Digital outputs (DO) are typically used to provide on/off control of valves, dampers, electric motors, lighting and external signaling devices, such as alarm bells and indicator lights. Digital outputs may also be used to control analog devices using tri-state or pulse width modulation (PWM) previously described in Chapter 1. The most common devices associated with digital outputs are relays, contactors, starters and two-position actuators.
A relay is a device where power applied to a coil or input terminal causes the path between pairs of separate, additional terminals to either allow electrical current flow, or stop current flow. Contactors and starters are essentially relays designed for interrupting and applying power to larger loads (i.e., integral horsepower motors) and significant resistance loads (i.e., lighting and heaters).
The most common types of relays are standard instantaneous control, latching, and timing. Contactors and starters can be considered common types of heavier duty relays with and without load protection.
Standard instantaneous control relays are electromechanical or solid state. Electromechanical control relays use a magnetic coil and armature to cause contacts to open or close when current is applied to the coil. Solid state relays use semi-conducting devices (such as transistors or triacs) that become electrically conductive between output terminals when a voltage is applied to the input.
Relays are typically used to switch AC and DC control signals with voltages from 0 to 600 volts and typically have contact ratings of less than 20 amps. Control relays come in numerous sizes and shapes. Relays used on printed circuit boards for pilot duty can be made very small, with the largest dimension under 1/2 inch (12.5 mm). Modular, miniature and sub-miniature rail mounted plug-in type relays are often used in shop or field-fabricated control panels because they are less costly and easy to mount and replace.
Latching relays are a variation of the standard instantaneous control relay where the contacts change position when initially energized, but do not revert to the normal state (when the input signal is removed) until a separate reset signal is applied. Latching relays may have mechanical latches using a set and reset coil, or they may latch magnetically. Latching relays are also available with manual reset latches.
Timing relays (also known as time delay relays) are a variation of the standard instantaneous control and latching relay where a fixed or adjustable time delay must occur following a change in the control signal before the switching action occurs. Common time delay relay configurations include on delay, off delay and on/off delay. Numerous other configurations are available.
Contactors are essentially large capacity relays specifically designed to control the flow of electrical power to electrical loads, such as motors, heaters and lights. Contactors are multi-pole devices typically arranged to interrupt all energized conductors serving an electrical load, thus removing all voltage from the load. Contactors can include normally closed and normally open contacts, but are most often of the single throw, normally open, double break configuration. Contactors do not include overload protection for the load they are serving. When contactors are applied to control motors, the power circuit must include thermal overload protection for the motor.
Starters are specially adapted contactors that include overload protection designed to sense motor overloads and interrupt the power circuit to the motor before severe damage can occur. Contactors and starters are rated according to national and international standards including NEMA/EEMAC (National Electrical Manufacturers Association/Electrical and Electronic Manufacturers Association of Canada) and IEC (International Electrotechnical Commission). Contactors and starters are listed by recognized testing agencies such as UL (Underwriters Laboratory) and CSA (Canadian Standards Association).
Ratings typically include maximum voltage, maximum continuous current and maximum single-phase and three-phase motor horsepower at voltage. NEMA/EEMAC standards for magnetic motor controllers designate two types of motor duty (non-plugging, non-jogging duty and plug-stop, plug-reverse or jogging duty) and a series of standard sizes with standard horsepower ratings for each size. The most commonly used starters and contactors in the United States conform to the NEMA standards.
The oldest and simplest motor overload protection scheme consists of a thermal overload for each power conductor. These power conductors consist of a resistance heating element and fusible metal or bimetallic temperature switch wired in the starter coil control circuit. The resistance heating element heats in proportion to the current flowing to the motor, creating a rise in temperature at the switch element that is proportional to the motor current and the time over which the current has been applied. If the motor overload is severe, heat will build up quickly, and the switch will open in a few seconds or less. If the overload current is just above the overload rating, the switch will take a longer time to open. Thermal overloads are typically non-adjustable, or adjustable over a very narrow current setting range.
In recent years, solid state overload relays have been developed that sense the motor current in each phase, digitize it and apply digital logic to determine when an overload or unsafe operating condition exists. Solid state overload relays can typically sense phase failure, asymmetrical current loading, severe overload or locked rotor conditions. Solid state overload relays typically allow for the adjustment of motor full-load current values. They also allow for setting a variety of time-current trip characteristics to provide optimal protection for the motor they are protecting.
Two-position control is commonly used in a wide variety of control schemes for HVAC applications. Fluid flow, damper position and fuel flow are commonly controlled (depending upon application) to open/closed positions through the use of a two-position actuator.
Two-position actuators are used to control the linear or rotary motion of a controlled device (such as a valve or damper) to one of two positions, usually open or closed. The two most common types of two-position actuators are the solenoid type and rotary type.
One of the simplest actuators is the solenoid, which consists of a coil wound around a fixed core and a movable core that is usually enclosed in a non-magnetic case. When the coil is energized, the movable core is attracted to the fixed core, causing a rapid linear motion. Solenoid actuators are most commonly applied to small valves for control of water and air flow in pipe and tubing. Solenoid valves are available in pilot-operated models, where fluid pressure of the fluid being controlled actually provides the motive force for operating the valve. The solenoid is used to control the internal flow of the pilot fluid within the valve, causing the operation of the valve. Non-pilot type solenoid valves open and close very quickly and may cause water hammer when used for controlling flow in liquid systems. Pilot-operated valves may be designed for slower opening and closing time to reduce this tendency.
Solenoid valves are also commonly applied to the on/off control of pneumatic control air supply (sometimes referred to as EP Relays). Two state, on/off control of pneumatic dampers and actuators is almost universally accomplished using the electrical signal to operate a solenoid valve that turns air supply to the pneumatic actuator on or off.
Rotary actuators typically are based on rotary electric motors combined with a gear train that may be reversible, or combined with a spring, such that the position is reversed by the energy stored in the spring when the motor is de-energized. Spring-return actuators are commonly applied where a device must be returned to a safe or normal position when the power supply or control signal fails. Linkages, rack and pinion configurations, cams and various other mechanisms are used to convert the rotary actuator motion to linear motion when applied to devices (such as globe-type control valves) requiring linear motion for actuation.
With the exception of the solenoid type, most two-position electric actuators can also be used for modulating control with the appropriate analog control circuitry.