Application Notes: High Accuracy for VFD's

High Accuracy for VFD's

The 720 analog (4-20 mA) sensors from Veris provide high accuracy across a wide frequency range, making them the perfect choice for variable frequency drive applications. These 3 wire sensors provide >.25% accuracy from 10 to 80 Hz and can be mounted on the load side of the motor.

Installation instructions:

Mount the 720 series sensor on one phase of the motor between the starter and the motor (between the inverter and the motor for VFD's)
Connect power and output terminals as shown below
Reconnect the sensed conductor to the motor
With the motor off, adjust the zero potentiometer until the output equals 4 mA
Start the motor and operate at normal full load (60 Hz)
Using a hand-held clip-on amp probe, measure the amperage flowing through the primary conductor
Calibrate the span adjustment on the 720 until the output equals 20 mA
Determine the software scaling using the following formula:


: Amps/mA ratio = Max amps (from #6 above) / 16
: Primary amps = (mA output - 4) *( Amps/mA ratio)

Typical Hawkeye 720 installation:

Troubleshooting tips:

If the sensor provides no output when the motor is turned off, the most likely cause is that the power supply is not connected properly. The 720 is a 3 wire device which requires a separate power supply (~25 mA at 12-30 VDC), either from the automation panel or from an external source. If the panel does not provide an acceptable power supply, the following wiring system can be used:

Applications are suggestions. Field applications vary; since manufacturer does not install the devices, it is up to the installer to ensure suitability and compliance to appropriate codes.