What year is the machine?

When you start the spindle, are you watching the DC bus voltage on the Diagnostics screen? Does it go down on that screen or are you measuring it with a voltmeter as it drops? Whatever it shows on the diagnostics screen is what the control is going to respond to. If that dips below--I think--300V, it triggers the low voltage alarm.

It gets that signal from the Vector Drive and--depending on the year--it goes to a connector on the MoCon board. If you have a later machine with a MainCon, I'm not as familiar but, the basics will be the same.