Electric Machines: Types, Operation, and Applications
Change of Polarity
To change the direction of the poles in a generator, you can change the direction of rotation of the generator, swap the two poles with two contactors, or permute the rotor. This applies to series, parallel, or loose excitation, with the latter being the most effective method.
DC Machines
The basic principle of electric power generation involves a magnetic field, movement, and the creation of current. This current is generated when alternating field lines cut through a coil. With two magnets, the current reaches its maximum point when the coil is perpendicular to the field. As the coil rotates, the current decreases to practically zero when it becomes parallel to the field. If the coil continues to rotate, the current reverses direction and becomes negative. Finally, the current reaches its maximum negative point, completing a full cycle.
Excitation of Inductors
The magnetic field necessary for an inductor to function can be generated in two ways: using a permanent magnet or an electromagnet powered by DC current.
Rectification of Current Collector Thin by a Dynamo
Starting with a single ring and brushes, the current flows in the same direction. As the number of rings increases, the current flow becomes more frequent and appears smoother (ripple current).
In a DC generator, several configurations are possible. Generators typically have four windings: two on the rotor and two on the stator.
Both sides can operate independently, in series, parallel, or compound, where half of one winding is connected in series with the other half, and the other half is connected in parallel.
Synchronous Machines
Synchronous machines are electromechanical energy conversion devices. They convert mechanical energy into electrical energy. Their speed depends directly on the grid frequency.
Types of Synchronous Machines
Synchronous machines consist of a rotor (moving part) and a stator (fixed part).
Rotor
The rotor is located inside the stator and consists of a mesh of steel plates forming a cylinder integral with the motor shaft.
Stator
The stator is made up of a casing that houses a crown of steel plates with slots. These slots accommodate the second electric circuit.
Salient Pole Alternators
Salient pole alternators have a flow of magnetized iron mass, creating poles at their extremities. The magnetic circuit is completed by a metal armor equipped with slots that house the second electric circuit.
Smooth Pole Alternators
Smooth pole alternators are formed by areas where the magnetic flux flows out from the cylinder head (north) or enters it through the armor (south).
4-Pole Alternators
Over the four arms of the rotor, two separate coil currents, each with N turns, are connected in series so that the orientation of the two windings of each arm is opposite. When the machine starts, current circulates continuously on the rotor, creating two opposing N and S poles, 180 degrees apart.
Synchronous Motors
In synchronous motors, the speed is constant and depends on the frequency applied, the number of pole pairs, and the synchronization. This constant speed makes synchronous motors very useful in automation applications. The speed is determined by the following formula:
n = (f * 60) / p
where:
- n is the speed in revolutions per minute (rpm)
- f is the frequency in Hertz (Hz)
- p is the number of pole pairs
Asynchronous Machines
In asynchronous machines, the rotor rotates at a speed slightly different from the stator magnetic field. This difference in speed is called slip. The speed of these machines varies slightly from the synchronous speed. As the engine load increases, the slip and power consumption increase. Slip is measured as a percentage.
Single-Phase Motors
Single-phase motors require an auxiliary element to start spinning.
Types of Induction Motors
Split-Phase
Split-phase motors have two windings: a main winding and an auxiliary winding. The auxiliary winding is responsible for starting the motor and has a two-step winding for starting. They use a squirrel-cage rotor, similar to three-phase motors.
Capacitor
Capacitor motors have two windings: a main winding and an auxiliary winding. A capacitor is placed in series with the auxiliary winding.
With Coil Shorted
Instead of an auxiliary winding, a loop is placed around one of the poles. The speed depends on the number of poles. The starting torque is much lower than that of a split-phase motor, about 60%.
Types of Collector Motors
Universal
Universal motors can be powered with AC or DC. The stator windings and rotor are connected in series through brushes. The torque is 2-3 times the normal torque. The speed varies depending on the load.
Types of Stepper Motors
Hybrids
Hybrid stepper motors combine the operation of two other types: variable reluctance motors and permanent magnet motors.
Synchronous Machine Operation
The operating principle of synchronous machines is the same as that of synchronous motors, based on the interaction of fields with variable currents.
Types of Machines
The electric machines are classified into three groups: generators, motors and transformers.
Transformed generators mechanical energy into electricity, motors and transformers backwards retains the same type of energy but transforms its characters.
From a mechanistic vision machines can be classified in rotating and static. Rotaries are the generators, motors and alternators. And the transformers static.
In rotary machines to and a fixed phone call and another call stator rotor.