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Electrical Control Devices Questions Answers

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Herein, we have covered several questions answers related to Pushbutton, Selector Switch, Joystick, Limit Switch, Foot Switch, Pressure Switch, Temperature Switch, and Level Switch in detail.

Industrial Pushbuttons

  1. Describe the different parts of pushbuttons and their functions.
  • Pushbuttons are the most common control switches used on industrial equipment. An industrial pushbutton consists of a legend plate, an operator, and one or more contact blocks (electrical contacts).
  • A legend plate is the part of a switch that includes the written description of the switch’s operation. A legend plate indicates the pushbutton’s function in the circuit.
  • An operator is the device that is pressed, pulled, or rotated by the individual operating the circuit. An operator activates the pushbutton’s contacts. Standard pushbutton operators include the flush, half-shrouded, extended, and jumbo mushroom buttons.
  • A contact block is the part of the pushbutton that is activated when the operator is pressed. A contact block includes the switching contacts of the pushbutton. Contact blocks include normally open (NO), normally closed (NC), or both NO and NC contacts.
  1. Identify the NEMA and IEC enclosure location rating for each service location environmental condition.
  • Every basic NEMA and IEC enclosure size is available because pushbutton stations need to be mounted where they can be conveniently operated.
  • Pushbuttons are often required to operate in environments where dust, dirt, oil, vibration, corrosive material, extreme variations of temperature and humidity, as well as other damaging factors are present. The correct components and enclosure should always match the environment in which they will operate.

Selector Switches

  1. Identify two-position and three-position selector switches.
  • A selector switch is a switch with an operator that is rotated (instead of pushed) to activate the electrical contacts. They are normally used to select either two or three different circuit conditions.
  • A two-position selector switch allows the operator to select one of two circuit conditions.
  • A three-position selector switch allows the operator to select one of three circuit conditions.
  1. Explain switch operation given a switch’s truth table.
  • Contact position on a selector switch may be illustrated using truth tables (target tables) or solid lines, dashed lines, and a series of small circles. In truth tables, each contact on the line diagram is marked A, B, etc., and each position of the selector switch is marked 1, 2, etc.
  • An X is placed on the table if a contact is closed in any position. The table is easily read as to what contacts are closed in what positions. An O is placed in the table if a contact is open.

Joysticks

  1. Define joysticks and describe their most common positions.
  • A joystick is an operator that selects one to eight different circuit conditions when the joystick is shifted from the center position into one of the other positions.
  • The most common joysticks can move from the center position into one of four different positions (up, down, left, or right).
  • The most common circuit condition controlled by a joystick is in controlling a hoist (or crane) in the raise, lower, left, right, or OFF position.
  • Two methods are used to indicate in which position the joystick must be placed to operate the contacts.
  • In the first method, a dot is placed in the symbol of the joystick to indicate the position the joystick must be in to switch the contacts.
  • In the second method, a truth table is used to indicate which contacts are switched in each position.

