Rotating Electrical Machines

Almost all electrical machines have several similar properties and features. The following discussion will explain the basic common features of rotating electrical machines. Where, a rotating electrical machine is one which has a moving (rotating) part, called rotor. The common examples of rotating electrical machines motors and generators.

In a rotating electrical machine, the torque produced can be considered in terms of the instantaneous flux pattern. According to this concept, a torque is produced in an electrical machine when the net magnetic field has asymmetry or distortion.

In any rotating electrical machine, the mechanical forces (torques) are produced due to the following two magnetic field effects −

• Alignment of magnetic field lines

• Interaction between magnetic fields and current-carrying conductors

In practical electrical machine, magnetic fields are produced by energizing a coil system. It is because, this method of magnetic field production relatively versatile and economic.

Basic Structure of Rotating Electrical Machines

The basic construction and structure of all rotating electrical machines is similar. A typical rotating electrical machine consists of two main parts namely,

• Stator

• Rotor

The stator and rotor are separated by an air gap. As the name implies, the stator is the stationary (non-movable) part of the electrical machine. In general, the stator is the outer frame of the machine. The rotor is the rotating (movable) part of the machine. Both stator and rotor are constructed by using laminated ferromagnetic materials to reduce the reluctance in the path of magnetic flux.

All rotating electrical machines consist of two windings, one placed on the stator part and another on the rotor part. The winding of the machine in which voltage is induced is known as armature winding. The winding which is used to produce the main working magnetic flux in the machine is known as field winding. Sometimes, instead of field winding, permanent magnets are used to produce the main magnetic flux.

Rotating Magnetic Field

The resultant magnetic field which revolve in the space and is produced by a system of windings (coils) symmetrically placed and supplied with poly-phase currents is known as rotating magnetic field (RMF).

The rotating magnetic field is such as that its magnetic poles do not remain in a fixed position, but go on shifting their positions. The speed of rotation of the magnetic field is known as synchronous speed and is denoted by NS. Mathematically, the synchronous speed is given by,

$$\mathrm{\mathit{N_{s}}\:=\:\frac{120\mathit{f}}{\mathit{P}}}$$

Where, f is the supply frequency in Hz and P is the number of poles. It is measured in RPM (Revolution per Minute).

Machine Torques

Torque is defined as the turning movement of force. The torque is the main factor which rotates the rotor of the machine. In electromechanical devices, there are two types of torques developed −

• Electromagnetic Torque

• Reluctance Torque

Electromagnetic Torque

The electromagnetic torque is one which produced due to interaction of the magnetic fields produced by the currents in two coils which may move relative to each other. In a rotating electrical machine, under normal operating conditions, there are two magnetic fields present a magnetic field from the stator circuit and another magnetic field from the rotor circuit. The interaction between these two magnetic fields produces the torque in the machine. This torque is known as electromagnetic torque. The electromagnetic torque is also known as induced torque.

Reluctance Torque

When an object made up of a ferromagnetic material is placed in an external magnetic field experiences a force (torque) which causes the object to align it with the external magnetic field, it is known as reluctance torque.

The reluctance torque occurs because the external magnetic field induced an internal magnetic field in the ferromagnetic object, and a torque is produced by interaction of the two magnetic fields moving the object to align with the external magnetic field. Since, the reluctance torque on the object tries to position it to give minimum reluctance (or saliency) for the magnetic flux. Therefore, the reluctance torque is also known as alignment torque or saliency torque.