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Low-vibration running ensures performance and service life

The PMB Bobertag GmbH supports the Racing Team of the University of Kaiserslautern – KaRaT In the Formula Student 2021, the team competes against students from all over Europe with its new electrically powered e20 racing car. We use high-precision balancing technology to ensure that the drive motors run smoothly and with low vibration. The Formular Student was launched in Germany in 2006 as an annual event and international, interdisciplinary design competition. Every year students from all over the world present their self-developed and constructed formula cars. They compete in different disciplines. Starting with the project work, the business plan and the presentation, it goes all the way to the dynamic disciplines in which the car has to show what it can do. Driving dynamics, acceleration and endurance, all categories are included in the ranking. During the development and production of the vehicles, the teams rely heavily on the support of companies and suppliers. This is done as sponsoring in the form of services and consulting. This is also the case this season, which will continue well into next year due to Corona. The Racing Team’s new racing car “Elektronyte e20”, which is currently under construction, relies entirely on an electric drive in which each wheel is individually driven by a powerful electric motor. The first series of tests with the vehicle should take place in autumn this year. Decisive for the e-motors is the available installation space, the weight and the continuous output of 20kW per motor and 35kW at peak. The market for such engines is not large. So the team of the TU and HS Kaiserslautern decided to develop the best fitting motors for the construction of the vehicle to a part by themselves. The production of the motors and the blanks for the rotor rotors was taken over by well-known sponsors. All in close cooperation with KaRaT. The subsequent wire-cut EDM of the rotor rotors inevitably creates an unbalance due to production. Built into the electric motor, oscillations and vibrations are then generated during operation, which spread over the entire drive unit. In order to ensure high performance and reliability, i.e. the service life of the motor and all connected components, balancing is necessary. Our company was recommended to the Racing Team as a specialist in this field. The aim was to reduce the unbalance as far as reasonable and economically feasible. In the first step, the rotors were measured for their unbalance at our company in order to define the characteristics and size of the compensating rings to be manufactured. The correction of the unbalance is later performed on these rings.

In the first step we measure the original unbalance of the electric rotor

Each of the four rotor rotors showed a rather high unbalance, in extreme cases it was about 2 grams on the radius of 25mm, i.e. 50 gmm. That sounds like very little at first. If you look at the resulting centrifugal force, the result is a completely different situation. At an operating speed of 15,000 rpm, an imbalance force of 123 N (weight equivalent 12.5 kg) pulls outwards with each rotation of the rotor. This is very much and stands in the way of a long service life. After this first measurement we defined the values for the design of the shims. With the boundary conditions of high unbalance and magnetism of the rotors, brass was chosen as the material for the shims. The balancing is carried out with bearings already pressed on so that their influences on the unbalance are directly compensated.

Electric rotor with compensating rings made of brass and pressed-on bearings on the CAROBA® Balancer OnDesk500


Measurement of the unbalance in 2 planes

We chose radial drilling on the rings. The result of the balancing: We reduced the unbalance on all 4 rotors to 1/200 of the initial unbalance and a residual unbalance of a few milligrams.

Removal of the unbalance mass by radial drilling on the compensation ring

Finished balanced. The residual unbalance is now only in the range of a few milligrams

With such low residual unbalances, a long service life and high reliability of the electric motors is ensured. We wish the Racing Team much success in the coming season of the Student form.    

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Markus Dörr - Marketing & Vertrieb
Hertelsbrunnenring 30
67657 Kaiserslautern

Tel.: +49 631 20 40 15 12

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