Tuesday, November 27, 2018

Key Components used in Industrial Control

Understanding the different components involved in a given process control is important especially for its proper application, and troubleshooting. In this article we look at the most common discrete components used in industrial control applications.
Electrical Symbols of Components commonly used in Industrial Control
COMMON ELECTRICAL SYMBOLS

Contactors
Power consuming devices like Motors are usually controlled by contactors having heavy duty contacts that can switch the power circuits safely. The contactor will be actuated by an electromagnetic solenoid (coil) which pulls the contacts closed when energized. They have an arc-suppressing cover to quench the arc formed when the contacts open under load. Note that AC contactors should never be used to break the current of a loaded DC power circuit. It is more difficulty to quench the arc from a DC power load because there is no AC  ''zero crossing'' to interrupt the current flow. DC contactors are designed to specifically handle DC current. You will find they have embedded magnets, or special blow-out coils that are used to stretch the arc long enough to break the DC current flow.
Devices using AC solenoids will have one or more single turn coil (s), called a shading ring, embedded in the face of their magnetic armature assembly. When the solenoid is energized, the magnetic field has continuous reversals matching the alternating current that is been applied. This will produce a noticeable hum or chatter because the magnetic structure is not energized continuously. As the magnetic field reverses, it induces a current in the shading ring, which in turn produces a secondary reversing magnetic field. The secondary field is out of phase with that of the applied power, and holds the armature faces sealed continuously between power reversals, and minimizes the noise. Over time, shading rings tend to crack from the pounding of the armature faces. When this happens, the solenoid will become very noisy, coil current will increase, and premature failure will result. In an emergency, one can remove the damaged ring and replace it temporarily with a shorted turn of copper wire.
Solenoids
Solenoids are used to actuate brake/ clutch mechanisms, hydraulic valves, air valves, steam valves or other devices that require a push-pull button. Some solenoids can be quite large, requiring contactors rated for their high current draw. Smaller pilot valves may draw no more current than a simple relay. Some heavy duty units operate on DC current. The DC solenoids are often specified for operation in dirty or corrosive areas because current is controlled by circuit resistance, and will not rise if the air-gap is fouled. AC solenoids depend upon the impedance of the circuit. If the air-gap is not sealed properly, inductance reactance is reduced and coil will draw excess current and over-heat. Shading rings must also be switched for possible failures.
Relays
Relays have a similar construction to contactors, but since they switch low-current logic signals, they do not have a requirement for the heavy-duty contacts and arc-suppression hardware. Most relays contacts have AC continuous ratings of no more than 10 A. They can close on an inrush current of 150%, but only break 15% at 120 volts AC (vac). A NEMA A600 rating limits the in-rush to 7200 volt-amperes (va), and a circuit breaking rating of 720 volt-amperes. As higher voltages are used, the current capacity goes down proportionately. This difference in make and break ratings closely matches the typical ratio of inrush and holding currents of AC control coils. AC coils have relatively low resistance, allowing high in-rush currents. As the coil is energized, the AC current builds up inductive reactance. Total impedance (ohms), the vector sum of resistance and reactance, limits the continuous holding current. The ratio of in-rush to holding current is often 5:1 or more. Maximum impedance is attained when the air gap in the magnetic armature assembly has sealed closed. A failure to close this gap will reduce the inductive reactance, allowing more current, which will overheat the coil, causing premature coil failure. A shading ring fracture will also lead to overheating and coil failure.
The same 10A contacts are only rated at 0.4 A DC, at 125V, and 0.2 A DC at 250 V (50 va) because the small air-gaps are not adequate to break a sustained DC arc. Voltages for DC logic controls seldom exceed 24 volts with typical current in the milliamp ranges.
Relays may have multiple coils for latching and unlatching of the contacts. Contacts may be normally open (NO), and or normally closed (NC). The number of contacts usually varies from 1-8. Some relays use contact cartridges which can be converted for either NO or NC operation. Most standard relays will have totally isolated contacts. Some miniature relays have type ”C” contacts where a NO and NC contact share a common terminal. This construction requires careful planning to match the schematic wiring diagram to actual relay construction.
Occasionally the required load on relay contacts may be slightly higher than their normal rating. One can increase the current capacity by connecting contacts in parallel, and improve the arc suppression by connecting multiple contacts in series. When multiple contacts are used this way, they should be on the same relay, because relay coils operating in parallel may have a slight difference in their time-constant.  If one relay closes late, its contacts will take all the inrush punishment. If a relay opens early, it will suffer more damage from breaking most of the full load current.
Timers
Timers are a type of relay that have pneumatic or electronic, time contacts to meet various sequencing requirements. They may be, ”stand-alone” relays or attachments to standard relays. On-delay timers actuate the contacts at a preset time after the coil is energized, and reset instantly with power off.  Off-delay times actuate the contacts instantly when energized, and reset a preset time after de-energizing. NO and/or NC contacts may be time-closed or time-opened. Many timers also include instantaneous contacts, actuated with no time delay. Instantaneous contacts may also be added as a modification kit in some cases to stand-alone timers.
The coils of contactors and relay may be rated at a different voltage than the circuits being switched. This provides isolation between the low voltage logic circuits and higher voltages power circuits.
Push Buttons
Push-buttons may have a single contact block, or an assembly of multiple contacts depending upon the complexity of the requirement. Most push-buttons have a momentary change of state when depressed, then return to normal when released. Some may have a push-pull actuator that latches in each position. They are often used as a control-power master-switch with a Pull-on/Push-off action.
Selector Switches
Many selector switches have the same construction as push buttons, except that the contacts are actuated by rotating a handle or key-switch. The rotating cam may be arranged with incremental indices so that multiple positions and contact patterns can be used to select exclusive operations. Contacts of push-buttons, selector switches, and limit switches usually have the same rating as the logic relays 10A continuous at 120 (vac).
Limit Switches
Mechanical limit switches have many configurations. Most will have both NO and NC contacts available. The contacts are switched when the layer arm is rotated a few degrees by a moving cam or slider. The conventional drawing will show the contact conditions when the machine is un-powered and at rest. It is assumed that the cam will normally strike the arm on the switch to change the state of the contact(s).
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Non-Contact Limit Switches
There are a number of electronic limit switches that are used where it is not practicable to have an actuator arm physically contact a product or machine part. The switches include: Photocells, and Proximity switches (inductive, magnetic, and capacitive). In each case, a control signal is activated whenever an object enters its operating field. These devices require additional wiring to energize their power.

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