Automotive Engineer

The push to electrify the powertrain has seen huge investment in battery technology, but it isn’t the only component that OEMs must focus on. Motor technology needs to be equally reliable to make electrified powertrains durable and efficient, especially as some figures indicate that there could be as many as two million hybrid and electric vehicles on the roads worldwide within the next two years.

But technologies differ. Although the three-phase motor is used in both hybrids and pure electric vehicles, different designs are needed if the motor is used only to assist the combustion engine, or if it is used for electric-only driving. 

Power outputs can range from 5kW to more than 120kW. Permanent magnet motors use rotors equipped with magnets while synchronous machines use a magnetised winding. Asynchronous machines or induction motors are the lower-cost option.

Tier One suppliers that manufacture motors, such as Continental, face many challenges, including packaging.

Short axial length will help OEMs to house the technology more easily, but motors will need to be developed to work in vehicles as small as A-segment city cars as well as in premium sedans and SUVs.

Cost is also an issue. Magnet technology can be expensive, so designing systems that can minimise its use or do away with it completely, without affecting performance, will allow motors to be used in a very wide range of applications.

While battery technology will remain the priority for OEMs and suppliers, as it plays such a large part in overall costs, motors will need continued development to make sure the powertrain can run efficiently and the drivability of hybrid and electric cars remains close to that of the combustion engine variants.

1. Stator copper winding
Hairpin winding technology is used for belt-alternator-starter technologies, which provides a high copper filling factor, improving the efficiency.

2. Stator lamination stack
Standard electrical steel is used with a thickness below 0.5mm. Because steel is widely available, it keeps costs down. There is a constant balance to be made between lower-cost materials and performance reduction.

3. Rotor lamination stack
Like the stator lamination stack, it uses standard electrical steel to reduce costs and is welded. Both parts are stamped from the same coil to minimise scrap.

4. Rotor copper winding
A single needle winding with round wire is used to produce the rotor.

5. Motor shaft
The motor’s maximum operating speed for running axle drives is 12,000rpm. For belt applications it is up to 17,000rpm. The shaft is made from case-hardened steel. The surface hardness is needed for contact surfaces while core strength is needed for torque transfer. Back-to-back test set-ups are used to determine lifetime durability. On each test stand, two machines are connected at the output shafts; one electric machine acts as motor, the other as generator, and vice versa.

6. Housing/cover
Possibly the simplest of the motor components. Both are made from diecast aluminium. Critical areas such as sealing surfaces and pilot diameter are machined to reach the required tolerances.

7. Brush system
Externally excited motors have two main advantages:
efficiency over a wide range (drive cycle efficiency) and a simple and reliable safety concept. But to get these advantages, a brush system for the rotor excitation is needed. Brushless energy transfer is theoretically possible, but needs too much space and is very expensive. For permanent magnet motors the challenges are safety and cost, and for synchronous machines the challenges are brush wear and rotor burst speed.