I thought this was too important not to put here in the GGMG.......Thanks David! It may take more than one post due to the 1000 character limit...this was in response to William Rogers, adding an emergency stop -
#2 - William, this actually highlights one of the biggest risks of a do-it-yourself double pole-double throw (DPDT) switch wired into the input 220v AC power side of the VFD between the 220v wall receptacle and the lathe. It sounds like that's what you have in mind, rather than wiring a switch into the low voltage circuitry of the lathe's existing switch. If the input power is cut off, but then cut back on as you describe, without waiting at least two minutes, you risk damaging the VFD. Here is an excerpt from an article by Thomas Robbins, a machine systems Applications Engineer, titled "Feed the Right Power to Your VFD." The whole article can be viewed and downloaded at http://www.machinedesign.com/motorsd...fd-right-power
#3 - Excerpt: Do not cycle the input power more than once every two minutes. In fact, drive manuals specifically warn that switching a drive off and on without waiting two to three minutes is detrimental: Applying input power more quickly causes a buildup of voltage in the input pre-charge circuit, and eventually burns it out. Why? Here, the dc bus capacitors don't have enough time to discharge, and the input circuit needs time to stabilize. Otherwise, additional input can damage the charge relay circuit, or at the very least, blow the input fuses or circuit breaker. In other words, the pre-charge circuit allows a certain time limit for the inrush limiter to send current through to charge the dc bus capacitors. The inrush limiter resistance changes with temperature. The hotter the limiter gets, the lower the resistance value. When that pre-charge time ends, the relay cuts off and the capacitors hold the charge.
#4 - When the drive is powered down, this voltage bleeds off through resistors in the discharge circuit. Power reapplied too quickly meets an inrush limiter that hasn't had time to cool down to an acceptable resistance level, so the current will be higher, and consequently, could blow the fuses or possibly damage the pre-charge circuit.
#5 - The problem that Robbins describes is in addition to the two that others have already mentioned: Interrupting all power to the lathe (1) defeats the VFD's cool-down cycle, and (2) defeats any built-in dynamic braking feature, thus causing the lathe to go into free-wheel. In an emergency, stopping the lathe is the important thing. If you can't reach the lathe's built-in E-stop switch safely, then of course pulling the plug, flipping the circuit breaker, or using a do-it-yourself 220v DPDT switch is better than nothing. But we should be aware of the unintended consequences.
#6 - BTW, if we happen to be turning at the time of a power failure, the loss of the cool-down cycle and the dynamic braking feature occurs then, as well. And if the power comes back right away, the risk of damage that Robbins describes exists. But occasional power failures are pretty much unavoidable. Even if an automatic back-up generator system is in place the risk exists absent a very sophisticated setup, as the cut-over to standby power will likely involve delay and voltage swings. Robbins discusses that in the full article.
#7 - I'm not a sparky, but bottom line for me is that a remote switch intended for use as a "start/stop" switch, rather than simply to shut off all power in an emergency same as pulling the plug, should only be wired into the lathe's existing low-voltage start/stop circuit.
