The AC current transducer monitors the flow of alternating current on a conductor and outputs a proportional DC voltage at its positive (+) and negative (-) screw terminals. The CT is especially useful for monitoring AC motor drives. As most of the motors used in CNC machines are three-phase AC, the CT is a very useful sensor. When used in integration, the AC current transducer is most commonly used to monitor the:
- Spindle drives
- Axis drives
- Work conveyor motor
The most common application for the AC Current Transducer is to monitor the spindle drive of a machine to determine active and idle states.
Topics covered in this article
Locate Your Machine's Spindle Drive
The spindle drive pictured below is flanked by the power supply to the right, and the axes drive to the left. The spindle drive is responsible for sending power to the spindle motor. Most often the power it delivers is three-phase AC. The three legs labeled U, V, W go to the spindle motor. Clamp the CT around any one of the three-phase legs.
If you're unsure where the spindle drive is, refer to the machine's wiring diagram. One way to verify you've got the CT on the correct wire would be to use a multimeter (set to DC Volts), and test the positive (+) and negative (-) terminals of the CT. If you note a change in voltage when the spindle is turning vs when it is stationary, you're on the right wire.
Connect the CT to the I/O Module
Connect the positive (+) terminal of the Current Transducer to AIN0 of the LabJack.
Connect the negative (-) terminal of the Current Transducer to GND of the LabJack.
The Range Jumper has three settings:
- Low: 10 AMPS: 10 VDC (Recommended)
- Mid: 20 AMPS: 10 VDC
- High: 50 AMPS 10 VDC
The Range Jumper is used to scale the DC Voltage signal from the CT's screw terminals. Depending on the size and application of the motor the CT is monitoring, you may want to use the Range Jumper in either low, mid, or high settings. The "low" setting can be achieved by removing it altogether; allowing a 1:1 ratio from Amperage to DC VOLTS.
*Below is the signal from the Current Transducer monitoring activity on the spindle. The signal from the CT monitoring a spindle motor gives a clear indication of whether the machine is active or idle.
The Adapter Script for a machine using a CT to determine active/idle states relies on setting a voltage threshold. In an idle state, the reading from the CT will be 0 volts. Depending on the size of the motor being monitored, the signal may rise up to 10 volts. If the signal from the CT crosses the voltage threshold, the machine is considered active.
*Below is an example of the signal from a CT above and below the designated voltage-threshold. Note that the machine is considered idle when the signal drops below the threshold.
Pictured below is an example of an Adapter Script using a CT to determine utilization. The positive (+) terminal of the CT is connected to AIN0 (pin: 0), and the voltage threshold is set to 1.5 volts:
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