How an Electric Motor Works: Stator, Rotor and the Principle Behind EV Drive Motors

An electric motor is a device that converts electrical energy into mechanical energy. It works by using energized coils — the stator windings — to generate a rotating magnetic field, which acts on the rotor’s closed conductive cage to produce magnetoelectric rotational torque. Motors divide into DC and AC types by power supply; most motors in the power system are AC, either synchronous or asynchronous (where the stator field’s speed does not stay synchronized with the rotor). An EV drive motor is built around the same two fundamental parts: a stator and a rotor. The direction of the force on a current-carrying wire in a magnetic field depends on both the current direction and the magnetic-field direction — in short, the working principle of the motor is the magnetic field acting on current to make the rotor turn.

Basic Structure of a Three-Phase Asynchronous Motor

A three-phase asynchronous motor consists of the stator, the rotor, and auxiliary components. The stator is the stationary part, and its construction determines much of the motor’s performance.

Stator Core

Function: it forms part of the motor’s magnetic circuit and carries the stator windings.

Structure: the core is punched and laminated from 0.35–0.5 mm silicon steel sheets with an insulating layer on the surface. Its inner circle is punched with evenly distributed slots to house the stator winding. Three slot types are common:

• Semi-closed slots: higher efficiency and power factor, but harder to wind and insulate — used in small low-voltage motors.
• Semi-open slots: accept pre-insulated formed windings — used in large and medium low-voltage motors.
• Open slots: easiest to insulate formed windings — used mainly in high-voltage motors.

Stator Winding

Function: it is the circuit part of the motor; connected to three-phase alternating current, it creates the rotating magnetic field.

Structure: three identical windings, symmetrically arranged at 120 electrical degrees apart, with each coil embedded in the stator slots according to a fixed pattern.

Main insulation systems (ensuring reliable separation between the winding’s conductors and the core, and between windings themselves):

• Ground insulation: between the whole stator winding and the stator core.
• Phase-to-phase insulation: between the stator windings of different phases.
• Turn-to-turn insulation: between turns within each phase winding.

Junction box wiring: a terminal block arranges the six ends of the three-phase winding in two rows — upper row U1, V1, W1; lower row W2, U2, V2 — so the winding can be connected in star (Y) or delta (Δ). Manufacturing and repair must both follow this numbering.

Frame

Function: it fixes the stator core and the front and rear end covers, supports the rotor, and provides protection and heat dissipation.

Structure: frames are usually cast iron; large asynchronous motors use steel-plate welding; micro-motor frames are cast aluminum. Enclosed motors carry cooling ribs on the outside of the frame to increase heat-dissipation area, while protected motors have ventilation holes in the end covers so internal and external air flow directly for cooling.

From Principle to Product

This stator–rotor architecture scales from industrial machines all the way down to the compact BLDC motors in light electric vehicles. Modern single-shaft hub motors put the stator, rotor, and wheel in one sealed unit, while the electronics that generate the rotating field live in the smart hub motor control system. Understanding the basic structure above makes it much easier to read motor specifications — efficiency, torque, and thermal behavior all trace directly back to how the stator and rotor are built.