Limit Switches

  1. Explain the purpose of a limit switch.
  • A limit switch is a mechanical input that requires physical contact of the object with the switch actuator.
  • In industry, limit switches are used to limit the travel of machine parts, sequence operations, detect moving objects, monitor an object’s position, and provide safety by, for example, detecting guards in place.
  • Limit switch contacts are normally snap-acting switches, which quickly change position to minimize arcing at the contacts. Limit switch contacts may be NO, NC, or any combination of NO and NC contacts. Most limit switches include one NO contact and one NC contact.
  • Limit switch contacts must be connected to the proper polarity. There is no arcing between the contacts when the contacts energize and de-energize the load as long as the contacts are at the same polarity. Arcing or welding of the contacts may occur from a possible short circuit if the contacts are connected to opposite polarity.
  • Contacts must be selected according to the proper voltage and current size according to the load and manufacturer specifications. A relay, contactor, or motor starter must be used to interface the limit switch with the load if the load current exceeds the contact rating.
  1. Define the actuator and describe its typical applications.
  • An actuator is the part of a limit switch that transfers the mechanical force of the moving part to the electrical contacts. The basic actuators used on limit switches include the lever, fork lever, push roller, and wobble stick.
  • A lever actuator is an actuator operated by means of a lever that is attached to the shaft of the limit switch. The adjustable lever is used in applications in which the length of the arm, or actuator travel, may require adjustment. A typical application is on an assembly line conveyor system.
  • A fork lever actuator is an actuator operated by either one of two roller arms. Fork lever actuators are used where the actuating object travels in two directions. A typical application is a grinder that automatically alternates back and forth.
  • A push-roller actuator is an actuator operated by direct forward movement into the limit switch. Push-roller actuators are commonly used to prevent over travel of a machine part or object. A typical application is on a milling machine or an automatic turret lathe because the travel of the work surface needs to be monitored.
  • A wobble-stick actuator is an actuator operated by means of any movement into the switch, except a direct pull. Wobble-stick actuators are used in applications that require detection of a moving object from any direction such as in the robotics section of an automated manufacturing facility.
  1. Explain the importance of properly installing limit switches.
  • Limit switches must be placed in the correct position in relation to the moving part. Limit switches should not be operated beyond the manufacturer’s recommended travel specifications.
  • A rotary cam-operated limit switch must be installed according to manufacturer recommendations.
  • Limit switches installed where relatively fast motions are involved must be installed so that the limit switch’s lever does not receive a severe impact.
  • Limit switches using push-roller actuators must not be operated beyond their travel limit in emergency conditions.
  • A stop plate should always be added to protect the limit switch and its mountings from any damage due to over travel.
  • Limit switches should be mounted so that a technician cannot accidentally activate the limit switch.
  • A limit switch must be mounted in a location where machining chips or other materials do not accumulate. These could interfere with the operation of the limit switch and cause circuit failure.
  • Heat levels above the specified limits of the switch must also be avoided. A limit switch should always be positioned to avoid any excessive heat.

Foot Switches

  1. Explain the purpose of a foot switch.
  • A foot switch is a control switch that is operated by a person’s foot. A foot switch is used in applications that require a person’s hands to be free or that require an additional control point. Footswitch applications include sewing machines, drill presses, lathes, and other similar machines.
  • The OFF position is normally spring-loaded so that the switch automatically returns to the OFF position when released. Foot switches with three positions include a pivot on a fulcrum to allow toe or heel control.
  • Like a two-position foot switch, a three-position foot switch is normally spring-loaded so that the switch automatically returns to the OFF position when released.

Pressure Switches

  1. Identify and draw the symbols for normally open (NO) and normally closed (NC) pressure switches.
  • A pressure switch is a switch that detects a set amount of force and activates electrical contacts when the set amount of force is reached.
  • Depending on the application, NC or NO contacts are used for a pressure switch. NC contacts are used to maintain system pressure. NO contacts are used to signal an overpressure condition.
  1. Identify the different types of pressure switch sensing devices and how they work.
  • Most pressure switches use a diaphragm, bellows, or piston sensing device.
  • A diaphragm is a deflecting mechanism that moves when a force (pressure) is applied. The diaphragm moves against a spring switch mechanism that operates electrical contacts when the source pressure increases.
  • A bellows is a cylindrical device with several deep folds that expand or contract when pressure is applied. The expanding bellows move against a spring switch mechanism that operates electrical contacts when the source pressure increases.
  • A piston is a cylinder that is moved back and forth in a tight-fitting chamber by the pressure applied in the chamber. A piston sensing device (pressure switch) uses a stainless steel piston moving against a spring tension to operate electrical contacts. The piston moves a switch mechanism that operates electrical contacts when the source pressure increases.
  1. Define dead-band (differential) as applied to pressure and temperature switches.
  • Deadband (differential) is the amount of pressure that must be removed before the switch contacts reset for another cycle after the setpoint has been reached and the switch has been actuated.
  • Deadband is inherent in all pressure, temperature, level, and flow switches and most automatically actuated switches. Deadband is not a fixed amount and is different at each setpoint. Deadband is minimum when the setpoint is at the low end of the switch range. Deadband is maximum when the setpoint is at the high end of the switch range.
  1. Explain the advantage and disadvantage of different dead-band range settings.
  • Deadband may be beneficial or detrimental. Without a dead-band range, or too small of one, electrical contacts chatter on and off as a pressure switch approaches the setpoint. However, a large dead-band is detrimental in applications that require the pressure to be maintained within a very close range.

