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Guide to DDC

Chapter 2

Input Devices and Sensors

Electrical Measurements

Monitoring of electrical system attributes is performed by DDC control systems to protect system components, determine power and energy consumption of various components, and implement usage and demand control strategies to conserve energy. A variety of hardware and techniques are applied to these measurements.

Types of Electrical Measurement Devices

There are many devices that measure electrical attributes on the market today. The two most common electrical measuring devices used for DDC are current transducers and power measuring devices.

Current Transducers

Current transducers are used in DDC control systems to monitor current flow to motors, heaters, or electrical distribution systems. Their input may be used for demand limiting purposes, control, or energy accounting. The sensing element of a current transducer is typically a current transformer. It transforms the current being monitored into a higher voltage, lower current. Additional circuitry reduces this voltage to the desired level. Current transducers may have line and load terminals for the monitored current, or they may be arranged as a coil that the current carrying conductor passes through. With this arrangement, the load conductor induces the current in the transformer via the electromagnetic field surrounding the conductor. Current transformers and transducers are available with solid or split cores. The split core device may be installed without disconnecting the power conductor provided that there is sufficient slack in the conductor and room in the enclosure. Accuracy of 0.5 % of full scale is readily available.

Power Monitoring Devices

Commonly monitored characteristics of a power system include:

  • Power Demand (typically measured in kW)
  • Power Consumption (typically measured kW per hour)
  • Voltage (typically measured in Volts)
  • Current (typically measured in Amps)
  • Frequency (typically measured in Hertz)
  • Power Factor
  • Reactive Power - (typically measured in kVAR)

Many panel level monitoring devices measure all or most of these characteristics and can communicate to the DDC system through a gateway. These are typically used to monitor whole building power systems. Other devices measure power and power consumption only and provide both analog and pulse signals for input to the DDC system. These sensors are typically installed at the terminal use point of power systems, such as variable speed drive controlled pump and fan motors. Accuracy 0.2% of reading and 0.04% of full scale are available.

There are other methods of monitoring demand and consumption. One of the simplest methods is to obtain a pulse signal output from the utility company's metering equipment. This can be input directly to a controller with pulse input capability, or a pulse to analog signal transducer may be used. The pulse represents a set number of kilowatt-hours. Average demand is calculated using a rolling time average of the number of pulses over the stipulated time period. Average demand is typically calculated for billing purposes over a 5, 15, or 30 minute period. Power consumption and demand may also be calculated using current transformers to measure current flow and voltage transducers to measure voltage on the selected load or system. The DDC controller calculates the demand from these values, and integrates this value over time to determine power use.

Other Electrical Measurement Devices

Transducers are available to provide a standard voltage or current input to a controller based on measured frequency, reactive power, or power factor. Available devices for load protection are available that monitor three phase voltages and provide a relay signal to disconnect loads if the power supply becomes unsuitable for continued operation due to conditions such as phase loss, phase imbalance, low or high voltage, or phase reversal.

Load protection for motors may be incorporated into the motor starter through the use of a solid state overload device. These devices provide the required time-current protection to protect the motor from overload conditions, as well as power monitoring to protect the motor from unsatisfactory power supply.

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