Input Devices and Sensors
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 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
Power Monitoring Devices
Commonly monitored characteristics of a power system include:
- Power Demand (typically measured in
- Power Consumption (typically
measured kW per hour)
- Voltage (typically measured in
- Current (typically measured in Amps)
- Frequency (typically measured in
- 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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