminium profiles. The resistors were connected in series and mounted on a heat sink to protect against overheating. "These resistors in particular could no longer meet the adapted requirements in the regulations. The danger of overheating was too high. So we had to find a new solution that met the changed requirements and, if possible of course was lighter than the old discharge board." The search for possible products did not only take place online. The Formula Student teams are networked with each other and exchange information at numerous events in different countries. "Another team recommended Krah's resistors to us. We looked around on the website and then shortlisted one product: an enamelled wire resistor."

Roger Renfordt, Key Account Manager for E-Drive Technology, handled the enquiry from Lemgo. "In order to find the best possible solution together with our customers, we have to understand the respective application exactly," says Renfordt. "In direct conversation, we quite quickly identified one requirement as central: In the event of a defective control unit, it must be possible to permanently apply 600 volts to the resistor - without damage or overheating. In addition to the requested wire-wound resistor, a PTC resistor is a very good solution in such a case. Since a PTC resistor is more effective under the conditions, we recommended this solution."

PTCs as discharging resistors

A PTC resistor is a temperature-dependent resistor which, in contrast to wire-wound resistors, consists of a single ceramic body. Ceramic as such is not conductive, but if it is deliberately mixed with foreign atoms it acquires the properties of a PTC resistor with a positive temperature coefficient. This means that the resistance value increases as the temperature rises. Above a certain temperature, this increase is exponential. The resistors then no longer absorb any further power and thus protect themselves from overload. Thus, the resistors can also safely withstand the voltage of 600 V without being damaged. The breakdown voltage of these products is only reached at 1100 V. The transition temperature of the RXLG PTC resistors is around 140°C. 

Krah developed the ceramic of the resistor together with renowned suppliers from the PTC industry. The ceramic body is surrounded by an aluminium profile, which is made from one piece and was developed according to the needs of industrial customers. This also does not have to be mounted on heat sinks like the previous solution, because Krah has made the aluminium profile body from one piece for optimum cooling of the ceramic. Both of these factors contribute to the RXLG-PTC series' excellent performance. The compact design allows for a reinforced mounting footprint without exceeding an overall height of 11mm. The ceramic can dissipate heat particularly well via the mounting surface of 98 mm x 34 mm (for the 35W model) and the aluminium profile. Therefore, with this solution, no separate heat sink is necessary. In addition, the number of necessary resistors has been reduced from three to one. All together, this saves weight and space, but also means that the discharge device had to be completely reworked. However, mounting the PTC resistor did not pose any particular challenge. "A redesign of the inverter is planned for the coming season anyway. In the process, we were even able to uncover further optimisation potential. We are currently in the process of making better use of them. However, we already did quite well with the new solution at the last scrutineering."

RXLG-PTC resistor

Summary

The predecessor model of the OWL 1.9 had a discharge board with three resistors connected in series and mounted on a heat sink. The solution was prone to overheating, could no longer meet the requirements of the FSE since 2021 and burdened the vehicle with unnecessary weight. In the search for a better solution, several types of resistors were considered. In the consultation with the manufacturer, an RXLG-PTC type resistor was identified as the optimal solution. In the new discharge device, a PTC resistor replaces three wire-wound resistors. The weight of the heat sinks is completely eliminated. In addition, the PTC resistor reliably prevents overheating. "With the RXLG-PTC from KRAH, we have found a significantly better electrical solution. It is more robust. We have gained space and saved weight; so an improvement in all areas."