Alternating and Direct Current: Motors and Alternators
Production of Alternating Current in a Rotating Coil
The production of alternating current occurs in a coil that rotates within a magnetic field. Each terminal of the coil is connected to a metal ring conductor, where two brushes collect the induced current supplied to the circuit and determine how it is applied externally. For the induced current, the right-hand rule of three fingers is applied.
Rectifying Current Through the Commutator Bars
We get DC current in the generator output. Converting AC into DC is achieved through the collector of thin connected on two washer conductors isolated from each other. We connect two brushes, which collect the current. The current flowing through the coil is alternating, but the collector, comprising the isolated washer, rectifies the current and makes it continuous. DC has many variations; it has a smaller ripple with four thin, and with four loops and eight thin, the stream is more linear.
Workmen Winding
In a powerful machine, a compensating winding is placed into the slots on the pole. Its mission is to eliminate distortions of the magnetic field. It is connected in series with the switch and the armature.
Parts of a Dynamo
The fundamental parts of a dynamo are the inductor, the induced, and the collector. The inductor is fixed and is located in the stator. It is formed by a two-pole electromagnet in bipolar magnetic machines and several pairs of poles in multipolar machines. The armature is movable and is the rotor. It is located in the compound of a cylindrical magnetic core comprising stacked magnetic plates. The collector consists of sheets of copper with the ability to connect the different circuits reflected in the induced. The current is collected with the help of two graphite sliding contacts.
Dynamo Magnetic Circuit
Lines of inductor magnetic field strength close through the pole pieces of the electromagnet, the armature, and the housing of the dynamo. The power lines run through a non-ferromagnetic, small space between the pole pieces and the air gap. The air gap is air, and we must minimize its size.
Armature Reaction
The armature creates a magnetic flux. The resultant flux between the inductor and induced causes spark problems in the brushes. To limit this problem, there are two solutions:
- Deviation of blades: Deviation depends on the intensity.
- Pole switching: It creates a field that compensates the induced field (this is the best method).
Excitation of Inducing
The inductor is fed by:
- Independent Excitation: The coil is fed from a source (battery) different from where the energy comes.
- Self-Excited: Excitation by shunt, series, and compound excitation.
Principle of Operation of DC Motors
It is based on the forces that appear in conductors with tours of electric currents and, in turn, are subjected to the action of a magnetic field of the magnet poles. The magnetic poles located in the stator are responsible for producing the magnetic field inductor. To get it to turn in one direction or another, a pair reverses the direction of force. This is achieved by reversing the direction of the rotor current and keeping the inductor magnetic field fixed.
Constitution of DC Motors
A DC motor is exactly like a DC generator and can function as a generator. Three parts are needed:
- Inductor circuit
- Armature circuit
- Collector with brushes
Armature Reaction in DC Motors
When passing through the motor armature, a transverse magnetic field develops, deviating from its original position to the main field in the inductor, causing sparks in the collector. To reduce the detrimental effect of armature reaction, switching poles are used, connected in series with the induced. The polarity is reversed both by the armature current and by the switching poles.
Performance of a DC Motor
Power is lost by the Joule effect because of the rush by drivers of other windings, by friction in the bearings and ventilation, and other circuits in current hysteresis and parasitic currents. It indicates the percentage ratio between the power of the engine and the total power or electric power.
Torque in DC Motors
The torque is obtained from the product of force by the radius. The useful torque produced by the engine can also be expressed between the useful power developed in the rotor and the angular velocity of the same. It is proportional to the armature current and field flux magnetic inductor.
Connection of DC Motors
- Separately Excited Motor: Very easy speed regulation, as are the operating characteristics (shunt).
- Shunt Motor: Includes a rheostat to limit the privacy of boot and one for regular B-inductor. The speed remains practically constant in any load regime. In high-power machines, easy winding compensation is needed. Speed regulation is easy. Torque is directly proportional to the induced current. Application: machine tools.
- Series Excited Motor: Uses a starting rheostat. It usually takes a very high starting torque. Dangerous operation in a vacuum is used to pack. Application: train traction, trams. Speed-regulating rheostat in parallel with the excited winding.
- Compound Excitation Motor: Field winding in two parts, one series, and one parallel. It does not pack. Velocity is stable with the load, varying a little. Best pair of starting the shunt application. Application: lifting device.
Regulation and Control of DC Motors
The rheostat is no longer often used as regulatory elements for modern DC motors. Regulators are able at all times to determine the operating point of the motor. Control and regulation of all variables are obtained with maximum effectiveness. At boot, the computer provides the engine around the time of the process and automated voltage values and currents required. Today, the evolution of electronics allows the fabrication of variable speed drives for AC motors at competitive prices.
Principle of Alternator Operation
Its operation is based on the general principle of electromagnetic induction. When drivers are set to rotate within a magnetic field, an induced EMF of sinusoidal character is produced, which connects to an external circuit through two slip rings and a pair of brushes. In practice, it is much more interesting to rotate the pole pieces of the magnetic field (inductor) and allow fixed drivers (induced).
Establishment of a Fixed Armature Alternator
The alternator consists of an inductor circuit and an armature circuit.
