On Tue, May 3, 2016, at 10:00 AM, andy pugh wrote: > If there is no reason not to leave the pre-charge relay closed during > operation, I could even use the existing mechanical relay that brings > in the discharge resistor. Is there a reason to switch-out the > precharge relay once the main contactor is closed? I guess the danger > is that if the main contactor fails to operate the machine would run > through the precharge resistor. Though I imagine that that would cause > an easily-detected undervoltage fault in the drives.
You can certainly leave the pre-charge relay closed after closing the bypass (main) contactor. The vast majority of the AC drives on the market don't even have a pre-charge contactor. When power is applied, the bus immediately charges thru the resistor. The low voltage power supply for the electronics runs off the DC bus (great for power dip ride-thru.) Once the bus is up, the low-voltage power supply starts and the microcontroller comes alive. Then the micro-controller makes the decision to close the bypass contactor. If the bypass contactor opens during normal operation, the drive typically trips on low bus voltage. Although the worst case situation is a motor load that's not quite enough to drag the bus down to the low bus trip point, but is high enough to cook the pre-charge resistor. Pre-charge resistors are usually sized for intermittent duty, so they need to be a type that fails safely under continuous overload (no arcing or fire). On large (expensive) drives we sometimes use an auxiliary contact on the bypass contactor as an interlock. -- John Kasunich [email protected] ------------------------------------------------------------------------------ Find and fix application performance issues faster with Applications Manager Applications Manager provides deep performance insights into multiple tiers of your business applications. It resolves application problems quickly and reduces your MTTR. Get your free trial! https://ad.doubleclick.net/ddm/clk/302982198;130105516;z _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
