From a simple electrical input signal, an AC motor can offer a comparably efficient method of producing mechanical energy. An AC motor commonly consists of two basic parts: a stator and a rotor. The stator stays outside which is the stationary part of the motor. It has coils and is supplied with alternating current to produce a rotating magnetic field.
The rotor stays inside which is the rotating part of the motor. It produces a second rotating magnetic field by being attached to the output shaft. Permanent magnets, reluctance saliency, or electrical winding may produce the rotor magnetic field. To gain a basic understanding of how an AC motor exactly works, we need to know its key characteristic.
An AC motor is distinct from many other motor types, especially DC motors. AC Induction motors are Asynchronous Machines meaning that the rotor does not turn at the exact same speed as the stator's rotating magnetic field.
Some difference in the rotor and stator speed is necessary in order to create the induction into the rotor. The difference between the two is called the slip. Slip must be kept within an optimal range in order for the motor to operate efficiently. A contactor allows the control of toggle power to an AC motor.
Manual starters have a manual switch that allows the operator to switch or change the power. This type of starter is known as across the line meaning the motor is wired directly to the power source. It directly connects the contacts of the motor to the full supply of voltage, which is normally six to eight times the rated current.
Star delta starters are common types of starters, which use a reduced supply of voltage in starting. The stator is connected in a star configuration, which switches to a delta configuration once the motor reaches a certain speed. By doing this, the line current drawn at starting is reduced. An auto transformer starter uses a similar method as a delta starter. Again, the initial current is limited to reduced voltage being applied to the stator. The advantage of an auto transformer starter is that the torque and current can be adjusted by the correct tapping.
A rotor impedance starter is connected directly to the rotor through the slip rings and brushes. At first, the rotor resistance is set to its maximum but gradually decreases as the motor speed increases.
A rotor impedance starter is very bulky and expensive. Since single phase motors produce a pulsating magnetic field they are unable to be self starting since a pulsating magnetic field torque cannot produce. Soft starters are a complex version, which allow for the control of acceleration and deceleration for stopping and starting the motor smoothly and evenly, which is not possible with across the line versions.
The advantage of soft starters is the reduction of the wear on the motor and the devices to which it is connected. The stator produces a rotating magnetic field. It has a solid metal axle, a loop of wire, coils, squirrel cage, and interconnections. Though a squirrel cage is not found in all AC motors, it is the most common type.
In AC motors, electricity is sent directly to the outer coils of the stator. The stator has multiple plates that extend out from its center with copper magnetic wire. For a three phase AC motor, it has three phase windings with a core and housing.
The windings are o apart, which can be six or twelve windings. The windings are placed on a laminated iron core. The construction of the core can be seen in the diagram below. Unlike a DC motor, the rotor on an AC motor does not have any connection with the external power source.
It receives its power from the stator. In a three phase induction motor, the rotor can be a squirrel cage or wound version. In the squirrel cage version, the rotor consists of rotor bars with end rings at both ends. There are several versions of the squirrel cage rotor, which include split phase, capacitor start, capacitor start and run, permanent split phase capacitor run, and shaded pole with classifications of A, B, C, D, and E. In the majority of cases, the squirrel cage is made of aluminum or copper.
As the current fluctuates, the EMF does the same causing the rotor to rotate producing rotational motion. A key factor in the motion is that the rotor does not turn at the same frequency as the AC current and is constantly trying to catch up, which is how the rotation is produced. If it did have the same frequency, the rotor would freeze, and there would not be any motion.
A wound or slip ring AC motor is a special type of AC motor. It contains the exact same parts as all AC motors but is always three phase. The cylindrical laminated core of the rotor is wound exactly like the windings on the stator with wire. The terminal ends of the wires are connected to slip rings on the output shaft. The slip rings connect to brushes and a variable speed resistor. The slip rings provide control of the speed and torque of the motor, which is the main positive feature of a wound rotor.
Wound motors are asynchronous where there is a difference between the stator speed and the output speed. When generating current in the rotor, the motor will have slippage between the rotating field and the rotor.
As the motor is powered, the rotor lessens the strength of the stator, which allows the control of the rotation and the ability to choose torque and running characteristics.
Now for this special arrangement, the magnetic field produced by 3 phase A. C current will be as shown at a particular instant. The components of A.
C current will vary with time. Two more instances are shown in the following figure, where due to the variation in the A. C current, the magnetic field also varies. It is clear that the magnetic field just takes a different orientation, but its magnitude remains the same. The speed of rotation of the magnetic field is known as synchronous speed. Assume you are putting a closed conductor inside such a rotating magnetic field.
Since the magnetic field is fluctuating an E. The E. F will produce a current through the loop. So the situation has become as if a current carrying loop is situated in a magnetic field. This will produce a magnetic force in the loop according to Lorentz law, So the loop will start to rotate, this is clearly illustrated in Fig A similar phenomenon also happens inside an induction motor. Here instead of a simple loop, something very similar to a squirrel cage is used.
A squirrel cage has got bars which are shorted by end rings. A 3 phase AC current passing through a Stator winding produces a rotating magnetic field. So as in the previous case, current will be induced in the bars of the squirrel cage and it will start to rotate. You can note variation of the induced current in squirrel cage bars.
This is due to the rate of change of magnetic flux in one squirrel bar pair which is different from another, due to its different orientation. This variation of current in the bar will change over time. To aid such electromagnetic induction, insulated iron core lamina are packed inside the rotor. Such small slices of iron layers make sure that eddy current losses are at a minimum.
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