Question 1

What does the normal status of an electrical switch refer to? Specifically, what is the difference between a normally-open switch and a normally-closed switch?

 

Question 2

A special class of electromechanical relays called time-delay relays provide delayed action, either upon power-up or power-down, and are commonly denoted in ladder logic diagrams by “TD” or “TR” designations near the coil symbols and arrows on the contact symbols. Here is an example of a time-delay relay contact used in a motor control circuit:



In this circuit, the motor delays start-up until three seconds after the switch is thrown to the “Run” position, but will stop immediately when the switch is returned to the “Stop” position. The relay contact is referred to as normally-open, timed-closed, or NOTC. It is alternatively referred to as a normally-open, on-delay contact.

Explain how the arrow symbol indicates the nature of this contact’s delay, that delay occurs during closure but not during opening.

 

Question 3

Match the following time-delay relay contact type symbols and labels:



Normally-open, timed-closed
Normally-open, timed-open
Normally-closed, timed-closed
Normally-closed, timed-open

 

Question 4

Match the following time-delay relay contact type symbols and labels:



Normally-open, on-delay
Normally-open, off-delay
Normally-closed, on-delay
Normally-closed, off-delay

 

Question 5

Time-delay relays are important circuit elements in many applications. Determine what each of the lamps will do in the following circuit when pushbutton Ä” is pressed for 10 seconds and then released:



Timing diagram:



 

Question 6

A simple time-delay relay may be constructed by connecting a large capacitor in parallel with the relay coil, like this:



Explain how this circuit works, and also determine what type of time-delay relay function is provided by it (NOTO, NOTC, NCTO, or NCTC).

 

Question 7

An electric motor is used to power a large conveyor belt. Before the motor actually starts, a warning siren activates to alert workers of the conveyor’s forthcoming action. The following relay circuit accomplishes both tasks (motor control plus siren alert):



Study this ladder logic diagram, then explain how the system works.

 

Question 8

Large electric motors are often equipped with some form of soft-start control, which applies power gradually instead of all at once (as in äcross the line” starting). Here is an example of a simple ßoft start” control system:



Analyze this ladder logic diagram, and explain how it starts up the electric motor more gently than an äcross-the-line” starter would.

 

Question 9

The following ladder logic diagram is for a reversing motor control circuit:



Study this diagram, then explain how motor reversal is accomplished. Also, identify the function of each “M” contact in the control circuit, especially those normally-closed contacts in series with the motor starter coils.

Now consider the following modification made to the reversing motor control circuit (motor and power contacts not shown here):



What extra functionality do the time-delay relays contribute to this motor control circuit?

 

Question 10

An electric motor is used to power a large conveyor belt. Before the motor actually starts, a warning siren activates to alert workers of the conveyor’s forthcoming action. The following relay circuit is supposed to accomplish both tasks (motor control plus siren alert):



Unfortunately, there is a problem somewhere in this circuit. Instead of activating the siren before starting the motor, there is silence. The motor’s start is still delayed by the correct amount of time, but the siren never makes a sound. Identify some possible causes of this problem. Also, identify portions of the circuit you know to be functioning properly.

 

Question 11

An electric motor is used to power a large conveyor belt. Before the motor actually starts, a warning siren activates to alert workers of the conveyor’s forthcoming action. The following relay circuit accomplishes both tasks (motor control plus siren alert):



However, this circuit is poorly designed. Although it works just fine under normal conditions, it may not do what it should in the event of an overload heater trip (if the normally-closed ÖL” contact opens). Explain what is wrong with this circuit.

 

Question 12

Suppose you needed to build a circuit that pulsed a lamp on and off (once) when a pushbutton is pressed and held. In other words, you wanted the lamp to do this:



Draw a ladder logic diagram for a circuit that would fulfill this function, using at least one time-delay relay.

 

Question 13

Suppose an engineer draws the following timing diagram for a time-delay relay circuit and then hands the diagram to a technician to figure out how to build it:



The technician, being well educated in the ways of time-delay relays, takes one look at this timing diagram and begins to laugh. Explain why this diagram is funny.

 

Question 14

Determine what sort of time-delay relay this circuit is:



Also, calculate the amount of delay, in seconds. Hint: the 555’s timing capacitor will charge from 0 volts to [2/3] supply voltage during the charging cycle.

 

Question 15

Explain what a time-delay relay is, and how it functionally differs from a regular electromechanical relay.

 

Question 16

A simple time-delay relay may be constructed by connecting a large capacitor in parallel with the relay coil, like this:



Explain how this circuit works, and also determine what type of time-delay relay function is provided by it (NOTO, NOTC, NCTO, or NCTC).

 

Question 17

Suppose you needed to build a circuit that pulsed a lamp on and off (once) when a pushbutton is released. In other words, you wanted the lamp to do this:



Draw a ladder logic diagram for a circuit that would fulfill this function, using at least one time-delay relay.

 

Question 18

Suppose you needed to build a circuit that delayed the energization of a lamp when the button was pushed, and also delayed its turning off when the button was released. In other words, you wanted the lamp to do this:



Draw a ladder logic diagram for a circuit that would fulfill this function, using at least one time-delay relay.

 

Question 19

When the pushbutton is pressed, the relay immediately energizes and sends power to the electric horn. When the pushbutton is released, the horn remains on for a few moments before turning off, due to the capacitor’s stored charge continuing to power the relay coil. So, the capacitor and relay form a time-delay control circuit for the horn:



Suppose this circuit has functioned as designed for quite some time, then one day develops a problem. The horn sounds immediately when the pushbutton is pressed (as it should), bit it immediately silences instead of continuing to sound for a few more moments when the pushbutton is released. Based on this information, identify these things:

Two components or wires in the circuit that you know must be in good working condition.
Two components or wires in the circuit that could possibly be bad (and thus cause the off-delay action to fail).

 


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