Investigative Approach
The first steps were to identify the sensitivity of the CNC/Machining Centers by inducing sags of various depths
and durations. When these created sags were applied, the critical internal power supply points would be
recorded. The closest pole mounted capacitor bank would also be manually switched with monitoring in place.
A visual inspection of the machine made available by the owner was performed to identify critical control
voltages. Baseline measurements were recorded. All three phases of the incoming 230V input terminals were
sagged. No motors were initially in operation. In the unloaded condition, the machine was found not to be
overly sensitive to sags.
The CNC/Machining Center was next adjusted to run the
spindle at 500rpm. At this loading the machine behaved
similarly to the unloaded condition. The DC bus voltage
response to a 20-cycle sag for 70% (shown below) did not
upset the machine.
The 5V DC power supply that provides the logic for the CNC
was also found to be very stiff with respect to input sags.
(shown on next page)
Ideally the sag testing could have been performed with the
machine actually performing a milling operation with the
spindle drive under maximum load. Concern regarding the
breakage of the expensive tooling prevented this testing from
being conducted.
The next step of the investigation observed the reaction of the CNC/Machine Center to the manual operation of
the nearest pole mounted capacitor bank. This waveform is shown on the next page.