Electric Motors

To propel an electric vehicle, electricity must be converted to mechanical force. Electric motors use current flowing through wires to create magnetic fields that interact with other magnetic fields in the motor to create motion.

Motor Types

DC Motors

DC motors are electro-mechanical machines which take DC inputs to generate torque. These machines include Brushed DC, Brushless DC machines (BLDC), and homopolar machines among others. While some use mechanical commutators like brushes, some DC machines commutate electrically based on the rotor position.

Brushed DC Motor

Brushed DC Motors are the conventional hobbyist motor and come in two main varieties: Permanent Magnet Based (PMDC) and Field-Winding Based (FWDC). They use mechanical brushes to commutate the the flow of current through the rotor to generate a torque. Because of this, they are easy to control, and do not require a position sensor to track the rotation of the motor. For the PMDC machine, the flux generated by the rotor interacts with the permanent magnet flux to generate torque. For the FWDC machine, the permanent magnets are replaced by another set of coils which generate the stator flux. This allows the machines to operate at high speeds using flux weakening techniques.

For FSAE applications, Brushed DC machines can reduce system complexity; however, at higher power levels, these machines have low efficiencies and must have regular maintenance of brushes. In addition, these machines have relatively low torque and power density which can increase system mass and cause packaging issues. For these reasons, they are not commonly used in FSAE powertrains.

Brushless DC Motor

Brushless DC motors (BLDC) are another form of DC machines that commutate electrically (i.e. without brushes). BLDC machines typically consist of a rotor with permanent magnets and a stator which is wound such that the EMF has a trapezoidal shape. While the structure shares many similarities with the AC Synchronous machines, the distinction is in the control methodology and corresponding winding structure.

Rather than using a high resolution control algorithm like Field Oriented Control, BLDC motors use a heavily discretized, six-step control method. This is implemented by commanding a DC voltage (averaged through PWM) through two of the motor phases based on the rotor position. This is implemented by Hall Effect sensors embedded in the machine which determine which of the six possible phases to regulate current in.

In addition to the Hall Effect sensor used for commutation, BLDC motors often use an encoder to track the rotational position and velocity of the motor for control purposes.

BLDC motors are advantageous for their high torque density (higher than SMPM machines); however, the six-step method generates a large amount of torque ripple which reduces efficiency. BLDC machines also cannot flux weaken with conventional techniques, so problems can arise at higher speeds if voltage limited.

AC Synchronous Motors

AC Synchronous Machines a class of machines characterized by the fact that their rotor flux is synchronous with the rotor angle. These category includes surface mount (SMPM), synchronous reluctance (synRel), and interior permanent magnet (IPM) machines among others. These machines use PM or mutual torque along with reluctance torque to generate torque over a wide speed range. AC Synchronous machines also have sinusoidal EMFs due to their winding distribution. These machines typically use high performance control aglorithms like Field Oriented Control which require a high resolution position sensors like a resolver.

Surface Mount Permanent Magnet Machines

Surface mount permanent magnet machines (SMPM or SMPMSM) are the most straight-forward type of synchronous machines. The AC stator consists of three phase wired in either a WYE or Delta configuration and usually has a sinusoidal winding distribution to achieve a sinusoidal Back-EMF. The rotor contains permanent magnets which are fixed on the surface of the rotor iron.

Four pole SMPM Machine Cross Section

Because the permeability of the magnets is close to air, the ideal SMPM has no rotor saliency and therefore no reluctance torque. The resultant torque expression is dependent solely on the current orthogonal to the rotor flux. I.E The only current that produces torque in the machine is that which generates a magnetic field in the quadrature axis in the rotor reference frame. The resultant torque expression is below.

$\tau ={\frac {3Np}{4}}\Lambda _{PM}I_{q}^{r}$

Where...

$Np$ = Number of Motor Poles

$\Lambda _{PM}$ = Permanent Magnet Flux Linkage

$I_{q}^{r}$ = Quadrature Current in rotor reference frame

For FSAE, SMPMs are good choice because of their high torque density and efficiency. At low speeds SMPM machines can also achieve unity power factor which is ideal when the vehicle is power limited. Using Field oriented control, SMPM's can flux-weaken as well to achieve higher speeds. Depending on the inductance of the machine, can achieve constant power after a certain speed. One problem with the SMPM design is that at high speeds the magnets experience high amounts of tensile stress. Because of non-idealities in the magnetic material, this stress worsens the performance of the material and inhibits high speed operation. This is not usually a problem for FSAE vehicles however.

