Electric Motors

1) The electric motor is a device employed for transforming electrical energy into mechanical energy. We know it to rotate machinery and various appliances. The two main types of electric motors are direct current (DC) motors and alternating current (AC) motors. Direct current motors, as the name implies, use direct-unidirectional current. Alternating current (AC) motors use an electrical current, which reverses its direction at regular intervals. The main advantage of DC motors over AC motors is that speed is more difficult to control for AC motors.

2) You know from the course of physics that the generator converts mechanical energy into electrical energy. Now, the process is reversed. It is electricity that is supplied to the machine and it is motion that we obtain. From what has been said about our getting magnetism from electricity and about the generation of electric current by using magnetism, it is obvious that generators and motors are similar in certain respect. There is certainly some difference in detail but in both of them we find an armature with windings, a commutator and brushes combined with an electromagnet for producing the magnetic field.

3) However, in an electric motor one shunt winding is not sufficient and a second one called a series winding should be added. “Why is it necessary?’ one might ask. The fact is that the motor should have a powerful effect at the very moment when the current is switched on, as for instance, in an electric tram or a train. A very strong magnetic field is needed to obtain a so-called powerful starting torque. This is achieved by adding a series winding to the magnetic coils. It is connected with the armature not in shunt but in series. Thus, heavy starting current, passing through the armature winding, now passes through the series field coil and provides a strong field necessary for starting, the shunt field winding providing the running conditions.

4) No appliance ever created by man has probably such a wide range of size and such a variety of application as a motor. In fact, on the one hand, there are all kinds of mighty giants in the motor world. These giants are known to perform innumerable operations wherever required. On the other hand, there exist all kinds of small-sized and even minute motors, which are able to power various complex machines and operate equally well under any conditions.

5) So far nothing was said of what a motor does in our homes. In a modern home there are many different electric motors in machines and devices utilized to meet our daily needs: to tell the time, to wash clothes, to cool the refrigerator, to clean things, to shave, to circulate air on a hot summer day and so on. In effect, vacuum cleaners, washing machines and modern refrigerators do work thanks to electric motors. It follows that in the electric motor we have the valuable and powerful appliance capable of fulfilling the required operations exactly and with just the desirable power and rate of motion. It is readily switched on, at will, and it continues running until we switch it off. There are often cases when it is simply impossible to replace it by any other means. In short, the motor finds application in industry and engineering, in agriculture and transport, in medicine and our homes.

Match each of these diagrams with the correct description A, B, C or D. One of the descriptions does not match any of the diagrams. The diagrams are in the correct sequence, but the descriptions are not.

Дата добавления: 2015-09-04