The Problem

A variable frequency drive was installed to regulate the pressure of water to maintain a constant 55 PSI. The drive would receive a 4 to 20 mA signal from a pressure transducer which would signal the drive to increase or decrease pump speed as necessary. The drive would ramp up in speed on start but would then run continuously at a speed lower than what was required by the pressure transducer.

At times the drive would also make an instantaneous change to run at maximum speed at a time when the transducer was sending a steady signal. The drive would then drop from top speed instantaneously to some speed that was lower than what was called for by the transducer.

The facility maintenance personel tested the drive by running it on a 4 to 20 mA signal derived from separate test equipment. The action of the drive remained the same.

Five service personnel had been contracted to inspect the drive during a period of four years. None could find a problem with either the drive or the transducer.

The Data

The pump manufacturer’s performance curves showed that the drive and motor were properly sized for the pump. Voltage fluctuations within the facility were reported as high as thirty volts on the 480 volt system.

The drive required and did have a set-point potentiometer wired to the drive’s terminal strip to be used to set the desired pressure point.


* Inspect the drive’s power input, power output, and signal wiring. That wiring should be in three separate conduits to eliminate the coupling of harmonic frequencies and EMI/RFI between the three circuits.
* Inspect the grounding and bonding within both the drive system and the feeding circuit breaker panel.
* Engage the keypad to read and evaluate all the drive settings and monitored functions.

Solution To The Problem

The PID had not been programmed. The Proportional Gain was found to be set at 0.00, the Integral Gain at 0.20, and the Differential Gain at 0.65. The uncontrolled changes in speed were the result of the fluctuations in incoming power voltage and the resulting variations in potential at the wiper arm of the set- point potentiometer. Thus, the drive speed output was being controlled by the input power voltage fluctuations and its own internal time constants.