ALTERNATING CURRENT ELECTRICAL MOTOR

alternating current electric motor  using an electric current that reverses direction at regular intervals. AC electric motor has two basic electrical parts: a "stator" and "rotor"

The stator is the stationary electrical component. The rotor is the rotating electrical component rotates the motor shaft. The main advantage of DC motors over AC is that the speed of an AC motor is more difficult to control. To overcome this disadvantage, AC motors can be equipped with a variable frequency drive to increase control while reducing its speed. Motor is an induction motor is the most popular in the industry because of its reliability and easier maintenance. AC induction motors are inexpensive (half or less than the price of a DC motor) and also provide power to weight ratio is quite high (about twice a DC motor).

Types of Motor AC / Alternating Flow-Back

a. Synchronous motor. Synchronous motor is an AC motor that works at a fixed speed frequency of the system. These motors require direct current (DC) for power generation and has a low starting torque, and therefore synchronous motor is suitable for applications that start with low loads, such as air compressors, motors and generators frequency changes. Synchronous motors are able to improve the power factor of the system, so it is often used in systems that use a lot of electricity.

The main components of a synchronous motor is
• Rotor. The main difference between synchronous motor with an induction motor is that the rotor synchronous machines running at the same speed as the rotation of the magnetic field. This is possible because the magnetic field of the rotor is no longer induced. The rotor has permanent magnets or DC-excited, which was forced to lock in a certain position when confronted with another magnetic field.
• Stator. The stator produces a rotating magnetic field which is proportional to the frequency supplied.

This motor rotates at synchronous speed, which is given by the following equation (Parekh, 2003):

Ns = 120 f / P

Where:
f = frequency of the supply frequency
P = number of poles

b. Induction motor. Induction motors are the most common motors used in various industrial equipment. Its popularity is due to the design, which is simple, inexpensive and easily available, and can be directly connected to an AC power source.

Component of induction motor has two main electrical components:
• Rotor. Induction motors use two types of rotor:
- Squirrel cage rotor consists of a thick conductor rods are embedded in parallel slots plots. These bars are short-circuited at both ends by means of short-circuiting rings.
- A wound rotor has three-phase windings, double layer and distributed. Many poles as the stator. Three phases digulungi wire on the inside and the other end connected to a small ring attached to the stem as a brush attached to it.
• Stator. The stator is made of a number of stampings with slots to carry three-phase windings. It is wound for number of poles. The rolls were spaced at 120 degrees geometry.

Classification of induction motor

Induction motors can be classified into two main groups (Parekh, 2003):
• Single-phase induction motor. This motor has only one stator winding, operates with single-phase power supply, has a squirrel cage rotor, and require a tool to turn the bike. So far this bike is the most common type of motor used in home appliances, such as fans, washing machine and clothes dryer, and for the use of up to 3 to 4 Hp.
• three-phase induction motors. Rotating magnetic field produced by a balanced three-phase supply. These motors have high power capabilities, can have a squirrel cage or wound rotor (although 90% had a squirrel cage rotor), and the ignition itself. It is estimated that about 70% of the motor industry uses this type, for example, pumps, compressors, conveyor belts, power lines, and a grinder. Available in sizes 1/3 to hundreds of horsepower.

Induction motor speed

Induction motor works as follows: Electricity is supplied to the stator, which produces a magnetic field. This magnetic field moves around the rotor at synchronous speed. Rotor currents produce secondary magnetic field, which is trying to fight the stator magnetic field, which causes the rotor to rotate. However, in practice, the motor never runs at synchronous speed but the "base rate" is lower. The difference between the two speeds is the "slip / slide" that increases with increasing load. Slip only occurs in an induction motor. To avoid slip ring can be fitted a sliding / slip ring, and the motor is called "motor slip ring / slip ring motor".

The following equation can be used to calculate the percentage of slip / slide (Parekh, 2003):

% Slip = (Ns - Nb) / Ns x 100

Where:
Ns = synchronous speed in RPM
Nb = base speed in RPM

The relationship between load, speed and torque
 graph torque vs. speed three-phase AC induction motor with the current set. When the motor (Parekh, 2003):
• Start turns lights are flame high initial currents and low torque ("pull-up torque").
• Achieve 80% of full speed, the torque is at the highest level ("pull-out torque") and the current begins to drop.
• At full speed, or synchronous speed, torque and stator currents down to zero.
also read article about direct current electrical motor