Electric Motor

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An electric motor is a device which converts electrical energy into kinetic and mechanical energy. Most electric motors utilise the motor effect in the collective deflection of a flow of electrons through a wire to turn a coil, however other models employ more ingenious devices such as the induction of eddy currents.

Contents

Basic Motor Design

A basic motor consists of a coil placed in a magnetic field. As current passes through the coil it will experience a force such that one side of the coil will be deflected upwards and the other side downwards (as the current is running in opposite directions), resulting in the coil turning. When the coil turns such that is is parallel to the magnetic field (and hence no force of deflection) the direction of the current is reversed, both sides of the coil will now experience a force in the opposite direction, resulting in the coil continuing to turn. As the current is reversed every 180O turn of the coil it will continue to turn provided there is a power input.

The basic motor, whether DC or AC, consists of the following components:

  • An armature, consisting of an axle, a coil and a commutator
    • An axle, the rod containing the coil and the commutator which is also used to turn other components (such as a wheel). In some motors the rod will contain an iron section around which the coil is wrapped, increasing its induced magnetic field and hence the strength of the deflecting force
    • A coil, or a length of wire looped many times, the current travels through this is deflected causing the coil to turn
    • A commutator, through which current is fed to the motor and whose role is to keep current travelling in the same direction with respect to the stator field (and hence keeps the coil turning in the same direction). It is essentially a cylindrical conductor which receives current from the brushes. The type of commutator (split ring or slip ring) depends on the motor
  • A stator component, consisting of a power supply, brushes and stator magnets
    • The power supply is fed through the brushes, these push up against the commutator such that as it turns there is sufficient contact between the two surfaces to allow a decent current to flow
    • Magnets, either permanent magnets or electromagnets, which provide the magnetic field (commonly called the stator field) which interacts with the induced field of the current, causing it to turn

The force on either side of the coil is given by F = nBIlsinX where n = number of coils, B = magnetic field density (tesla), I = current (amps), l = length of the wire (meters) and X is the angle of rotation of the coil (0O when the coil is parallel to the stator field, 90O when it is perpendicular to the field. This may seem contradictory, as in the motor effect a charged particle travelling parallel to a field will experience no force. However, the force on the coil is along the sides which are always perpendicular to the stator field, and hence will always experience a force, the reason the coil doesn't turn any further when it is at 90O is because it has been deflected as far as it can go, and needs to have the current in the coil reversed to keep turning.

Torque is defined as the turning effect of the force, in this scenario it is the ability of the motor to do work (eg turn another object). Torque = Fs where s is the distance of the application of the force to the turning point (in this case the side of the coil to the axle). However, as there are two sides to consider the value of s is the length of the coil not affected by the field, hence allowing the derrivation of Torque = BIAcosX where A is the area of the coil. A motor with high torque will be able to exert a larger force on another object, in contrast a motor with low torque may not be able to carry much of a load. Torque is not related to speed, a motor can have a high turning speed and low torque, and vice versa.

DC Motor

A DC electric motor converts a DC input current into kinetic energy. It uses a split ring commutator to ensure the current in the coil is maintained in the same direction as the stator field (and hence keep the coil turning in the same direction). A split ring commutator is a ring of conductive material around a cylinder base with two slits on either side of the ring, positioned such that the chord between the two splits is perpendicular to the brushes when the coil is perpendicular to the stator field, meaning that the direction of the current will be reversed in the coil (although kept constant with respect to the stator field) when the coil is deflected to its maxima, allowing the motor to keep turning.

AC Synchronous Motor

An AC synchronous motor uses a slip ring commutator instead of a split ring commutator, in which a fixed circuit feeds power to the coil. The current is reversed in the coil as the current from the power source reverses direction. However, in order to keep the current in the coil travelling in the same direction with respect to the stator field (and hence keep the coil turning in the same direction) the motor needs to turn at the same frequency as the frequency of the current (eg in countries where alternating current operates at 50Hz the motor would have to turn exactly 50 times per second, or at 3000 rpm). Although this is a major drawback of AC synchronous motors it can be overcome using gears to allow larger objects to turn at a lesser speed.

AC Induction Motor

An AC induction motor indirectly uses the motor effect, but it primarily relies on the induction of eddy currents and Lenz's Law to turn a motor. The motor consists of a squirrel cage (picture here) placed inside a ring of electromagnets. Half the electromagnets are connected such that their poles are south when the poles of the other half are north. An AC current is used to power the electromagnets so that their poles are constantly swapping. This results in a changing magnetic field with respect to the conducting bars of the cage, which induces a current such that its magnetic field interacts with the stator field to produce a force (motor effect) which opposes the relative motion between the two fields (Lenz's Law). As the electromagnets of the stator field are fixed in position the squirrel cage experiences the force and begins to turn to reduce the slip velocity (the relative motion between the cage and the stator field), as a result the limiting speed of an induction motor can reach is the frequency of the current. Because of their simplicity in design AC induction motors are in regularly use, and up to 95% of household appliances which use motors use AC induction motors.

See Also

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