The auto industry reinvents the electric motor — Part 1 of 2
Since its invention nearly 200 years ago, the electric motor has become so popular that its use is ubiquitous in many fields. Is there nothing more to invent? Well, yes actually because the auto industry has been adapting the electric motor — improving its efficiency (which is not as great as so often advertised), increasing its power density, and reducing its weight, form factor, and cost — to meet growing demand for electric and hybrid cars. Of all the types of electric machine technologies, only a few have been chosen by auto manufacturers and suppliers.

Permanent-magnet synchronous motor and hairpin stator (BorgWarner).
This article recaps the operating principle of electric motors used in automobiles and presents major developments in stator design. Part 2 will deal with the development of rotors, cooling systems, and drive systems.

ZF motor used by the Formula E Venturi team during the 2017 season.
History and technical reminders
The history of the electric motor began in 1821, when the Danish chemist Hans Christian Oersted discovered electromagnetism. Just a year later, English scientist Michael Faraday built on Oersted's work and invented a rotary device.
A multitude of inventors followed, such as Ányos Jedlik (commutated motor, 1827), William Sturgeon (the first DC commutated motor, 1832), Werner von Siemens (motor reversibility [generator], 1867), Walter Baily (the first induction motor, 1879), Galileo Ferraris (the three-phase induction motor, 1885), and Nikola Tesla (the generating dynamo, 1886). And still many more scientists improved on all these inventions.

Ányos Jedlik invented the first commutated motor, in 1827..
An electric motor converts electrical energy into mechanical energy. It can also be used in reverse as a generator, particularly to recover kinetic energy during deceleration. Motor–generators (M–G sets) are used in hybrid and fully electric vehicles.

An electric machine has a coaxial stator and rotor. The stator is fastened to the motor housing and consists of coils wound around a core. To propel a vehicle, the rotor is connected to the transmission. It has either magnets or a winding that generates a magnetic field. The most common arrangement consists of a rotor inside a ring-shape stator, but the reverse is possible as well.
Electricity is usually supplied by a battery, but it may also come from a fuel cell or a generator. Remaining within the scope of the technologies used in the automotive industry, direct current from the battery is converted to alternating current by an inverter. The inverter modulates the frequency, voltage, amperage, the shape of the distorted sinusoidal waveform, and the number of phases. This current then feeds the stator coils to create a magnetic field that varies according to the characteristics controlled by the inverter.
Magnetic attraction explained
https://www.youtube.com/watch?v=bCEiOnuODac&feature=emb_logo
Motor torque is created by a series of attractions and repulsions created between the rotor and stator by the magnetic field using the principle that unlike poles attract each other while like poles repel each other. Thus, the south pole of the magnetic field generated in the stator attracts the north pole of the rotor, causing it to rotate. Before two poles align with one another, the inverter switches off the energised coil in the stator and switches on the following coil. This allows the rotor to produce torque by 'trying to catch' the constantly changing magnetic fields in the stator.

Three-phase power supply of the stator coils of the VW ID.3 motor.
The vast family of electric motors makes use of a wide range of technologies. The scope of this article is limited to those used in the automotive industry.

The family tree of electric machines (J.R. Hendershot, 2013).
The synchronous motor and the induction motor
The terms synchronous and asynchronous (or induction) refer to the speed of the rotor in relation to the rotational speed supplying the stator coils one after the other. Both technologies are reversible (motor or generator).
The synchronous motor

Permanent-magnet synchronous motor on a rotor.
In a synchronous motor, the rotor speed is equal to that of the stator coils. One pole of the rotor always 'runs' after a rotating pole of the stator. Both speeds are therefore identical. The rotor is made up of either a winding or, as in most cases, permanent magnets. The torque is proportional to the magnetic intensity of the rotor and the current flowing through the stator.

Coil supply according to the Motor Design simulation tool.
Because synchronous motors deliver high efficiency (peak of 93–97%) and a high power density, most automakers opt for this technology. An extremely accurate rotor position sensor, such as a resolver, is required.
The induction motor
The difference with the induction (or asynchronous) motor lies in its speed ratio. The rotor 'comes unlocked' from the stator's magnetic field and its speed is therefore lower. This difference in speed, known as 'slip speed', creates torque. In generator mode, the rotor rotates faster than the stator's magnetic field.

Induction motor of the Tesla Model S (photo credit: Motor Design).
While some induction motors are built with wound rotors, most use squirrel-cage rotors. These consist of longitudinal conductive bars that are shorted at their ends and extend along the length of the rotor to carry induced currents. The bars are often slightly angled relative to the rotor's longitudinal axis so that they receive the magnetic field from the stator at all times, regardless of the rotor's position.

