Basic concept of rotating electric machines

Introduction

All electric machine have many similar properties and feature. The basic common feature in rotating machine are given in the following section.

The torque production in a machine can be considered in term of the instantaneous flux pattern. In this concept torque is produced in a machine when the resultant magnetic field has distortion or a symmetry.

the two basic magnetic field effects resulting in the production of mechanical forces are;

Alignment of flux lines
interaction between magnetic fields and current carrying conductors.

The magnetic field in practical device and machine hour are invariably produced by a current Android coils system to make a versatile and economic arrangement. With a single and raised coil, the device is called singly excited system. double excited system have two coil which are arranged with one on the stationary part and one on the moving part.

In a singly excited system torque is exerted on the magnetic material to align it with the magnetic field aur move it into a stronger field. Stock ok is known as saliency torque. In a doubly excited system, a torque is produced by the core element of two magnetic field.

Basic structure of rotating electric machine

A rotating electric machine has two main parts, stator and rotor, separated by the air gap. the stator is the stationary part of the machine. Normally, it is the the outer frame of the machine. The rotor is the rotating part of the machine. solid or laminated ferromagnetic material are used for the stator and rotor to reduce the reluctance of the flux path. Most rotating machine have winding on the rotating and stationary members. The winding in which voltage is induced is called the armature winding. The winding through which current is passed to produce the main flux is called the field winding. Permanent magnets are used in some machine to produce the main flux of the machine.

DC machine

in the DC Machine the field winding is placed on the stator and armature winding on the rotor. Direct current is passed through the field winding to produce flux in the machine. Voltage induced in the armature winding is alternating. mechanical commutator and a brush assembly function as a rectifier or inverter, making the armature terminal voltage uni-directional.

Induction machine

in an induction machine, the stator winding serve as both armature winding and field windings. When the stator winding are connected to an AC supply, flux is produced in the air gap. This flux rotates at a fixed speed called synchronous speed. The rotating flux induced voltage in the stator and rotor winding if the rotor circuit is closed, current flow through the rotor winding and react with the rotating flux and the torque is produced in the steady-state, the rotor rotate at a speed very close to synchronous speed.

Two types of induction motor rotors are used;

Squirrel cage rotor or simply cage rotor.
Wound rotor or slip ring rotor.

Synchronous machine

in a synchronous machine testator carried armature winding, and rotor carries the field winding. The field winding is excited by direct current to produce flux in the air gap. When the rotor rotates, voltage is induced in the armature winding. The armature current produce rotating flux in the air gap. The speed of this flux is the same as the speed of the router there are two types of rotor construction, namely, the silent pole type and the cylindrical pole type.

MMS space wave of a concentrated coil

Consider a full pitch coil on the stator of two pole uniform gap AC machine. The coil consists of n turns and each turn carries a current I is on in figure.

The direction of current in the two coil side is known by a crossantidote police Stop the magnetic flux setup by this coil current is shown by dotted line in figure...

the developed view as shown in figure. In north and corresponding South pole are induced in the stator periphery. The magnetic axis of the coil is from the stator not to stators south.

The following adoption are made to determine the distribution of coil MMF.

the relative permeability of the stator and rotor course is infinite and the reluctance to the magnetic flux is offered by the air gap alone.
the air gap flux is radial and the field distortion in the unity of the coil is neglected.
the gap length is a small with respect to the rotor diameter and the flux density is assumed to be constant along a radius.

MMF of distributed single phase winding

Let us consider the fact of distributing a winding in several slot. In figure so phase a of the armature winding of two pole, three phase AC machine. Phases b and c have been put in empty slots. the windings of the three face are identical and are located with their magnetic Axis 120 degree apart. Let us consider the MMS of phase a alone. the winding is arranged in two layer each coil of NC having one side in the top of slot and the other coil side in the bottom of a slot, nearly one pole pitch away in figure b show one pole of this winding lead out flat. The MMF wave is a series of step of height 2πncia equal to the ampere conductors in the slot, where i a is the coil current. The distribution of winding produce a closer approximation to a sinusoid as compared to concentrated coil. The MMF wave can be split into fundamental component and higher order harmonics. the space fundamental component is known by the sinusoid in figure.

The MS office lot is displays from that of adjacent slot by slot angle beta. The resultant MMF is found by Fraser addition of slot MMF the phase difference between adjacent phaser being Alpha degree electricals. Desi factor similar to that consider in secondary for finding out EMF.

Thus, the effect of distributed winding on MMF waves can be accounted 4 by using a multiplication factor called distribution factor in a manner similar to the considered in finding EMF. effect on the MMS wave of short pitched coil can be taken into account by using the multiplication factor KP called the pitch factor.

MMS of 3 phase winding, rotating magnetic field

In this figure shown the status of two pole, three phase machine. The three phase winding, represented by aa', bb', and cc',are displaced from each other by 120 electrical degree in space around the inner circumference of the stator. The concentrated full pitch coil represent the actual distributed winding.

latest now considered the three phase of an AC winding carrying balance alternating currents.

When this current flow through the respective phase winding, is produce a sinus radially distributed MMF wave in space along its axis and having a peak located along the axis. each MMF wave can be represented by a space phasor along the axis of its phase with magnitude proportional to the the instantaneous value of the current. The resultant MMF wave is the net effect of the three-component MMF waves.

Machine Torque

There are essentially two kinds of force developed in electro mechanical devices. the first is due to interaction of the field produced by the current in two windings which may move relative to each other. This torque is called the electromagnet torque aur induce torque. II, usually called reluctant force or reluctance torque coma is dependent on the current in only one winding and is the result of variation in the reluctance of the air gap in the magnetic circuit carrying the flux which links that windings. Both this phenomena are often active simultaneously.

Electromagnetic torque or induced torque in AC machines

in AC machine under normal operating condition there are two magnetic field present a magnetic field from the rotor circuit and another magnetic field from the stator circuit. The interaction of these two magnetic field produced the torque in the machine.

Reluctance torque or alignment torque

reluctance torque is a torque experienced by a ferromagnetic object placed in an external magnetic field, which cause the object to line up with the external magnetic field. This talk occur because the external magnetic field induced and internal magnetic field in the object, and the torque is produced between the two field twisting the object around two line up with the external magnetic field. Thus, a torque is exerted on the object so that it tries to position itself to give minimum reluctance for the magnetic flux. Reluctance torque is also called the alignment torque or saliency torque.

a reluctance motor depend on reluctance torque for its operations.

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