This is a VGD control assembly on a YK chiller. The first picture shows the actuator and armature. The second picture is the Optiview graphic and control screen of the VGD. The third is inside the actuator head with the wiring terminals.
There is not an potentiometer on the VGD like there is on the PRV. The chiller only knows the actual position when the actuator hits the end switch internal to the actuator. The VGD itself assists with lift across the impeller to help prevent stall and surging.
On the VGD screen the chiller is monitoring the stall detector voltage to decide how it needs to adjust the VGD. The higher the voltage, the greater the pressure instability or "noise", the closer to stall and surge you are. This voltage is taken from the stall detector board. The stall detector board monitors the stall transducer (discharge transducer) for pressure instability. This will indicate a unsafe condition that is more accurate than waiting on a major current fluctuation like other chillers will do. The armature slides a steel ring in the diffuser of the compressor to adjust the diffuser gap. In most circumstances you want to stay below .4v on the detector voltage for normal operation. If the chiller has recently stalled or surged there will be a count down timer to protect the compressor by locking the ring in place. Otherwise the display will show probing when operation is normal.
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