Squirrel-cage rotor of an induction motor.
Induction motors offer a good power-to-cost ratio, especially since they do not require expensive magnets. Induction motors that do not use magnets in their rotors do not generate resistance torque during so-called freewheeling operation and can be operated without a position sensor or do not require a high-accuracy sensor. This design enables them withstand high overloads over short periods.
Their main drawback is a lower efficiency under partial load (peak of 88–93%) due to high energy loss in the rotor (20–35% of total losses).
Among mass-market vehicle manufacturers, only Tesla and VW (ID.3 front-wheel drive) use induction motors.
The synchronous reluctance motor

Typical rotor shape used in variable reluctance motors (Tenneco starter-alternator).
Synchronous reluctance motors are also known as variable reluctance motors or switched reluctance motors. Their rotors have an even number of teeth made of non-polarized soft iron and their stators work using coil pairs spaced 180° apart. Reluctance is the ability of a magnetic circuit to oppose the flow of a magnetic field. Just as a magnetic field will pass through metal more easily than through air, a coil's magnetic field will try to pass through a tooth of the rotor, creating torque when the tooth is not located opposite. The field will then continue through the rotor and on to the coil located 180° opposite (and which also has the opposite pole) via the opposite tooth, generating torque again.
Remember that the torque generated by synchronous and induction motors is based on the attraction between coils and magnets. Just as with synchronous motors, synchronous reluctance motors are rotated by energising their coils in a specific direction.
Synchronous reluctance motors offer the advantage of being affordable, as they do not use costly permanent magnets. As a result, they are used for small accessories. The simple shape of their rotors makes them easy to balance and enables them to reach high speeds at reasonable cost. Because of this advantage and their low inertia, Valeo uses synchronous reluctance motors to power its superchargers. When the stator is no longer energised, the rotor is free of any magnetic field, which prevents the generation of resistance torque when the vehicle has to run on its inertia alone.

Valeo supercharger powered by a variable reluctance motor.
Synchronous reluctance motors do not offer the best efficiency or the best power-to-weight ratio. Their air gap (the distance between the rotor and stator) must be small to reduce the magnetising current. Their rotor geometry induces high torque variations and generates vibrations that can be reduced by controlling the stator.
Tesla uses this type of motor on the rear (main) axle of its Model 3 but with a major change: it added an arrangement of permanent magnets, known as a Halbach array (described in our article on this car), to the rotor. The company has announced that the asynchronous motor used to drive the front axle achieves a peak efficiency of 97% instead of 93%. This motor also confirms the advantage that reluctance rotors are easier to balance, despite the addition of magnets, since it reaches speeds of 19,000 rpm on Tesla's Performance model.

Extract from our article on the Tesla Model 3:
A Halbach array is an arrangement of permanent magnets that creates a stronger field on one side of the array while virtually cancelling it out on the other side. This is achieved by placing the magnets such that their poles are out of phase with one another, usually at 90 degrees. This essentially redirects the magnetic field from one side of the structure (the 'non-working' surface) to its other side (the 'working' surface), thereby strengthening the magnetic field of the active surface and reducing the field of the opposite surface to almost zero.
Stator technologies
A stator is made up of coils of copper wire through which current flows to create a rotating magnetic field to drive a rotor. However, when opposite a moving component, this magnetic field creates another magnetic field that passes through the coil mount. This phenomenon (known as eddy currents) is responsible for some of the losses in magnetic circuits, particularly at high frequency (high speed). These losses are largely eliminated by using stator cores made with stacks of up to 300 ferromagnetic laminations coated with an insulating varnish. These stacks are either bonded or welded together. The same solution is also used for rotors, and Siemens & Halske were the first to market a laminated rotor, in 1973.

Stator plates bonded together in the Vedecom's electric machinery prototyping workshop.
To further reduce the diffusion of eddy currents, the number of plates can be increased by reducing their thickness. For instance, the thickness of the plates making up the laminate was decreased from 0.35 mm on the Toyota Prius 1 to 0.30 mm on the Prius III, and then to 0.25 mm on the Prius IV (0.27 mm on the VW ID.3).

Stator of the Audi e-tron's front axle motor–generator: laminated core, round wires and winding heads.
A stator usually consists of a core with slots into which multiple coils of wire are wound. Each winding is connected to the others in either a delta or wye (or star) configuration. The wires are made of copper, which is highly conductive, and are coated with a layer of electrical insulating resin. An insulating layer is also sandwiched between the wires and the core.

Schaeffler alternator with stator winding heads.
Many stators with round wires have a winding head on each side. The area where the wire passes through the core in a slot is turned 180° to one side so that it passes through the core again in the opposite direction. Winding heads reduce efficiency because their wires are not facing the rotor and cannot create torque and they cause power losses due to their electrical resistance. They are especially large in alternators because their manufacturing costs must be kept low.
BMW i3 electric motor production: https://www.youtube.com/watch?v=dJZagXcLo1w&feature=emb_logo
Stator with radial coils

Motor with radial coils for a BMW hybrid powertrain.
Since it is difficult to reduce the volume of the winding head, the narrower the stator, the more it contributes to winding-head losses. Narrow stators are notably found in so-called 'pancake' motors of hybrid vehicles, which are mounted between the engine and the transmission.
In such cases, it is preferable to place radial coils around the stator's periphery. This does away with winding heads, but the large empty volumes (at the centre of each coil and between the coils) reduce the power density.
An important improvement in stator efficiency is the copper filling ratio, which affects the strength of the rotating magnetic field.

