Differentiating Stator and Rotor: A Simple Guide

This article describes the differences between stator and rotor. Before the comparison let’s have a look at their definitions.

What is Stator?

The stator is the assembly of the fixed parts performing the function of supporting – at least partially – the motor, but fundamentally it constitutes the part of the magnetic circuit that includes the inductor windings housed in special slots made in correspondence with its internal surface.

The stator is constituted by silicon steel alloy or by steel laminations, insulated one from the other. From its structure, it depends on how much it is affected by time-variable magnetic flows which cause losses due to hysteresis (linked to the nonlinear magnetization of the material) and to induced “eddy currents”.

In the slots obtained in the structure of the laminations, three primary windings are inserted (each of them constituted by more coils differently connected between them), to which the supply voltage is applied and which generates the magnetic field. The three-phase stator windings can be star- or delta-connected; this can be achieved with motors equipped with a terminal box with 6 terminals so that it is possible to feed the same motor with different three-phase network voltages. An example of double indication could be 230VΔ – 400VY or 400VΔ – 690VY, where the symbol Y or Δ refers to the connection of the stator windings; for example, taking into consideration the second case (400VΔ – 690VY), the indication means that the delta-windings of the motor can be connected to a three-phase network at 400V (phase-to-phase voltages), whereas, if for the same motor the windings are star-connected, the motor itself can be connected to a supply network at 690V (the star-windings shall be subjected to the network voltage reduced by 3 times).

What is Rotor?

The rotor is the rotating part of the motor and can be found inside the stator.

As with the stator, also the rotor core consists of electrically laminated steel sheets. Inside the rotor slots, there are aluminum windings that are cast together with short-circuit rings.

This is done by making holes in the lamination so that when they are stacked, channels will be formed through the rotor core. During the casting, these channels will fill up with aluminum and form the windings that together with the short-circuit rings are shaped like a squirrel cage. Hence the name squirrel cage induction motor.

The windings inside the rotor do not go straight but are skewered to reduce electrical noise and vibration. The rotor core helps to conduct the magnetic field from the stator to the rotor windings.

There is an air gap between the stator and the rotor and since it is known that air conducts magnetic fields poorly the gap can’t be too big. The airgap cannot either be too small since metallic objects expand when heated and as the rotor gets warm there will not be enough space for it to rotate inside the stator. The rotor windings can also be called rotor bars.

Differences Between Stator and Rotor

The differences between stator and rotor are:

• Stator is a stationary part of the motor. Rotor is the rotating part of the motor.

• The three main parts of the stator include the stator core, stator winding, and the outer frame. The two main parts of the rotor include rotor core and field winding.

• The three-phase supply is given to the winding of the stator. The rotor is connected to the DC supply.

• The winding arrangement of the stator is more complex. The rotor winding arrangement is simple.

• Stator winding is highly insulated because high voltage induces in it. The rotor winding has low insulation.

• The friction loss of the stator is high. The friction loss of the rotor is low.

• The cooling system of the stator is simple. The cooling system of the rotor is complex.

• The size of the stator winding is large for carrying heavy current. The size of the rotor winding is small.