Тема №1 AC and DC Electric Motors .

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DC MOTOR

DC motor is one of two basic types of motors: the other type is the alternating current(AC) motor.
A direct current (DC) motor converts electrical energy into mechanical energy. DC motors are the simplest of motor and type are used in household appliances, such as electric razors, andelectric windows and fans in cars.

In the above picture, see two magnets in the motor:The armature (or rotor) is an electromagnet and the field magnet is a permanent magnet.

DC motor has either permanent magnets or electromagnets (electromagnetic windings), which produce a magnetic field. When current passes through the armature (also known as the coil) placed between the north and south poles ofthe magnet, the field generated by the armature interacts with the field from the magnet and the armature rotates. The force rotating the armature is called torque.

In DC motor, the magnet forms the stator, the armature placed a on the rotor and a commutator switches the current flow from one coil to the other. The commutator connects the power source to the armature through the use of brushes. DС motors operate at a fixed speed for a fixed voltage.

DC Motor Types

Among DC motors, there are shunt-wound, series-wound, compound-woundmotors. There are also permanent magnet motors.

The field coils and the armature in a shunt-wound motor are connected in parallel, also known as shunt, so the field coil current is proportional to the load on the motor.

 In series-wound motors, the field coils and armature are connected in series and the current flows through the field coils and through the armature. The advantage of series-wound motors is that they have a large torque and they can be operated at low speed.

A compound-wound motor is hybrid of both the shunt-wound and series-wound types. It combines the characteristics of a series- wound motor and a shunt-wound motor. It has a greater torque than a shunt motor but a constant speed.

The function of the commutator is to transfer current to the rotating shaft. Brushes, electric contacts on the commutator, connect the power source to the armature. A brush is an electrical component which makes contact between a stationary and a moving electrical circuit.Brushes usually consist of a carbon/graphite composition.

At certain times during the rotation of a DC motor, the commutator must reverse the current. This can lead to decreased motor life because of the wear tear that occurs through friction and arcing.

 

AC MOTOR

   AC motors are electromagnetic devices that have an alternating current as a power source. Their function is to convert mechanical energy into electrical that can perform work. They operation the principle ofFaraday law of induction, which states that a rapidly changing magnetic field produces an induced emf or voltage.

Wires that carry current produce magnetic fields. Michael Faraday discovered that when a wire is in a changing magnetic field, a current is induced just as if there were a source of emf. The rapid is the changing of field, the greater is the generated more emf.

 

The two most common types of AC motors are induction and synchronous.

 A large percentage of small AC motors are induction motors. There is no current supplied to the rotating coils on the armature. These coils are closed loops which have large currents induced in them because of their low resistance. Induction motors obtain their torque from currents induced in these loops by the changing magnetic field produced in the stator (stationary) coils.

A rotating magnetic field in an induction motor exerts a torque on the armature coils. In the illustration, the rotating field is achieved by the extra coils on the pole pieces. Induction AC motors are found in refrigerators, trains, computers, hard drives, industrial machinery, household appliances and many other electronic devices. Induction motors are simpler to make, less expensive to produce and they have big life-time.

Synchronous AC motors turn at the same speed as the magnetic field. The stator may use permanent magnets or wound magnetic poles. The current in the coils of the rotor is changing to run same direction. Speed ​​of synchronous AC motors is determined by the number ofpairs of poles and the frequency of supplied alternating current.