The fill factor of this stator has been improved by the use of rectangular wires.
The stators of the simplest and cheapest machines, such as alternators or actuators, consist of round-wire coils wound around a core. Round wires facilitate automated winding operations but create voids when they stacked on top of one another, thereby reducing the fill factor. As early as 2006, Honda began mass producing a flat-wire stator for the motor–generator of its second-generation Civic IMA Hybrid.

Schaeffler pancake motor with round-wire radial coils.
Motor-generator of the second-generation Honda Civic IMA. The stator has radial windings made up of square wires.
The hairpin stator

Hairpin stator for the Chevrolet Bolt.
Automating the manufacture of flat-wire windings directly onto stator cores is complex because it is difficult to prevent them twisting.
The most common technology used on recent high-efficiency motors is hairpin stators. The core is covered with a large number of pins made from precisely preformed copper flat wire. Each pin is inserted around the core through the same side and welded together on the opposite side.
This results in denser packing — impossible with round wires — that increases both the fill factor and the machine's power density. The winding head is also reduced to its maximum on one side. Toyota multiplied the fill factor of the round-wire stator on the motor–generator of its first-generation Prius and its fourth-generation model with hairpin stator by 1.3.
According to the DriveMode study, the copper fill factor ranges from 0.4 to 0.45 for wire windings and 0.6 to 0.7 for hairpin windings.

The motor–generator on the fourth-generation Toyota Prius uses a hairpin stator while the three preceding generations used round-wire stators.
There's no such thing as perfect technology. The welds used to connect each pin together can provide electrical resistance, making production more complex and extending assembly time. As with the winding heads, the side where the pins are welded creates electrical losses because this portion of the wires is not facing the rotor magnets and generates resistance.
Efficiency-wise, it is difficult to know if this technology is better than radial coils because all the applications are different and not easily comparable. However, Schaeffler points out that large flat wires induce energy losses through eddy currents and skin effects. The latter phenomenon occurs at high frequencies where current is unable to penetrate the conductor material and only diffuses along their surface.

Pins being inserted into a core by Tecnomatic.
General Motors revealed that it reduced eddy current losses on the Bolt motor by reducing the size of the pins and compensating for this reduction by stacking six pins in each slot instead of four, as in its Chevrolet Spark motor.
Hairpin stators can be used on both synchronous and induction motors.

The welded ends of the hairpins are visible on this side of this BorgWarner stator.
Stator with wave windings — Part 1 of 2

Wave winding design developed by Schaeffler.
To reduce the disadvantages of hairpin coils, Schaeffler has a wave winding that offers a few compromises. Square wire is wound into a flat, mat-like jig so that it forms a shape that matches the stator slots.

Schaeffler's wave winding stator.
After each length equivalent to the width of the stator core, the wire is turned over towards a different slot. The result is one surface without any welds and virtually no winding head.
This 'mat' of wires is then inserted into the stator slots via the inside of the core. Multiple mats are stacked one after the other and their wire ends are then welded together to make the necessary number of poles.
Schaeffler says that this design reduces eddy current losses due to the smaller wire cross sectional area and thus lowers stator temperature. It also says that the lower filling factor due to the smaller cross sectional area can be compensated by increasing the number of slots.
BorgWarner offers the same solution. Called S-wind, it is already being used on 12 V alternators but its suitability for high voltages up to 350 V required additional development to avoid placing mechanical stresses in the square wires.
The S-wind stator with round wires is now available on pancake motors mounted in P2 hybrids (between the engine and the transmission).

BorgWarner's S-wind stator.
Stator with wave windings — Part 2 of 2

Four-point comparison (rpm/torque) of a motor with wave windings and a motor with hairpin windings.
To compare the performance of a stator with wave windings with that of a hairpin stator, Schaeffler simulated a motor delivering 147 kW at 5300 rpm and 123 kW at a maximum speed of 18,000 rpm and fitted with a stator of each technology. While both stators had an outside diameter of 220 mm and a width of 110 mm, the hairpin stator had 72 slots and the stator with wave windings had 96 slots.
The simulation results showed that losses in the stator with wave winding were lower at low engine speeds but significantly higher at high engine speeds due to the high frequency. In addition, the rotor losses were higher for each measurement point due to 'higher harmonics' caused by the passage from one coil to the other, which generates a less continuous magnetic field than with hairpin stators. The engineers at Schaeffler added that the higher number of slots facilitates heat dissipation.
When compared during a Worldwide Harmonized Light-Duty Vehicles Test Cycle (WLTC), the average efficiency of the motor with wave windings was 94%, while that of the motor with hairpins was 89%. Schaeffler also said that its stator can deliver a motor torque of 230 Nm/kg versus 105 Nm for that of a Tesla (unspecified model) and just 100 Nm/kg for the BMW i3 (stator weight only).
Schaeffler will be able to produce stators with wave windings starting in 2020. They will be fitted on synchronous and induction motors whether with permanent magnets or controlled excitation.

Comparative analysis of three stator technologies (Schaeffler).
Written by: Yvonnick Gazeau
Sources: Audi, BMW, BorgWarner, Chevrolet, Honda, Schaeffler, Tesla, Toyota, Valeo, VW, ZF