Coil Current Feedback
The drive measuring the actual current through the coil and feeding that value back into the control loop. Measurement is usually done with a shunt resistor on the order of milliohms or with a current sensor; the reading is used both to keep the flow rate constant and to catch faults early.
Without feedback the drive only knows "what it delivered," not "what happened." Current measurement closes this gap and serves two separate purposes.
- Regulation: if the measured current deviates from the target the PWM duty is corrected. Even if the coil heats up or the supply drops, the flow characteristic is preserved.
- Diagnostics: if the current is zero the cable is broken or the coil has burned out; if it is far above expectations there is a short circuit. In that case the drive can cut the output and issue a warning.
For the measurement to be read correctly the PWM ripple must be averaged; otherwise the value read varies depending on where in the period you sample. Good drives fix the sampling at a specific instant in the period.
The same idea is seen in AXI's retrofit kits: a disconnected motor connection is detected and reported with an LED and a buzzer. Electrical feedback is needed not only for performance but also to make the operator aware of a silent fault.
Where this term is used: Connect Plus and Minus, Run: Installation That Does Not Depend on Technical Staff