Basic Circuit Troubleshooting

Determine the statuses of all lamps and relay coils in this circuit, given the following process conditions:

{\bullet} Flow = 3.5 GPM
{\bullet} Pressure = 41 PSI
{\bullet} Temperature = 155 °F
{\bullet} Level = 1.3 ft
{\bullet} Pushbutton A = unpressed
{\bullet} Pushbutton B = unpressed

A form of liquid level switch called a tilt switch is often used for detecting sewage level in “lift stations” where sewage collected from homes via gravity is pumped out of the collection sump to the wastewater treatment plant (usually located miles away):

Tilt switches often use a small glass vial containing liquid mercury as the tilt sensor. Explain how a glass tube partially filled with mercury works as an electrical tilt switch, and also perform a “thought experiment” where you describe this system’s function from start to finish through a complete start-stop cycle of the pump motor:

{\bullet} What would happen if the OL switch failed open in this system?
{\bullet} What would happen if the LSL switch failed open in this system?
{\bullet} What would happen if the LSH switch failed open in this system?
{\bullet} What would happen if the LSL switch failed shorted in this system?
{\bullet} What would happen if the LSH switch failed shorted in this system?
{\bullet} What would happen if the LSH switch failed shorted in this system?
{\bullet} What would happen if the M1 seal-in contact failed open in this system?
{\bullet} What would happen if the M1 seal-in contact failed shorted in this system?

Draw a ladder logic control circuit for the electric motor of an air compressor, controlled by two pressure switches: one switch turns the motor on when the pressure falls to 80 PSI, while the other switch turns the motor off when the pressure rises to 105 PSI:

Be sure to include the overload (OL) contact in the 120 volt control circuit (L1 & L2), and include a manual on/off switch as well.

Some common components of three-phase motor control circuits are shown here in the following illustrations. These include fuses, a contactor, and an overload assembly:

Fuses protect the power wiring from gross overcurrent conditions such as what might happen if there were an accidental phase-to-phase short-circuit inside the motor. The contactor is nothing more than a big relay with three normally-open contacts to send power to the motor, serving to start and stop the motor on command with a 120 volt signal to its coil.

The overload block, however, is a little more mysterious. Its three “heater” elements (looking like back-to-back “question mark” symbols in the schematic diagram) carry the motor’s current from the contactor to the motor terminals. These resistive heaters are designed to become warm under normal operating conditions, just as the motor itself will become slightly warm under normal conditions from resistive power losses in its windings.

If the motor ever becomes too warm as a result of overloading (slight overcurrent), the overload heaters (which will also be too warm due to the overcurrent) will trigger a small thermally-operated switch contact to spring open. Connection terminals for this small switch contact may be seen on the right-hand side of the overload block in the pictorial diagram.

Explain how the overload heaters may be used to automatically shut the motor off and prevent damage.

Explain the operation of this circuit:

Sketch the necessary wiring to make this float-type level switch control a pump and a lamp in the following manner:

{\bullet} High liquid level: pump on and lamp off
{\bullet} Low liquid level: pump off and lamp on

Hint: remember that the “normal” status of a switch is defined as the status of minimum stimulus when the switch is exposed to the lowest possible degree of process stimulation (in this particular case, to the lowest possible level).

Sketch the necessary wiring to make this pressure switch control two lamps in the following manner:

{\bullet} High process pressure: green lamp off and red lamp off
{\bullet} Low process pressure: red lamp on and green lamp on

Hint: remember that the “normal” status of a switch is defined as the status of minimum stimulus when the switch is exposed to the lowest possible degree of process stimulation (in this particular case, to the lowest possible pressure).

Examine the following electronic level switch circuit:

Identify what kinds of process liquids this level switch would be applicable to, and why. Also, identify which ladder-logic switch symbol would be appropriate for this particular level switch:

Qualitatively determine the following component voltage drops in the circuit with low level and with high level (i.e. write “low” or “high” voltage rather than try to calculate actual values):

$$\begin{array} {|l|l|}\hline Component & Low-level~condition & High-level~condition \\ \hline R1 & & \\ \hline Q1~(between~drain~and~source) & & \\ \hline \end{array}$$

Sketch the necessary wiring to make this pressure switch control two lamps in the following manner:

{\bullet} High process pressure: green lamp on and red lamp off
{\bullet} Low process pressure: red lamp on and green lamp off

Hint: remember that the “normal” status of a switch is defined as the status of minimum stimulus when the switch is exposed to the lowest possible degree of process stimulation (in this particular case, to the lowest possible pressure).

Determine the statuses of all lamps and relay coils in this circuit, given the following process conditions:

{\bullet} Flow = 7.9 GPM
{\bullet} Pressure = 36 PSI
{\bullet} Temperature = 210 °F
{\bullet} Level = 7.1 ft
{\bullet} Pushbutton A = pressed
{\bullet} Pushbutton B = unpressed

Determine the statuses of all lamps and relay coils in this circuit, given the following process conditions:

{\bullet} Flow = 4 GPM
{\bullet} Pressure = 24 PSI
{\bullet} Temperature = 190 °F
{\bullet} Level = 2.5 ft
{\bullet} Pushbutton A = pressed
{\bullet} Pushbutton B = pressed

Switches, whether they be hand-actuated or actuated by a physical process, come in two varieties: normally-open (NO) and normally-closed (NC). You are probably accustomed to seeing both types of switch represented in pushbutton form on schematic diagrams:

Normally-open pushbutton switches close (pass current) when actuated (pressed). When un-actuated, they return to their “normal” (open) state.