Temperature Switches

  1. Explain the purpose of a temperature switch.
  • A temperature switch is a control device that reacts to heat intensity. In most applications, temperature switches react to rising or falling temperatures.
  • A heating system maintains a set temperature when the ambient temperature drops. In a heating system, as ambient temperature drops, the switch contacts close and turn on the heat-producing device.
  • A cooling system maintains a set temperature when the ambient temperature rises. In a cooling system, as the ambient temperature rises, the switch contacts close and turn on a cooling device.

Flow Switches

  1. Explain the purpose of a flow switch.
  • A flow switch is a control switch that detects the movement of a fluid.
  • Flow switches use different methods to detect whether the fluid is flowing. The methods used to detect whether the fluid is flowing include the paddle and transmitter/receiver methods.
  • Both NO and NC electrical contacts can be used with flow switches.
  • A NO contact on the flow switch could be used to sound an alarm.
  • An NC contact is used to signal when a fluid is not flowing. The NC contact may be used to sound an alarm if fluid stops flowing. When fluid is flowing, the NC contacts are held open by the fluid flow.
  • A flow switch may also be used to detect airflow across the heating elements of an electric heater. The flow switch is used as an economical way to turn the heater off anytime there is not enough airflow.
  • Flow switches may be used to detect the proper airflow in a ventilation system.
  • A flow switch may be used to advance a clogged filter based on restricted airflow.

Level Switches

  1. Explain the purpose of a level switch.
  • A level switch is a switch that detects the height of a liquid or solid (gases cannot be detected by level switches) inside a tank.
  • All level switches are designed to detect a certain range of materials. Some level switches can only detect liquids; others can detect both liquids and solids. Some level switches must come in direct contact with the product to be detected; others do not need to make contact.
  • The different level switches include mechanical, magnetic, conductive probe, capacitive, optical, and ultrasonic level switches.
  • A mechanical level switch is a level switch that uses a float that moves up and down with the level of the liquid and activates electrical contacts at a set height.
  • A conductive probe level switch is a level switch that uses liquid to complete the electrical path between two conductive probes.
  • A capacitive level switch is a level switch that detects the dielectric variation when the product is in contact (proximity) with the probe and when the product is not in contact with the probe.
  1. Explain the difference between charging and discharging level control circuits.
  • Charging a tank is also known as pump control and discharging a tank is also known as sump control.
  • In a charging application, the level in a tank is maintained. As liquid is removed from the tank, the level switch signals the circuit to add liquid.
  • In a discharging application, the liquid in a tank is removed once it reaches a predetermined level. The liquid is removed until the level switch detects the tank is empty.
  1. Explain the difference between one- and two-level control circuits.
  • In level control applications, the distance between the high and low level must be considered. This distance may be small or large.
  • In applications using a one-level switch, the distance is small. In applications using a two- level switch, the distance may be any length.
  • Product type must be taken into consideration when determining the distance and time between the high and low level.
  • One-level control is best when the liquid is emptied very slowly from the tank.
  • Two-level control is best when the liquid is emptied at a fast rate.

Preventing Problems When Installing Control Devices

  1. Explain how mechanical contacts can be protected for longer operating life when switching higher currents.
  • Contact protection should be added when frequently switching inductive loads to prevent or reduce arcing.
  • A diode is added in parallel with the load to protect contacts that switch DC. The diode conducts only when the switch is open, providing a path for the induced voltage in the load to dissipate.
  • A resistor and capacitor (RC network or snubber) are connected across the switch contacts to protect contacts that switch AC. The capacitor acts as a high-impedance (resistor) load at 60 Hz, but becomes a short circuit at the high frequencies produced by the induced voltage of the load. This allows the induced voltage to dissipate across the resistor when the load is switched off.
  1. Explain how mechanical contacts can be protected for longer operating life when releasing higher pressure.
  • Protection for a pressure switch should be added in any system in which a higher pressure than the maximum limit is possible.
  • A pressure relief valve is installed to protect the pressure switch.
  • A pressure relief valve should be set just below the pressure switch’s maximum limit.
  • The valve opens when the system pressure increases to the setting of the relief valve.
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About Ahmad Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.