What are the stator and rotor in a motor?

In an electric motor, the stator is the stationary part, while the rotor rotates with the shaft. Their magnetic interaction produces the turning force. Their construction depends on the motor design: Not every rotor has wound coils, and not every rotor receives power through an external electrical connection.

What does the stator do?

In a typical three-phase AC motor, the stator has a core made from thin electrical steel laminations and windings placed in its slots. Three-phase current in these windings creates a rotating magnetic field. The laminated construction helps reduce eddy-current losses in the core. The motor's rated current and star or delta connection are directly relevant to supplying these windings correctly.

A stator does not always have windings. Some brushed DC motors use permanent magnets to provide the stationary field. In an outer-rotor motor, the rotor surrounds the stator. What defines the stator is therefore that it remains stationary, rather than its position on the outside of the motor.

What does the rotor do?

The rotor transfers electromagnetic turning force to the load through the shaft. Depending on the design, it may contain conductive bars, windings, permanent magnets or a specially shaped iron structure. The shaft is usually supported by bearings; describing it as turning on pulleys is misleading. Its torque can drive the load directly or pass through a gearbox.

Do the stator and rotor touch?

During normal operation, an air gap separates the stator from the rotor. Magnetic interaction takes place across this gap without the two parts rubbing against each other. Bearings and other components still create losses, so non-contact magnetic operation does not make the entire motor frictionless. An uneven air gap or a rotor rubbing against the stator requires investigation.

How do different motor designs compare?

Squirrel-cage induction motor: In this common induction motor design, conductive rotor bars are joined by short-circuiting end rings. No external power cable supplies the rotor. The rotating stator field induces voltage and current in the rotor circuit, allowing torque to develop. Even without a shaft load, the motor draws no-load current to establish its magnetic field and cover its losses.

Other induction and synchronous designs: A wound-rotor induction motor can connect its rotor windings to an external circuit through slip rings. This differs from the squirrel-cage arrangement. Synchronous motors may use permanent magnets or an excitation winding to create the rotor field. A synchronous reluctance motor instead uses a rotor structure with preferred paths for magnetic flux; rotor windings and magnets are unnecessary. Permanent-magnet rotors are common in AC servo motors and BLDC motors. The term “servo” describes a controlled-motion application, rather than one universal rotor construction.

Brushed DC and stepper motors: In a brushed DC motor, brushes and a commutator deliver current to the rotor windings. In a stepper motor, driving the stator phases in sequence makes the rotor move through defined angular steps. Stepper rotors may use permanent-magnet, variable-reluctance or hybrid construction.

Linear motion: A linear motor develops electromagnetic force between stationary and moving sections, producing travel along a straight axis instead of shaft rotation. Consequently, a moving motor section is not always a rotating rotor.

Does the rotor turn at the speed of the magnetic field?

The field's synchronous speed depends on supply frequency and pole count: n_s = 120 × f / P. Here, f is frequency in hertz and P is the total number of poles; the result is in rpm. At 50 Hz with four poles, the field rotates at 1500 rpm. This helps explain why the motor's rated frequency matters. In normal motoring operation, an induction rotor turns slightly slower than this field; the relative difference is called slip.

If the rotor runs at 1440 rpm, slip = (1500 − 1440) / 1500 × 100 = 4%. This difference is necessary for an induction motor to produce torque and does not, by itself, indicate a fault. A drive's slip compensation can help reduce the speed drop. To assess a load, consider the relationship between torque and speed together with the difference between torque and power. Three-phase power and current calculations also require voltage, power factor and efficiency.

How does a drive affect the stator and rotor?

A suitable variable frequency drive controls the voltage and frequency applied to the stator. V/f control and vector control are different methods; the selected mode must suit the motor and application. Auto-tuning can help identify the motor model used by the drive, but it does not correct inaccurate nameplate entries or an incorrect motor connection.

A PWM drive's carrier frequency is its switching frequency. It is not the motor's fundamental supply frequency or a direct measure of rotor speed. Switching settings can affect sound and heating, so changes must stay within the motor and drive manufacturers' operating limits.

What should maintenance checks consider?

High current, vibration, overheating or unusual sound does not prove a stator or rotor fault on its own. Supply imbalance, an incorrect connection, bearing problems, mechanical loading and inadequate cooling can cause similar symptoms. Readings from a suitable clamp meter should be assessed alongside load, speed, voltage and temperature. Check the meter's suitability separately when measuring a PWM output. Motor overload protection must also match the nameplate information and operating conditions.

Stator insulation, winding balance, rotor bars, bearings and the air gap require different checks. Do not work on an energized motor; electrical supplies and sources of mechanical movement must be safely isolated. The motor's efficiency classification or a healthy-looking exterior cannot replace these checks. Assess symptoms using measurements and manufacturer information together.