Tuesday, January 13, 2015

Control Circuits : Interlocking , Jogging and Timers .

INTERLOCKING

Interlocking is used to prevent some function from happening until some other function or action has occurred. A good example of interlocking is shown in the circuit in Figure 19–12. This circuit is a forward–reverse control. The two normally closed contacts labeled F and R are used to interlock the system. In this circuit, the motor is reversed by changing two of the input lines to the stator. If both motor starters F and R are energized

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at the same time, there would be a direct short circuit across two of the phases. Forward– reverse starters are generally interlocked mechanically as well as electrically, but this example shows only electrical interlock.

It will first be assumed that the motor is to be operated in the forward direction. When the forward push button is pressed, a circuit is completed to F coil. This causes all F con- tacts to change position. The normally open F load contacts close and connect the motor to the line. The normally open auxiliary contact closes to complete the circuit around the normally open F push button when it is released, and the normally closed F contact opens. When the normally open F contact opens, it breaks the circuit to R coil. If R push button were to be pressed, R coil could not energize because of the now open contact connected in series with it.

Before the motor can be reversed, the stop button must be pushed and F coil de- energized. This permits all F contacts to return to their normal position. If the reverse push button is now pressed, current can flow through the normally closed F contact and energize R coil. When R coil energizes, all R contacts change position. The R load contacts connect the motor to the line, the normally open R contact closes to complete a circuit around R push button, and the normally closed R contact connected in series with F coil opens. This prevents F coil from being energized if F push button should be pressed.

JOGGING

Jogging in this context is moving a machine with short jabs of power. It is used to bring a machine into some particular position, generally for loading or unloading. Another term, inching, is very similar to jogging. The difference is that jogging applies full volt- age to the motor and inching applies reduced voltage to the motor. Regardless of which method is used, the basic control requirement for either is for the motor starter to energize when the jog button is pushed and deenergize when the jog button is released. This control function can be accomplished by several methods. One method is shown in Figure 19–13. In this circuit, a double-acting push button is used as the jog button. When the jog button is pressed, the normally open section of the button closes to complete a circuit to M coil. The normally closed section opens to prevent the normally open M contact from holding the coil in after the button is released. This permits M coil to deenergize and disconnect the motor from the line. If the start button is pressed, M coil will energize and close all M contacts. The normally open auxiliary contact can now provide a current path through the closed push button to keep the coil energized.

TIMERS

There are two basic types of time-delay relays: the on delay and the off delay. The on-delay timer is often referred to on a schematic as DOE, which stands for delay on energize. Off-delay timers are often referred to as DODE, which stands for delay on de- energize. Timers may contain only time-operated contacts or a combination of both time- operated and instantaneous contacts. Time-operated contacts are operated by the timer mechanism, and instantaneous contacts operate like normal relay contacts.

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On-Delay Timer

The operation of an on-delay timer is as follows. When the coil is energized, the contacts delay changing from their normal position. When the coil is deenergized, they change back to their normal position immediately.

The circuit shown in Figure 19–14 illustrates the operation of an on-delay timer. It is assumed that the timer has been set for a delay of 10 seconds. When the normally open push button is pressed, a circuit is completed to TR coil. The normally open contacts connected around the start push button are instantaneous contacts. These contacts close immediately and act as holding contacts to keep the relay energized. A normally open time-operated contact is connected in series with a pilot light. This contact will close 10 seconds after TR coil has been energized. When the stop button is pressed, the circuit to TR coil is opened and the relay deenergizes. All TR contacts open immediately. This disconnects the pilot light from the line.

Off-Delay Timer

The off-delay timer functions as follows. When the relay coil is energized, the contacts change position immediately. When the coil is deenergized, the contacts delay changing back to their normal position.

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The circuit in Figure 19–15 illustrates the operation of an off-delay timer. A nor- mally open instantaneous contact is used as the holding contact around the start button. A normally open time-operated contact is connected in series with a pilot light. When the start button is pressed, TR coil energizes and both TR contacts close immediately. When the stop button is pressed, TR coil is deenergized, but the normally open time-operated contact remains closed for 10 seconds before it reopens.

Types of Timers

There are several methods that can be used to achieve a time delay. One type of timer is known as a pneumatic timer because it uses air to provide a time delay. Figure 19–16 illustrates this type of timer. When rod A pushes against the bellows, air is forced out the check valve. Retraction of the bellows causes TR contact to close. When rod A is pulled back, the spring pushes outward on the bellows. Before the bellows can expand, however, air must enter through the air inlet. The needle valve controls the rate at which air can

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enter the bellows and therefore the time required to expand the bellows and reopen the TR contact.

Dashpot timers use a piston moving through oil to create a time delay. Some timers use electric clocks to provide time delays. Electronic timers have become very popular because they are inexpensive and have good reliability and repeat accuracy.

The circuit shown in Figure 19–17 illustrates an electronic circuit that can be used as an on-delay timer. When switch S1 closes, capacitor C1 begins to charge through resistor RT. When C1 reaches about 10 V, the unijunction transistor turns on. This provides gate current

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to the silicon controlled rectifier (SCR). When the SCR turns on, relay coil K1 energizes. The SCR remains turned on until switch S1 is again opened. Two types of electronic timers are shown in Figures 19–18 and 19–19. One is adjustable and the other is not.

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