Normally-closed pushbutton switches are just the opposite: they open (stop current) when actuated (pressed) and return to their “normal” (closed, passing current) state when un-actuated.

This is simple enough to comprehend: the “normal”’ status of a momentary-contact pushbutton switch is the state it is in when no one is touching it. When pressed, the pushbutton switch goes to the other (opposite) state.

Things get more confusing, though, when we examine process switches, such as pressure switches, level switches, temperature switches, and flow switches:

Define “normal” for each of these process switches. In other words, explain what condition(s) each process switch must be in to ensure it is in the “normal” state; and conversely, what condition(s) need to be applied to each switch to force it into its other state.

Draw the appropriate pressure switch symbol in this ladder-logic diagram for a low-pressure alarm which turns on a lamp if the oil pressure of an industrial machine ever drops below 10 PSI:

Be sure to specify whether the pressure switch needs to be normally-open (NC) or normally-closed (NC).

Two pressure switches are plumbed together so as to receive the exact same pressure at all times, and they both sense the pressure of compressed air in a pneumatic system. Based on the wiring diagram for these switches, identify the function of the lamp:

Determine the functions of all pressure switches and relays in this steam boiler monitoring circuit, and what each of their designations mean:

Also, explain the significance of the switch symbols: normally open versus normally closed. The time-delay relay (TD1) is especially important here!

Finally, add a “Lamp Test” pushbutton switch to this circuit which will force all lamps to energize when pressed, in order to test the proper operation of the lamps without waiting for an abnormal process condition to occur.

{\bullet} Why do you suppose a time-delay relay is used in this particular control application?
{\bullet} Is the boiler shutdown solenoid energize-to-trip or de-energize-to-trip? Explain how we can tell from an examination of the schematic.
{\bullet} Identify a circuit fault that would cause the boiler to needlessly shut down (a “safe” fault).
{\bullet} Identify a circuit fault that would cause the boiler to not be able to shut down when it needs to (a “dangerous” fault).

Limit switches are often used on the doors of electrical enclosures and cabinets to automatically shut off power or shut down a machine’s function if anyone opens the door for maintenance purposes. The limit switch is typically mounted in such a way that a shut door holds the switch lever in the “actuated” position. When the door opens wide, the limit switch lever is released and the switch returns to its “normal” status.

Draw the appropriate limit switch symbol in this ladder logic diagram so that the control circuit (shown as a rectangular box) gets shut down if ever someone opens the cabinet door:

Be sure to denote whether this limit switch needs to be normally-open (N.O.) or normally-closed (N.C.).

Identify which lamp in the following ladder-logic diagram is the high-flow alarm and which is the low-flow alarm, given the flow switch symbols shown:

Explain what the following ladder logic circuit does, and identify the meaning of each symbol in the diagram:

{\bullet} Explain why the TSH uses a normally-open contact instead of a normally-closed contact.
{\bullet} Explain why the TSL uses a normally-closed contact instead of a normally-open contact.
{\bullet} Based on what we see in this diagram, determine whether the electric solenoid valve allows cooling water to flow when energized, or when de-energized.
{\bullet} What do the designations “L1” and “L2” refer to in ladder-logic electrical diagrams?
{\bullet} Suppose switch TSL has a trip setting of 105 °F (falling) and a deadband value of 2 °F. Explain how this switch will respond to a rising and falling temperature.

{\bullet} Suppose we wished to have switch TSHH activate two different alarm lights instead of just one. Modify the circuit diagram accordingly.

An alternative to the conventional schematic diagram in AC power control systems is the ladder diagram. In this convention, the “hot” and “neutral” power conductors are drawn as vertical lines near the edges of the page, with all loads and switch contacts drawn between those lines like rungs on a ladder:

As you can see, the symbolism in ladder diagrams is not always the same as in electrical schematic diagrams. While some symbols are identical (the toggle switch, for instance), other symbols are not (the solenoid coil, for instance).

Re-draw this ladder diagram as a schematic diagram, translating all the symbols into those correct for schematic diagrams.

An improvement over direct-contact limit switches for many applications is the inductive proximity switch. This type of switch actuates simply when an object gets near it—no direct physical contact necessary! Explain how these devices work, and what kinds of material they are able to detect.

Inductive proximity switches are powered devices by necessity. They usually require a DC voltage for power, and their output is usually not a dry switch contact. Instead, it is usually a transistor, with the output signal being standard TTL logic (0 to 5 volts). Inductive proximity switches are often manufactured as three-wire devices:

Assume the switch above is a sinking output. Show how you would connect the limit switch in the above illustration so that it makes the LED turn on when actuated, assuming the switch’s internal transistor is configured to sink current through the output lead.

{\bullet} Identify an object an inductive proximity switch would be able to detect.
{\bullet} Identify an object an optical proximity switch would be able to detect.
{\bullet} Identify an object a capacitive proximity switch would not be able to detect.
{\bullet} Identify an object an ultrasonic proximity switch would not be able to detect.

Limit switches are electrical switches designed to actuate based on the motion or position of an object, rather than the touch of a human operator. Simple limit switches rely on direct, physical contact, using a lever, sometimes tipped with a roller for low friction:

Show how you would connect the limit switch in the above illustration so that it makes the light turn off when actuated (i.e. the light will be on when no one touches the switch lever).