This is the worst case waveform captured across phases A-B at the machine terminals. The waveform shows the
effect of the transformer connection and the different closing points of the two oil switches used for capacitor
control. The capacitors are connected as normal in a phase to neutral configuration. The small perturbation
appearing in the middle cycle is the first pole closing. The more obvious transient in the third cycle is the
second pole closing. Note the initial negative going transient followed by a 1.1 to 1.2 per unit positive going
peak. No upset occurred during the manual capacitor switching. The observed switching transients were not
severe. Of course it is not possible to recreate all possible closing angles or combinations of system conditions.
The sag testing performed on this particular machine revealed a high level of sag immunity while the machine
was not heavy loaded. Under heavy loading conditions this immunity level would most likely be reduced due to
the effect on the spindle DC link voltage. A closer examination of the CNC machine that was made available
for sag testing revealed that the machine had a nominal input rating of 200 volts. As with any AC to DC power
conversion process, the higher the nominal peak voltage, the better the sag and interruption ride-through. An
examination of the eleven CNC machines at the facility found that three were rated at 200 volts while the
remaining eight were rated at 240 volts.
It is very common for CNC/Milling machines to have a very long useful life, and there is also a very active
market for used machines. Since the machine “shops” produce no product except under contract, the workload
of the industry following the economy and the equipment tends to follow the contracts.
Another point of concern is the milling apparatus is manufactured in the Americas, Europe and the Far East,
each having a different nominal working voltage. Although the same company made all the CNC units, they
Industrial Design Guide CNC Machine: Case Study #1
5
Typical CNC Local Panel
were not mated to identical milling machines. In fact the CNC were of different generations which further
complicates the issue.