Application Notes: Variable Frequency Drive Status

Monitoring Variable Frequency Drive Status

Special considerations for monitoring variable frequency drive status:

Unlike constant volume fans and pumps, motors with variable frequency drives generate stepped waveform outputs which are designed to appear as sinusoidal waveforms to the motor. These stepped outputs are generated by high frequency (6kHz +) signals switching on and off. Since most current sensors are designed to operate at 60 Hz, the VFD presents special challenges to the installer. There are several different installation options to consider for the installer to get reliable positive status indication on fans and pumps.

Option 1: Mount a standard 700 or 900 series sensor on the line side

If the motor speed will be greater than 25 Hz, the installer can use a standard 701-708 or 908 current switch. In this installation, the 700 (solid core) or 908 (split core) sensor is mounted on the line side of the inverter. Using the standard current switch will provide indication of status, but does not provide indication of the load on the motor to the automation system.

Installation Instructions:

Mount the current switch on one phase of the motor between the electrical panel and the inverter (line side)
Run the motor to its lowest operating frequency (if less than 25 Hz, see option #3)
Adjust the potentiometer until the "Output Status" LED just switches on;
Turn the potentiometer CCW ~ 1/2 turn;
he unit is now calibrated to detect mechanical or electrical failures

Hawkeye 701/708 or 908 installation:

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.