ЗНАЕТЕ ЛИ ВЫ?

Manual Electric Drive Controls.



Switches. Every electric circuit must have some type of switch. Therefore a switch is the most basic of all electrical controls. Its purpose is to open and close an electric circuit. It must start and stop the flow of electricity, and, in effect, it does this by connecting and disconnecting wires. The switch contacts always become a part of the electric wire that is used for supplying electricity to the appliance in question. Therefore, when the switch contacts are open, the wire is effectively broken, and when the contacts are closed, the wire is made to be continuous again.

In addition to having contacts that open and close, a' switch must have a second mechanical feature: The contacts must open and close quickly. This is known as the snap-action feature of a switch. A switch would operate with­out this feature, but it would not last very long. There is an arc when the flow of electricity is started and another arc when the flow is stopped. The latter arc causes the switch contacts to become pitted, burned, and worn. The greater the arc, the greater is the wear on the contacts. If the con­tacts open and close very quickly, the amount of arcing will be greatly reduced. For this reason a mechanical means of-causing the switch contacts to snap open or closed must be incorporated in the switch mechanism. There is an exception to this requirement in the case of a mercury switch, for its contacts do not move. The quick opening and closing is achieved by a sudden movement of a pool of mercury.

Approved nomenclature relative to the pole and throw designation of a switch is shown by illustration in Fig. 6.

The AC Relay. A relay is a type of switch having con­tacts which are operated by magnetism. The contacts of the relay open and close circuits just the same as if the con­tacts had been operated by hand. The magnetism is created by allowing an electric current to pass through a coil of wire, and the magnetism is removed when the flow of current through the coil is stopped. A switch, separate from the re­lay, is used to connect and disconnect the coil from the source of power.

The principle of operation of a relay is illustrated in Fig. 7. The separate switch which connects the relay coil to the source of power may be any type of manually operated switch, or it may be an automatically operated type. The magnetism created by the coil causes certain metal parts to snap together. The movement of one of the metal parts operates the contacts of the relay. The electrical input to the appliance is controlled by the relay contacts. When the coil of Fig. 7 is connected to the voltage source, the end moves upward and the contacts close. The movable con­tact is insulated from the movable metal part.

Although the relay contacts of Fig. 7 are normally open and close when the coil is energized, they could be arranged to be normally closed, and then would open when the coil was magnetized. Most applications use the normally open type of contact. A further variation in the contacts is ob­tained by using double-pole-switch contacts (four-contact button) instead of the single-pole type illustrated in the fig­ure. The current rating of the contacts is veryimportant,and proper selection depends upon the current flow to the appliance which is to be controlled.

The main advantage in using a relay is that the switch and wires which control the current flow to the relay coil do not conduct the current to the appliance. Since the current required for energizing the coil is quite small (less than 1 amp), the wire and switch can have low-current ratings and proportionately low costs. The large appliance current is conducted through the contacts of the relay. Also the switch may be placed at a distant position from the appliance since long lengths of small-size wire between switch and coil do not cause voltage drop in the wires leading to the ap­pliance. This type of arrangement is .well adapted for con­trolling motors, heaters, or large lighting loads from remote­ly located small-size switches.

Pushbuttons.1 The momentary-contact manually operat­ed pushbutton is the most common type of remote control device for starting and stopping electric motors. It is also used for .operating the electric-relay type of circuit.

This control is not the same as the pushbutton wall switch­es2 that-are sometimes used in the home for controlling lights. That type of pushbutton is not momentary contact; Instead its contacts remain open or closed until the opposite button is pressed. Another difference is that the pushbut­ton wall switch is a complete control in itself, whereas the momentary-contact pushbutton requires other equipment in performing its controlling function.

The standard-duty pushbutton control for farm use is usually the two-button type. The function of each button is clearly labeled on the button or on the metal cover. Push­button use for circuit control is illustrated in Fig. 8. The buttons and their related contacts are held in position by springs. The buttons are usually made of bakelite or similar moulded insulation, and the contacts are silver or silver alloy. The current rating of the button contacts is small (1, 2, or 3 amp) since the appliance current does not flow through these contacts. Knockout holes for the wiring are provided in the metal or bakelite cases.

The pushbutton control illustrated in Fig. 8 starts and stops the flow of current to a relay coil. The contacts of the relay, in turn, control the power input to the appliance. Which in this illustration is several electric strip heaters. When the start button is pressed downward, current flows from L1 through both pushbuttons, through the relay coil and back to L2. The contacts snap closed and the appliance is on. When the start button is released, a spring3 causes it to return to its normal position (the normal position is up ns shown in the figure). With the start pushbutton in the released position current cannot flow through its contacts, but notice that the relay contacts labeled x and у would be dosed and would conduct the current around the start button find on to the coil. When the stop button is pressed downward, the current flow to the coil is interrupted, the coil loses its magnetism, and the relay contacts open, thereby disconnecting the appliance.

Notes:

1pushbuttons – кнопки, клавиши

2pushbutton wall switch – настенный кнопочный выключатель

3a spring – пружина

Вариант № 4.





Последнее изменение этой страницы: 2016-12-12; Нарушение авторского права страницы

infopedia.su Все материалы представленные на сайте исключительно с целью ознакомления читателями и не преследуют коммерческих целей или нарушение авторских прав. Обратная связь - 3.219.31.204 (0.007 с.)