The article is about electrical machines of different types, providing AC (alternating current) or DC (direct current), converting electrical energy into mechanical energy, used for remote-control and etc.
Electric motors and generators are a group of devices used to convert electrical energy into mechanical energy or mechanical energy into electrical energy by electromagnetic means. The machines that convert mechanical energy into electrical energy are called generators, alternators, or dynamo, and machines that convert electrical energy into mechanical energy are called motors. Two related physical principles underlie the operation of electric machines. The first is the principle of electromagnetic induction discovered by British scientist Michal Faraday in 1831. If a conduct is moved through a magnetic field or the strength of the stationary conducting loop is made to vary then the current is set up, or induced in the conductor. The converse of this principle is that of electromagnetic reaction, first observed by French physicist Andre Marie Ampere in 1820. If a current is passed through a conducter located in a magnetic field, the magnetic field exerts mechanical force on it. Both motors and generators consist of two basic units, the field, which is the electromagnet with its coils, and armature, the structure that supports the conductors which cut the magnetic field and carry the induced current in a generator or the exciting current in a motor.
Many scientists contributed the development of electrical machines. Besides Faraday and Ampere, such engineers and inventors as Zenobe-Theophile Gramme, who built the continuous-current generator, Nikola Tesla, who introduced the prototype of the present-day AC motor, Alessandro Volta, by whom the electrical battery was invented, Robert Groove developed the fuel cell and etc.
If an armature revolves between two stationary field poles, the current in the armature moves in one direction during half of each revolution and in the other direction during the other half. In DC generators a steady flow of unidirectional, or direct current outside the generator is obtained by means of reversing, which is called commutator. It is a split ring mounted on the shaft of the armature and fixed brushes of metal or carbon are held against it. When the armature revolves each brush is alternately in contact with half of the ring to produce the direct current in the external wires. DC generators are operated at fairly low voltages to avoid sparking between brushes and commutator, the highest potential commonly developed by such generators is 1500 V.
In AC generators the conductors that carry induced current are fixed and field is one that in motion. They alternators are constructed with stationary armature within which the rotor revolves, also in these machines it is not necessary to provide unidirectional current in the external circuit that is why they don’t have a commutator. That makes possible to operate AC generators at rather high voltages (up to 13200 V). The current generated by alternators rises to a peak, sinks to zero, drops to a negative peak and rises to zero again a number of times each second, depending on the frequency for which machine is designed. If we have only one winding on the armature the current provided will be single-phase alternating current, if we have two or three windings mounted at certain angles the current provided will have double-wave and triple-wave form and will be called two-phase and three-phase alternating current. The larger number of phases can be obtained by increasing the number of the armature windings.
In general, DC motors are similar to DC generators in construction and may be described as generators “run backwards”. AC motors are nodivided into two types – synchronous motors and induction motors. Synchronous motors have the same construction as alternators and are same as AC generator operated in reverse. In synchronous motors the magnetic field revolves with the armature at a same speed. The simplest type of induction motors is that with squirrel-cage rotor. There is a difference between the speed of the magnetic field and the speed of the rotor in the induction motors. This difference is called a slip.
Also for special applications several combined types of dynamoelectric machines are employed. Motor generators, for example, consisting of an appropriate motor mechanically coupled to an appropriate generator, or selsyns, that consist of pairs of motors with stationary fields and armatures wound with three sets of coils similar to those of a three-phase alternator. In use, the armatures of selsyns are connected electrically in parallel to each other but not to any external source. The field coils are connected in parallel to an external AC source. When the armatures of both selsyns are in the same position relative to the magnetic fields of their respective machines, the currents induced in the armature coils will be equal and will cancel each other out. When one of the armatures is moved, however, an imbalance is created that will cause a current to be induced in the other armature. The magnetic reaction to this current will move the second armature until it is in the same relative position as the first. Selsyns are widely used for remote-control and remote-indicating instruments where it is inconvenient or impossible to make a mechanical connection. DC machines known as amplidynes or rotortrols, which have several field windings, may be used as power amplifiers. A small change in the power supplied to one field winding produces a much larger corresponding change in the power output of the machine.
Add new comment