General Purpose AC Sensing Applications

Overview 

The VTI21 can be employed as a general purpose, 2 channel Alternating Current sensor for monitoring currents of up to 25A RMS. In this regard, each VTI21 attached to an SA380TX acts as a 2-channel slave logger. (see SA380 series Master-Slave configuration).

Data derived from the VTI21-ACS can be utilized by the following SA380TX Applications:

• Acquire on Change
• Triggered Capture
• Generic Logger
  
  

Installation

For general purpose applications such as AC consumption of single-phase equipment or track circuits, the monitored circuit conductors can pass through either transducer in any direction.

Ensure all conductors carrying circuit current pass through the CT. This may often be two or more conductors in many railway applications.

VTI21-ACS Outputs

The VTI21-ACS sensor can output the following quantities:

For each input channel, the ACS outputs both RMS current (at a rate of 100Hz, with filtering (see below)) as well as the average RMS current each second. This allows the user to decide between the more finely detailed 100 Hz data or the "smoother" average data depending on the application and monitoring requirements.

How does the VTI21-ACS Work?

• The VTI21-ACS samples incoming waveforms from the Channel 1 and Channel 2 at 25 KHz with 10-bit precision.
• The true RMS of these samples is computed at 100 Hz.
• The 100 Hz RMS is fed through a five-point moving average filter (to compensate for beating effects at different frequencies) and presented as the "RMS Current" quantities.
• The average of these filtered values is calculated each second and presented as the "RMS Current (1 second average)"
• Data from all channels is recorded by the VTI21-ACS whenever a significant change is detected (Acquire on Change) and read by the master TX over RS485
• "RMS Current" data is requested from the VTI21-ACS by the master TX over RS485 whenever a Triggered Capture event occurs.
• The 4-20 mA output is continuously updated at 100 Hz with the value of the filtered output of Channel 1.
  • '4' represents no signal present
  • '20' represents a full-scale positive reading, i.e. 25 A

Sample Applications

AC Track Circuit Monitoring

The diagram below shows how the VTI-ACS may be used to monitor an AC track circuit:

• One input is being used to monitor the RMS output of an AC track circuit feed set.
• The second input is being used to monitor the RMS input to the control coil of an AC track circuit relay.
• Two outputs are produced. Each output is the RMS value of one of the input channels.
• Two "Acquire-on-Change" analogue channels should be configured on the SA380TX data logger to capture each channel whenever there is a significant change.

Single Phase Switch Machine Monitoring

The diagram below shows how the VTI-ACS may be used to monitor a single-phase AC switch machine

• Single phase switch machines typically employ a three-wire interface.
  • One "Live" wire is energised to motor the switch from Normal, to Reverse
  • A seconds "Live" wire is energised to motor the switch from Reverse to Normal
  • A third "Neutral" wire acts as a common current return regardless of the direction of motion.
• In order to determine motor current draw and direction of operation it is necessary to utilise two inputs to monitor a single switch machine.
• The first input measures RMS current on the Normal to Reverse "live" wire
• The second input measures RMS current on the Reverse to Normal  "live" wire
• Two outputs are produced. Each output is the RMS value of one of the input channels.
• Two "Triggered Captures" would be set up on the SA380TX data logger
  • The first capture logs moves in the N-R direction. It is triggered by the detection of current flow on the N-R live wire. During the capture, current flow on the N-R live wire is logged.
  • The second capture logs moves in the R-N direction. It is triggered by the detection of current flow on the R-N live wire. During the capture, current flow on the R-N live wire is logged.