EV-05 — Electric Motors

ASSUMPTION — This lesson introduces motor topologies at an engineering level. A specific model’s motor type is not presented as fact unless officially disclosed (EV-29).

1. Overview of Motor Types

Type	Key trait
PMSM (IPMSM/SPMSM)	High power density and efficiency; permanent-magnet rotor
Induction motor	No magnets; robust, but has rotor losses
BLDC	Trapezoidal drive; common in small/medium power
Switched reluctance (SRM)	Magnet-free, robust; noise/torque-ripple risk
Axial flux	Short/compact; high torque density

FACT — In passenger EVs the most common choice today is the permanent-magnet synchronous motor (PMSM), especially the interior-magnet IPMSM.

2. PMSM: IPMSM and SPMSM

• SPMSM — magnets on the rotor surface; torque comes mainly from magnet flux.
• IPMSM — magnets buried in the rotor; adds reluctance torque to the magnet torque.

FACT — The IPMSM adds reluctance torque through the magnetic asymmetry of its buried magnets, giving a wider speed range and better field weakening.

3. Induction Motor

Its rotor contains no permanent magnets; the stator field induces current in the rotor bars. Robustness and being magnet-free are advantages, but rotor currents produce losses, so efficiency is usually below a PMSM.

4. BLDC and Switched Reluctance

• BLDC — runs with trapezoidal back EMF; simpler control than a PMSM, but more visible torque ripple.
• SRM — magnet-free and robust; noise and torque ripple are drawbacks.

5. Axial Flux and Radial Flux

• Radial flux — flux flows perpendicular to the rotation axis; the conventional layout.
• Axial flux — flux flows along the axis; disc-shaped, short, high torque density.

6. Torque Production and Back EMF

Torque arises from the interaction between the stator’s rotating magnetic field and the rotor flux. As the motor spins, a back EMF is induced in the windings; it rises with speed.

FACT — Back EMF rises in proportion to speed. Since the supply voltage is limited, beyond a certain speed the flux must be reduced to raise speed further; this is field weakening.

7. Torque–Speed Characteristic

• Constant-torque region: at low speed the motor delivers maximum torque.
• Field-weakening (constant-power) region: after base speed the torque falls while power stays roughly constant.

The curve is shown in the “Technical Diagrams” section.

8. FAQ

What is the difference between PMSM and BLDC?

FACT — A PMSM runs with sinusoidal back EMF, a BLDC with trapezoidal; PMSM gives smoother torque, BLDC simpler control.

Why are induction motors still used?

FACT — Being magnet-free offers cost and supply advantages; they are chosen in some applications (notably auxiliary-axle motors).

9. Summary

• The most common EV motor is the IPMSM (magnet + reluctance torque).
• The induction motor is magnet-free and robust but usually less efficient.
• Back EMF rises with speed; field weakening is needed for high speed.
• Axial flux offers a compact shape and high torque density.

10. Sources and Verification Note

No model-specific motor-type claim is made. Topology knowledge is established textbook/IEEE-level material.

• DOE AFDC, SAE J1715, IEEE — electric machine topologies.

Next Lesson

• EV-06 — Inverter / Power Electronics: DC link, switching, SiC, and 800V systems.