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Guide to DDC

Chapter 3

Output Devices

Analog Devices

There are numerous analog devices used in the HVAC controls world. Typically, analog output devices are used to provide modulating control of valves, dampers, electric motors through variable speed drives and a wide variety of other devices. The most common devices associated with analog outputs are sequencers, variable speed drives, silicon controlled rectifiers and actuators.

Sequencers

Sequencing of multiple on-off devices based on a single analog output from a control loop is often required for items, such as cooling towers with multiple two-speed fans, multi-stage electric heaters and multi-stage refrigeration systems. This sequencing can be accomplished within the DDC controller, or it may be accomplished externally using a discrete sequencing device. These devices have two or more relay or digital outputs that are adjusted to spread the signal range that they turn on and off. For example, a two-stage sequencer might be adjusted so the stage one relay turns on at 37.5% analog signal level and off at 12.5%. The stage two relay would be adjusted to turn on at 87.5% analog signal and off at 67.5%. More advanced sequencers may incorporate adjustable inter-stage time delays, minimum on and off times, etc.

Variable Speed Drives

Variable speed drives are used to vary the speed of AC and DC motors in order to control the output of driven equipment. DC variable speed drives are costly and offer very precise control. They are widely used in industry for precise speed control of conveyors and printing presses, but are not widely used in the HVAC industry. AC variable speed drives are less costly and offer good control for equipment, such as centrifugal compressors, fans and pumps.

AC variable speed drives operate on the principle that the synchronous speed of an AC induction motor is directly proportional to the frequency of the AC power supplied to the motor. In the US, the standard frequency at which AC power is distributed and motors are rated is 60 cycle per second (hertz). Virtually all AC variable speed drives currently manufactured use solid state components to accept AC power at standard distribution voltages and 60 hertz frequency (50 hertz in Europe) and output a variable frequency power supply to the controlled motor(s). Commonly available drives have provisions for external on/off control by a contact closure, analog speed feedback signal for monitoring, and accept a standard analog voltage or current signal for speed input. Many drives are available with one or more drive status alarms. Some are also available with digital communication interfaces that allow detailed status and fault monitoring by DDC control systems.

Most drives use an AC to DC converter and a DC to AC inverter. The converter may consist of a diode rectifier, a diode rectifier with a DC chopper, or a silicon controlled rectifier (SCR) sometimes called a thyristor. The simple diode rectifier creates a constant DC voltage for input to the inverter. The addition of the DC chopper allows regulation of the voltage to the inverter. Silicon controlled rectifiers also allow regulation of the voltage to the inverter.

The inverter section of the drive consists of solid state switching devices that reconstruct an AC power signal with controlled frequency. The three most common types of inverters are variable voltage source (also called six step), current source and pulse width modulated (PWM). The six step inverter uses six solid state switching devices in combination with six diodes. The solid state switches are controlled to produce a six step voltage wave form for each phase. Changing the conducting time for each of the six switches results in a change in frequency of the output wave. The current source inverter operates much the same as the six step variable voltage source except that solid state switching devices construct a six step current wave for each phase instead of a voltage wave. Pulse width modulated inverters use solid state switching devices to produce a series of constant voltage pulses of various widths to produce an AC output. The timing and number of pulses are varied to produce the varying frequency.

Application Considerations For Motors and Drives. The following items should be considered for any variable speed drive application:

  1. Normally, NEMA Design B squirrel cage induction motors with continuous duty rating are used.
  2. Multiple motor loads can be controlled from a single AC variable speed drive, however the manufacturer's guidelines must be followed regarding operation if some or all motors are not connected. This applies in particular to drives with current source-type inverters.
  3. With current source and PWM-type inverters there is some additional stress on the motor insulation. These stresses are usually not significant.
  4. PWM inverters usually cause motors to produce more noise than normal.
  5. Any type of inverter produces a current waveform that contains harmonics that do not produce any additional torque, but do cause additional heating in the motor windings. This will typically produce 5% - 15% additional heating load and must be considered when operating motors controlled by drives near full load conditions.
  6. With current source inverters, an open circuit (such as a disconnected load) will cause an excessive voltage rise in the inverter. Unless appropriate protection is provided, this condition may cause inverter failure.
  7. Jerky shaft motion can result with any inverter type at low speed (typically below about 10 hertz) due to badly distorted waveforms at these frequencies. Some PWM drives are available that are optimized for operation at low speed and can reduce this effect.
  8. It is important to consider the torque - speed characteristic of the load to be imposed on the drive. Most HVAC applications are for centrifugal machines (pumps, fans and compressors) and are described as "variable torque" because the torque is low at low speed and rises according to the cube of the motor speed. Infrequent applications for HVAC, such as positive displacement pumps, may have constant torque characteristics.

Silicon Controlled Rectifiers (SCRs)

SCRs are used to regulate an AC power supply to a typically resistive electrical load, such as an electric heater, to provide continuously variable output. SCRs accept standard analog control signals (usually voltage or current) and regulate the output of their load proportionally.

With microprocessor-based controls, SCRs can be used in combination with sequenced contactors to provide vernier control that is continuous in proportion to the input signal, but does not require control of the entire load by a SCR and thus reduces the cost.

Actuators

Analog signal controlled actuators are one of the most important components of DDC systems today. Air temperature control is commonly accomplished with actuators of various types through the control of damper position and valve position. The majority of modern HVAC designs include actuators of one type or another.

Types of Actuators

With the invention and continual refinement of DDC systems, electric motor controlled actuators are steadily replacing pneumatic controlled actuators as the application allows. There are still a large number of both types available and in service today.

Pneumatic Actuators

The pneumatic actuator has been widely used for HVAC control for decades. With the inventions of the electric-to-pneumatic signal transducers and EP relays, DDC systems can readily integrate pneumatic actuators into the control scheme for steam valves, dampers, etc. Diaphragm- and piston-type actuators are the two most common pneumatic actuators.

Diaphragm-type actuators are most commonly used with low pressure pneumatic control signals in the range of 0 to 30 psig, but are available for industrial application at higher pressures. Diaphragm actuators typically have an opposing spring, with air supply to only the side of the diaphragm opposing the spring. The spring constant sets the range of air pressure over which the valve will operate and also provides for failure in an open or closed position, depending on orientation. The action of diaphragm actuators is normally linear, but may be converted to rotary motion approaching 180 degrees through the use of suitable links.

Piston-type actuators are most commonly used with higher air pressures in the range of 80 to 100 psig. Piston actuators are generally more compact than diaphragm-type actuators, particularly for larger valve sizes. Pistons may be single acting (air applied to piston on one side, spring pressure on opposite side of piston provides return pressure) or double acting (air pressure is applied alternately to either side of the piston to produce bi-directional motion). Piston actuators may have linear or rotary motion through the rack and gear or other mechanisms.

Positioners are commonly used with a pneumatic actuator to control the stroke or rotation of the actuator so that it positions the controlled device in a fashion that is linear in proportion to the control signal. Limit switches may be mechanical- or proximity-type actuators and are often mounted within a positioner enclosure.

Electric Actuators

A wide variety of sizes and shapes of electric actuators are available to meet the requirements for valve and damper actuation for HVAC systems. Most electric actuators are based on an electric motor and output mechanism. Some mechanisms are designed for spring return; others are designed so that the mechanism locks in place when the motor is off. Most actuators relate the analog control signal proportionally to the position or percent of total travel. Torque switches may be used on large electric motor-driven actuators to stop the valve motor when the valve has reached full open or closed position.

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