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Correspondence Lesson 2: Basic Wired Control Devices

Our coverage of specific control devices begins with basic wired controls- the most commonly used control items. You can find them in thousands of locations worldwide.Push button switches. Most of these switches consist of two parts: a contact unit and an operating mechanism. This allows for many combinations.You can get contact units in blocks that contain one normally open (NO) and one normally

Our coverage of specific control devices begins with basic wired controls- the most commonly used control items. You can find them in thousands of locations worldwide.

Push button switches. Most of these switches consist of two parts: a contact unit and an operating mechanism. This allows for many combinations.

You can get contact units in blocks that contain one normally open (NO) and one normally closed (NC) contact. (The designation "normal" is the position of the contact before it's acted upon.) You can also assemble multiple contact blocks to get up to four NO and four NC contacts. With some switches, you can add even more units.

The contact rating for a typical heavy-duty, oil-tight unit is: AC Volts: 110V to 125V Amperes Normal: 6.0A Amperes Inrush: 60.0A

In some cases, you can find the contact block mounted in a push button enclosure. Thus, it may be pre-wired, and you can install the operating mechanism later. The alternate method is panel mounting, where you mount the base, with the contact block attached, through an opening in the panel. You then secure it with a threaded ring, installed from the front of the panel. The ring is part of the operator assembly. This arrangement provides a space for a terminal block installation in the base of the enclosure. Thus, you can terminate all connecting circuits at a convenient location.

You may have to modify the hole when substituting one make of unit for another. Many types of operations are available to suit most applications.

Some people consider the easy identification of switches important, like how most of us associate certain functions with a specific color. For example, in the machine tool industry, we assign the following functions for the colors red, yellow, and black:

  • Red: Stop or emergency stop

  • Yellow: Return or emergency return

  • Black/Green: Start motors or cycle.

While this is a good pattern to follow, you should never assume anything. When working on an existing circuit, always verify which button is for which operation. This may slow you down, but it will also keep you safe.

Selector switches. You can get selector switches with up to four positions. They can be the maintained contact-type, a three-position switch arranged for spring return from the right, left, or both right and left. Units are available with up to eight contacts per device.

Available operators include standard knob, knob lever, or wing lever types. You can use a cylinder lock to lock the switch in any one position or all positions. The arrangement for opening or closing contacts in any one position or more depends on a cam in the operator.

A simpler form, called a double-pole, double-throw selector switch, comes with two contacts arranged for two positions. Similarly, a double-pole, double-throw with neutral selector switch has two contacts arranged for three positions. As the number of positions increases to four and the number of contacts (poles) increases to eight, the manufacturer generally codes its reference to a specific operation through a symbol chart or function table. Heavy-duty switches and special application switches are also available.

Indicating lights. Three basic types of indicating or pilot lights are available: full voltage, resistor, and transformer.

Due to the vibration normally present in machines, most usually prefer the low-voltage bulb. It operates at 6V to 8V, obtained through a resistance or transformer unit.

The lenses are either plastic or glass and come in a variety of colors. Again, as for push button operators, you would use colors to increase safety of operation. For example:

  • Red: Danger, abnormal conditions

  • Amber: Attention

  • Green: Safe condition

  • White or clear: Normal condition

The push-to-test indicating light provides another feature. Suppose the indicating lamp isn't lighted. It may be the bulb isn't energized, or is burned out. Depressing the lens unit connects the bulb directly across the control voltage source, providing a check on the condition of the bulb.

Like the standard line of devices, manufacturers make miniature oil-tight push buttons, selector switches, and pilot lights. This line covers about the same selection as the standard line, while offering advantages where space is limited.

The oil-tight designation. All oil-tight operators mount on the panel of an enclosure with some type of sealing ring, for the oil-tight rating. Nameplates identify the unit, and are available in different sizes, generally depending on the amount of information required to properly describe the function of the unit. Manufacturers will custom-engrave nameplates for special applications (for an additional fee, of course). Almost all control devices of this type are rated "oil-tight."

Solenoids. Solenoids are extremely important for controlling machines. Like the relay and contactor, the solenoid is an electromechanical device, using electrical energy to magnetically cause mechanical movement.

When you first energize a solenoid, you partially remove the iron core from the core of the coil, and the inductive reactance of the unit is very low. The initial current (called inrush current) is relatively high. The holding current is the current at the closed position: It's sometimes called sealed current. The ratio of inrush current to holding current generally varies from approximately 5:1 in small solenoids to as much as 15:1 in large solenoids.

A solenoid is made up of three basic parts: the frame, plunger, and coil. The frame and plunger are made up of laminations of high-grade silicon steel. The coil is wound of an insulated copper. Solenoids for AC use are available as oil-immersed types. You get better heat dissipation and wear conditions with this design. They're also available with a plug-in base.

When energizing the coil of a solenoid, you produce a magnetic field about the coil. This field produces a force called pull, which acts on the solenoid plunger. Due to this force, the plunger moves into the coil. The pull in solenoids varies, from as low as a fraction of an oz. to as high as nearly 100 lb.

