David,
Thanks. Well, I changed my mind after thinking about the potential internal circuitry inside of the PowerPro. As I understand it, there is an inverter driver that excites each stator sequentially. There are different topologies for these, but a common one for SRM motors is the asymmetrical bridge inverter topology (
https://en.wikipedia.org/wiki/Switched_reluctance_motor#/media/File:Asymmetric_Bridge_Converter.svg). Not knowing which one or being able to review the PowerPro schematics, it is difficult for me to identify potential leakage paths for the PowerPro design. When I wrote my previous response I had been thinking more along the lines of a simple induction motor load and/or simple consumer electronics type application...so I kind of dismissed the potential risk for ground fault. Having a HV driving circuit like this presents the opportunity for many more potential leakage paths to ground. Every FET has leakage, but the actual schematic and topology choices can either mitigate that leakage or exacerbate it. Passive components (R's, L', C's) have leakage too. Even with a good schematic design and topology selection, there can be damage in the way of EOS (electrical over stress) or ESD (electrostatic discharge) in the worst case, which could cause one or more of the components to start leaking. Another concern is how they did the substrate connections on those FET's, e.g. is unbalanced (120V operation) more or less dangerous for leakage paths than a balanced (240V operation) connection (due to where those substrate ties connect)? I can't say without seeing the schematics. Not trying to scare PowerPro owners here, but just pointing out that
it gets complicated to determine all the potential failure modes where a user might accidentally end up in the path of an undesired leakage current. All of these potential leakage paths could stay internal to the PowerPro and never complete a circuit through the operator, but anytime you have a leakage path in a circuit you need to identify possible paths it can take and the impedances involved. When the impedance to ground, through a human, is lower than through the internal circuit it is always a bad outcome... :(
Yeah, I wouldn't want to compromise other taps from the same branch circuit either. One should try to get the GFCI to work on the PowerPro, but if not should probably have at least a GFPE (breaker) on it. I would be curious to learn of what Shopsmith recommends on this if you find it. Thanks again.
