Greater comfort and better fuel economy are much in demand for today’s new vehi­cles. Auto­matic start/stop systems and hybrid vehi­cles are good exam­ples of devel­op­ments in this direc­tion. But in addi­tion to the engine, it is also neces­sary to consider compo­nents such as the auto­matic trans­mis­sion. In conven­tional vehi­cles, the internal combus­tion engine drives an oil pump that in turn builds up the neces­sary control pres­sure. When the engine is at rest, this pump also stops and the pres­sure drops. Newer models there­fore have an elec­tri­cally driven pump in the trans­mis­sion to contin­u­ally provide control pres­sure and make sure that it can change gears imme­di­ately at any time – even while the engine is not running. A robust, elec­tron­i­cally commu­tated DC motor that oper­ates reli­ably in the trans­mis­sion oil drives this pump.

Figure 1: Robust oil pump system with excel­lent dynamic oper­a­tion even at high oil temper­a­tures.

Modern drive concepts for cars increas­ingly rely on elec­tri­cally controlled oper­a­tion. It allows many drive train para­me­ters to be opti­mally adjusted to the current driving situ­a­tion and saves energy in the process – trans­mis­sions included. Conven­tional, purely mechan­ical systems are diffi­cult to inte­grate into the new concept, which is why elec­tri­cally oper­ated actu­a­tors are the means of choice. ebm-papst St. Georgen, the specialist for auto­mo­tive compo­nents, now offers a robust DC motor as the hydraulic pump drive for auto­matic trans­mis­sions (Figure 1). It is ready to provide the control pres­sure required for elec­tronic closed-loop control from a speed of zero, ideally suiting the new control and drive concept.

No more rigid coupling

In the past, only mechan­i­cally rigid coupling was possible, such as for camshaft drive via chain or gear­wheel using the crank­shaft. However, this prin­ciple always means a fixed ratio of speed to control time. The modern elec­tronics in today’s cars have allowed ebm-papst St. Georgen to loosen up this rigid connec­tion, making all the drive train control processes flex­ible. An advan­tage here is that clas­sical hydraulic coupling or brake band acti­va­tion is easy to inte­grate into modern trans­mis­sion manage­ment via elec­tron­i­cally actu­ated valves.

The dynamic internal rotor motors work at ambient temper­a­tures from far below 0 °C to over 100 °C.

However, the clas­sical process of pres­sure gener­a­tion via pump – which is conven­tion­ally located on the input shaft driven by the engine – is a signif­i­cant disad­van­tage. Pres­sure can only be gener­ated when the engine is running, which means minimum delay times that become rele­vant during startup. And the delivery rate, which increases with pump speed, quickly becomes too high. The choke must “dispose of” the excess air flow being deliv­ered. An elegant solu­tion to this problem: an elec­tri­cally driven auxil­iary pump that provides the required pres­sure imme­di­ately after the igni­tion is turned on.

Sophis­ti­cated site of oper­a­tion

Figure 2: The in-house test stand has all the equip­ment required for testing the motors in detail under real-world condi­tions.

The mechan­ical design of the pump depends on the indi­vidual trans­mis­sion design, and the drive motor para­meters must always be adjusted to the trans­mis­sion. The special­ists from the Black Forest solve the problem by using basic compo­nents that they select and modify according to the perfor­mance spec­i­fi­ca­tions. Users must observe this application’s special condi­tions of use: The dynamic internal rotor motors work in trans­mis­sion oil and must be able to with­stand ambient temper­a­tures from far below 0 °C to over 100 °C. The oil contains addi­tives that opti­mize the fric­tion behavior of couplings, brake bands, etc. but can also have an aggres­sive impact on non-ferrous metals. Unpro­tected copper coils or unsuit­able insu­lating enamels would corrode and fail.

Since oil is viscous in cold envi­ron­ments and is diffi­cult to squeeze through cracks and open­ings, a low air flow is required here. This means speeds must be low and torques high, since rela­tively high fric­tion must be over­come. When it is warm, oil is more fluid and the pumps must convey higher volumes. In this case, the motor must run at a higher speed and a rela­tively low torque. This requires a corre­sponding motor power curve, and here, the good field weak­ening capa­bility of the drive meets the required perfor­mance profile.

Robust motors

On the one hand, different motor powers are required depending on the hydraulic design of the trans­mis­sion. On the other, the existing instal­la­tion space often requires extremes: a short motor with a wide diam­eter or the oppo­site (an elon­gated, slim motor). Motors with stack diam­e­ters of between 40 and 76 mm and differing lengths are avail­able as basic compo­nents. They allow power, torque and design to be adjusted to user require­ments. In numbers, this means that the range from 100 mNm to 2000 mNm, or converted, 40 – 500 W output power is seam­lessly covered.

The motors can be produced in a special clean room to achieve the absence of dust.

Espe­cially temper­a­ture-resis­tant high-perfor­mance magnets permit oper­ating temper­a­tures of up to 140 °C. Selected mate­rials and varnishes are able to with­stand the corro­sive addi­tives in the oil. The flex­ible design of the motors permits sinu­soidal or block control. Despite the use of stan­dard­ized motor compo­nents, of course customer-specific elec­trical contacts or mechan­ical vari­ants, such as the inte­gra­tion of a pump flange into the motor bearing assembly to reduce the number of inter­faces, are possible.

Produc­tion know-how

Figure 3: Clean-room produc­tion keeps foreign bodies out of the motors.

An in-house labo­ra­tory with a variety of test stands supports the devel­op­ment team (Figure 2). The motors can be exam­ined in detail under the most rigorous condi­tions. When the devel­op­ment work is completed, the next step is imple­men­ta­tion as serial produc­tion. For modern auto­matic trans­mis­sions in partic­ular, the closest toler­ances between moving compo­nents must be main­tained. As a result, even the smallest foreign bodies can produce sensi­tive disrup­tions, damage, or even complete failure. Upon customer request, the rele­vant motors will be produced in a special clean room in order to achieve the required absence of dust (Figure 3). Harmo­nized process manage­ment and a dedi­cated analysis labo­ra­tory monitor produc­tion constantly in order to guar­antee 100% quality assur­ance.

The result is modern elec­tric motors that reli­ably resist the adverse condi­tions present in the oil baths of auto­matic trans­mis­sions. Opti­mally designed for the hydraulic issues of the corre­sponding trans­mis­sion, they allow for reli­able oper­a­tion and highly respon­sive gear changes – even when the drive motor speed is zero. This makes them the ideal supple­ment to modern drive systems.