There are two primary considerations in the application of solenoids:

  • Make sure the pull of the solenoid exceeds the load. If the pull is a little less, you'll get sluggish action, and the solenoid may not complete the stroke. You also have to consider conditions somewhat out of your control, such as low voltage or increased loading through friction or pressure. That's why it's generally advisable to overrate the solenoid by 20% to 25%. But don't oversize it too much; that would cause the plunger to slam, resulting in damage to the plunger and frame.

  • Make sure you know the duty cycle of the workload. Some applications require a duty cycle of only an occasional operation. Others require several hundred operations per minute. You have to be careful with the latter application because operating a solenoid above its maximum cycling rate will result in excessive heating and mechanical damage.

The pull-in force of a solenoid decreases rapidly as the voltage decreases below the coil nominal rating. On the other hand, as the voltage increases over the nominal value, the pull-in force increases. But, the solenoid temperature may also rapidly increase.

From a low-voltage standpoint, you should allow for adequate force at some arbitrary low-voltage level when selecting a solenoid size. This will prevent failure to pull out (as low voltage leads to minimal current and consequent coil burnout).

Design practices vary, but low-voltage levels are usually set at 85% or 90% of rated or nominal levels.

Relays. As mentioned in Lesson 1, the relay is a combination of a solenoid and one or more switches. A relay connects two circuits with differing characteristics. Every relay is connected to a controlling circuit that activates its coil, either leaving it in its normal position, or moving the coil and attached switch contacts to its second position. This opens or closes the contacts, as the case may be. This controlling circuit must be of the correct voltage, amperage, and characteristics for the relay coil. A relay can control one or more additional circuits, being opened or closed by the solenoid action of the relay's coil.

Relays vary from small panel-mounted units to huge high-voltage relays. However, the essential operations of all are the same. Manufacturers supposedly design coils on electromechanical devices (such as relays, contactors, and motor starters), so they don't drop out (de-energize) until the voltage drops to about of 85% of the rated voltage. Also, coils shouldn't pick up (energize) until the voltage rises to approximately 85% of the rated voltage. However, there is some "fudge factor" built into these numbers, and most electromechanical devices will not drop out until they "see" a lower voltage level. Also, most electromechanical devices will pick up at a lower rising voltage level. Generally, coils on electromechanical devices will operate continuously at 110% of the rated voltage without damage to the coil.

The two important factors in a relay are the coil and contacts. Of these, the contacts generally require greater consideration in practical circuit design. Most relays used in machine control have double-break contacts. The three ratings generally published are:

  • Inrush or "make contact" capacity;

  • Normal or continuous carrying capacity; and

  • Opening or break capacity.

A typical industrial relay may have the following contact ratings: 10A noninductive continuous load (AC); 6A inductive load at 120 V (AC); and 60A make and 60 A break, inductive load at 120V (AC).

A DC resistance is an example of a noninductive load. This may be a resistance unit used as a heating element. An inductive load is a coil (usually a solenoid, contactor coil, or motor starter coil). The important thing is: When determining the contact rating, you must have a clear understanding of which rating is given.

Latching relays. Manufacturers add mechanical latching attachments to control relays, with the resulting devices called mechanically held relays. These have two control coils, one to pull the relay into one position, and one to move it to the other position.

The latching relay operates electromagnetically: It's held by means of a mechanical latch. And by energizing a coil (called the latch coil), the relay operates. This results in the relay's NO contacts closing and the NC contacts opening. When you remove the electrical energy from the relay's coil (de-energize it), the contacts remain in their operated condition. To return the contacts to original condition, you must energize a second coil on the relay, the unlatch coil. The industry refers to this arrangement as a memory relay.

The latching relay has several advantages in electrical circuit design. For example, suppose you want to open or close contacts early in a cycle. At the same time, you may want to de-energize a section of the circuit responsible for the initial energizing of the relay latch coil. Later in the cycle, you can energize the unlatch coil to return the contacts to their original or non-operated condition. The circuit is then set up for the next cycle.

Another use for the latching-type relay involves power failure. Here, you may want the contacts to remain in their operated condition during the power-off period. Conditions in this case are the same after the power failure as they were before.

Quietness of operation is another feature of the latching relay. Since the coil is only energized momentarily, you don't have the usual hum.

Plug-in relays. Some industrial machine relays are available in plug-in types and designed for multiple switching applications at or below 240V. Coil voltages cover standard levels from 6V to 120V. These relays are available for AC or DC operation. Mountings include tube-type socket, square-base socket mounting, or flange mounting using slip-on connectors.

The plug-in relay has a distinct advantage when you want to change relays without disturbing the circuit wiring. In critical operations (where relay service is very hard and downtime is a premium), the plug-in relay may have some advantages. You'll have to assess the actual operating conditions in specific cases to determine their need.

Contactors. A contactor is a large relay, often designed for a specific use (as in controlling lighting circuits). The major difference is in the size range available with contactors. They're capable of carrying current in the range of 9A through approximately 2250A. For example, a size 00 contactor is rated at 9A (200V to 575V). A size 9 contactor is rated at 2250A (200V to 575V).