Synchronous Reluctance Machines

Rather than using permanent magnets, Synchronous Reluctance (SynRel) Machines rely on the magnetic saliency of the rotor to generate a reluctance torque. The rotors of these machines have complex flux barriers cut out of the rotor lamination to achieve these salient poles. These machines are not as common as there SMPM counterparts, but they allow easy control of the flux in the machine. Because they do not contain permanent magnets, they are often thought of as the synchronous analogy to induction machines. Furthermore, a variation of SynRel machines, Switched Reluctance Machines, are often thought of as BLDC Machines are to SMPM machines.

Four Pole PM Assisted Synchronous Reluctance Machine

$\tau ={\frac {3Np}{4}}(L_{d}-L_{q})I_{d}^{r}I_{q}^{r}$

Where...

$Np$ = Number of Motor Poles

$L_{q}$ = Quadrature Axis Inductance in rotor reference frame

$L_{d}$ = Direct Axis Inductance in rotor reference frame

$I_{q}^{r}$ = Quadrature Axis Current in rotor reference frame

$I_{d}^{r}$ = Direct Axis Current in rotor reference frame

As previously stated, SynRel machines have intricate flux barriers that generate salient poles in the rotor. While this allows for easy control of machine's flux levels, the flux barriers can cause large mechanical stress concentrations which can cause problems for high torque applications. With that said, these machines can still achieve high levels of efficiency and have started to become more common place. Specifically, variations like the switched reluctance machine have been used in some electric vehicle applications despite their noise and torque ripple issues.

Interior Permanent Magnet Machines

Interior Permanent Magnet (IPM or IPMSM) Machines are newer type of synchronous machine that embed the rotor magnets in the rotor iron. There are several different types of IPM machines including inset, embedded, spoke, etc. Because the magnets which have a low incremental permeability are embedded in the highly permeable rotor iron, IPM machines a substantial rotor saliency. IPM machines, therefore, can use both permanent magnet and reluctance torque.

Four Pole IPM Cross Section

$\tau ={\frac {3Np}{4}}((L_{d}-L_{q})I_{d}^{r}+\Lambda _{PM})I_{q}^{r}$

Where...

$Np$ = Number of Motor Poles

$\Lambda _{PM}$ = Permanent Magnet Flux Linkage

$L_{q}$ = Quadrature Axis Inductance in rotor reference frame

$L_{d}$ = Direct Axis Inductance in rotor reference frame

$I_{q}^{r}$ = Quadrature Axis Current in rotor reference frame

$I_{d}^{r}$ = Direct Axis Current in rotor reference frame

IPM machines are advantageous for multiple reasons. First, because the magnets are embedded in the rotor, the magnets see predominately compressive forces rather than tensile in SMPM designs at high speeds. Because high performance magnets can withstand higher compressive forces than tensile, IPM motors can typically run at higher speeds than SMPM machines. Second, because of IPM's saliency and large inductance's, these machines can easily flux weaken and achieve constant power over a wide speeds range.

AC Asynchronous Motors

Asynchronous machines are a class of electric motors which are characterized by the fact that their rotor flux is not necessarily synchronous with the rotor itself. The most common and pervasive type of AC asynchronous machines in the induction machine. There are several different configurations of induction motors (wound rotor, squirrel cage rotor, slip-ring, etc); however, they all have similar core operating fundamentals. That is, each machine generates torque by inducing currents on the rotor, thus generating a rotor flux density which interacts with the stator flux.

Induction Machine

The torque analysis for an induction machines is more complex than that of a synchronous machine as there are many synchronous reference frames to choose from (e.g. Stator Current Frame, Stator Flux Reference Frame, Rotor Flux reference frame, etc.). For control purposes, namely Direct Field Oriented control, the machine dynamics are represented in the rotor flux reference frame. Additionally, the induction machines must regulate both the stator currents and flux levels in the rotor.

$\tau ={\frac {3Np}{4}}{\frac {M}{L_{r}}}|\lambda _{r}|i_{sq}^{\lambda _{r}}$

Where...

$Np$ = Number of Motor Poles

$\lambda _{r}$ = Rotor flux linkage

$M$ = Mutual Inductance

$L_{r}$ = Rotor Inductance

$i_{sq}^{\lambda _{r}}$ = Stator Quadrature Current in rotor flux linkage reference frame

While Induction motors are widely used in industrial applications because of their low cost (no permanent magnets) and self starting features, induction machines are not often used in electric vehicles. This is because they are often not as torque dense as their PMSM counterparts and for efficiency purposes due to added rotor losses due to the induced rotor currents. In addition, even with Direct Field Oriented control methods, induction machines are not as responsive as synchronous machines.

Common FSAE Motors

When selecting a motor or motors for a car, one must consider the mass, size, power, efficiency, available controllers, and ease of integration. The most common motors in FSAE are those from Emrax, AMK, and Plettenberg.