- The inductor circuit: Consists of a number of electromagnets, whose winding is made so that the poles present a north polarity, with an even number. Feeding the inductor windings are made with DC through two slip rings and a pair of brushes. As the alternator does not generate DC, there are two methods for the DC feed inductor winding to generate the magnetic field:
- Through a dynamic exciter, coupled to the shaft of the alternator. An exciter dynamo self-excitation has a shunt resistor connected through a regulation.
- By auxiliary alternator coupled to the shaft of the alternator. The advantage of such excitement is that if you merge it with a three-phase rectifier bridge to the shaft of the alternator, it is not necessary to use brushes for collecting circuit power inductor.
- The circuit induced: Comprises three coils, located 120º from each other and housed in grooves, a hollow cylindrical core, and magnetic sheets. The connection is usually a star, connecting the neutral ground. The winding of each phase of the armature is composed of several coils so that the EMFs join.
Frequency of Alternators
For an alternator to produce a fixed frequency AC at normal operation, it must rotate at a constant rate, known as synchronous speed. The frequency or number of cycles per second produced by an alternator is directly proportional to the speed of the alternator and the number of pairs of poles of the inductor circuit.
Alternator Coupling
When you want to increase power, several alternators are coupled in parallel. Also to the power grid in order to provide electrical energy to the production system. To perform the necessary coupling, these conditions are needed:
- The tension of the alternator to couple must be equal.
- The frequency of alternators must also be the same.
- The order of succession of phases of the alternator must be equal.
- When connecting the alternator, the voltage must be in phase.
To achieve this, it acts on the speed and excitation current alternators. It must operate at a constant speed to maintain a constant frequency. When more power is taken from the alternator, it reacts, giving greater resistance to movement and tending to slow down. If the speed drops, synchronism is lost, and it should be unplugged. To avoid this, use an automatic speed control. The alternator also has to provide a lot of reactive power, so he has to build a system that controls the exciting current of the inductor, increasing it and producing more reactive power.
The Three-Phase Asynchronous Motor
Three-phase asynchronous motors are commonly used in industry because of their simplicity, robustness, and easy maintenance.
Principle of Operation of Three-Phase Asynchronous Motors
When the magnet is in motion, the field lines that cross the disc also set in motion in the disk, producing an EMF. According to Laplace’s law, a couple of forces originate in the disc that put the disc in motion. The disc can never reach the same speed as the magnet because, if this happens, the relative motion of the two would vanish, and the magnetic field would be variable with respect to the disk, which would eliminate the induced EMF, current, and with them, the force couple.
Rotating Magnetic Field in Three-Phase Asynchronous Motors
Three coils are 120º out of phase with each other. Each of these coils is connected to each of the phases of a three-phase system.
Rotor of Three-Phase Asynchronous Motors Short-Circuited
In the stator of these motors, the coils that produce the rotating magnetic field are placed. The three coils are 120 degrees out of phase if electrical, and the 6 terminals are connected to the motherboard connections, being able to connect later in star or triangle. The rotor is cylindrical, and aluminum conductors are placed in it, housed in the core slots and shorted at their ends by a conducting ring. It is given the name of a squirrel cage. The rotor conductors, which are initially stationary, are swept by the rotating magnetic field, so a FEM is induced in them. The rotor speed can never reach that of the spin, as if these dolls do not induce any voltage in the rotor. The slip of an engine varies with the mechanical load it has to drag. The induced current produces a torque increase with the load on the engine, and the slip also increases. The current produces a strong start and strong torque. As soon as current begins to flow through the stopped rotor, it begins to rotate with accelerated motion in the same direction as the rotating field.
Mechanical Features of a Three-Phase Asynchronous Motor
The engine feature indicates the relationship between the motor torque and speed, the relatives of the pair of rotor speed.
Wound Rotor Asynchronous Motor with Slip Rings
The stator has the same characteristics as the shorted rotor, but the rotor winding is constructed by inserting a three-phase winding in slots of a cylindrical core of magnetic sheets. A brush and rub these rings allow you to connect some external resistors in series with the power to limit the current rhetoric. An advantage is that it is necessary to reduce the voltage stress on the stator to reduce the flow and, with the current rhetoric, always brings a torque reduction. Booting is done in successive steps, starting with getting a gentle current in the stator with a good pair of boots. The disadvantage is that they are more expensive and require more maintenance.
Single-Phase Motors
The AC phase is not always available in all electrical installations. Given their simplicity, robustness, low cost, and spark absence, they have great application for small appliances. The trend is to use a universal motor. In any case, the use of single-phase motors will be feasible for small power applications.
Phase Induction Motor with Rotor in Short
It’s like the three-phase squirrel. It has a rotor and a stator where the windings are. Its principle of operation is similar to the asynchronous phase. If we put a winding stator phase and submit it to a sinusoidal alternating voltage, the magnetic field that is obtained is not rotating. The rotor conductors first develop a torque in one direction, and when you change the magnetic flux, they develop torque in the opposite direction, thus getting the engine not to start. If these conditions manually push the rotor in either direction, we get to shift the axis of the rotor magnetic field, and the motor starts to spin up to full speed.