Like the relay, it's an electromechanical device. The same coil conditions exist in that a high inrush current is available when you energize its coil (generally at 120V). The current level drops to the holding or sealed level when you operate the contacts. Generally, you find contactors in 2-, 3-, or 4-pole arrangements. Manufacturers generally supply one NO auxiliary contact as standard on most contactors. You would use this contact as a holding contact in the circuit, for example, around a NO push button switch. You can get additional NO and NC auxiliary contacts as an option from the manufacturer or ordered as a separate unit and mounted in the field.

You use a contactor for switching power to resistance heating elements, lighting, magnetic brakes, or heavy industrial solenoids. You can also use them to switch motors, if you supply separate overload protection.

Control circuits. The circuits we use to connect all of our controls are obviously important. Three things are essential for control circuits:

  • They must be properly designed for their jobs. (To control equipment properly.)

  • They must be safe to operate.

  • They must be reliable in the long term.

Control circuits usually use standard wiring methods, in normal circumstances-the most common being type THHN conductors in EMT (electrical metallic tubing or "thinwall" conduit). In more difficult environments, such as in or around industrial machines, you will find THHN conductors in heavy-wall (GRC) conduit.

Probably the three most basic applications of control circuitry are:

  • You operate a motor start push button switch, which energizes a motor starter coil, closes motor starter contacts, and then energizes a motor.

  • A NO thermostat contact closes, energizing the coil of a contactor, which in turn, closes contacts, and then energizes heating elements.

  • A NO limit switch contact connected to energize the coil of a relay is held closed (operated) at the start of a cycle. A NO relay contact then closes to energize a solenoid.

Obviously, these are very simple applications. Other control circuits can be very complex, requiring skilled professionals to design, install, and especially troubleshoot them.

Circuit diagrams. In the schematic diagrams used to depict control circuits, symbols represent each component, and every wire is either shown by itself or included in an assembly of several wires that appear as one line on the drawing. However, each wire in the assembly is numbered when it enters and keeps the same number when it emerges to be connected to some electrical component in the system.

Refer to the schematic and wiring diagrams (Fig. 2 and Fig. 3, original article) with this lesson. They show the various devices (in symbol form, as shown in Fig. 1, on page 59, original article) and indicate the connections of all wires between the devices.

The holding circuit. If you refer to the first motor starter control circuit drawing (Fig. 2, original article), you'll see one of the most basic yet important control techniques: the holding circuit (also called the stick circuit). Follow the current through this circuit, starting at the left (at L1). The line L1 represents one of the two power conductors that provide voltage for the control circuit's operation. Current in this control circuit flows from L1 to L2. Presumably, L1 and L2 are two of the three conductors feeding the 3-phase motor this circuit controls. In this case, the control circuit is operating at the same voltage as the motor. Otherwise, you would have a small transformer (commonly called a control transformer) installed to bring the voltage down to an appropriate level for the control circuit.

The current flows first through a STOP button. Notice two things about this device: First, it's an NC switch; second, all the current in the circuit must flow through it. (It's wired in series.) This arrangement ensures when you press the stop button, all current in the control circuit will stop.

The next component in the circuit is the START button. Notice this is a NO contact, and there is a NO set of contacts in parallel with the START button. This is the main component of the holding circuit. This set of contacts is marked "M," which means the "M" coil (further to the right in this control circuit) controls them. So, when there's current flowing through this circuit, the M coil is energized and the M contacts close, providing a path for current flow through the circuit, even without the START switch remaining in the closed position. You must push the START switch to start the circuit's operation, but once you have, the M contacts close and keep the circuit going without the START button remaining closed. This holds the circuit in its energized state.

The M contacts are separated from the M coil in this diagram. These two components, which are not connected electrically, are connected mechanically: The contacts make and break when you energize and de-energize the coil.

Just before this circuit reaches L2, you see three overload contacts. These are NC contacts located in the motor starter. If one of the three conductors exceeds its limit, the overload contact associated with it will open. This will cause the control circuit to open, the M coil to de-energize, the M contacts to open, and the main motor contacts (which are not shown in this drawing, but are operated by the M coil) to open, stopping the motor. Notice that the overload contacts are connected in series, so any one of them opening will de-energize the control circuit. Three overload contacts are shown in this drawing, which is required on all new installations. Keep in mind, however, there are old installations that have only two overload contacts.

A final note on this circuit: If you look at the START and STOP buttons, you'll see "1," "2," and "3" near them. These points represent the three wiring terminals inside the common START/STOP station. They're shown on some schematic diagrams, but are not fully necessary.

Schematic diagrams show the various components as they relate to each other electrically. Wiring diagrams (as shown in Fig. 3, original article) show the components as they relate to each other physically. Most electricians consider schematic diagrams better for understanding the circuit, its components, and its operation. Wiring diagrams, while generally less preferred, are helpful during installation and troubleshooting. This is especially true when trying to plan conductor routings for complex circuits.

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