I was involved in a conversation with a contractor this morning about small pumps in the context of sewage and water systems. He offered that in his experience small single phase capacitor start / capacitor run pumps are failure prone. His preferred solution is to replace these with three phase pumps driven by variable frequency drive (VFD) which is supplied by a single phase supply. This isn’t the first time I’ve heard the idea but I’ve always been a bit skeptical. This approach adds a fairly expensive and complicated component to an otherwise simple system.
My contractor friend referred me to a company that specializes in the supply and service of pumping systems, so I gave them a call. My question was quickly passed up the chain to Earl, who turned out to be very knowledgeable fellow.
I’d like to share a few things I learned from that conversation.
First, in his opinion single phase capacitor start / capacitor run pumps are quite reliable. Not an issue. This is based on their return/replace rate, which he feels is quite low considering the number of devices in operation.
His next point is quite interesting to me. He explained that the combination of grinder pumps and VFDs can be problematic. The issue is that when the grinder pump encounters material it needs to grind through then the supply current rises abrubtly. The VFD can pick up this spike as an overload and will react by slowing the pump down. When that happens the pump loses torque and can’t get through the material, and the end result is a clogged pump.
Earl advised me to try and stay away from variable drive entirely when driving grinder pumps if at all possible. However, as we both realize, in some cases variable speed is a legitimate requirement, and then Earl recommended that I make sure the VFD is programmed to filter out short duration current spikes in order to give the pump a fighting chance to get through whatever crap it is dealing with.
I'd like to thank Earl for his time this morning. As the only electrical consultant in a small market I need to be quite diverse, and it is hard to be an expert on everything! It it really helpful to have folks like Earl than I can turn to when I need to dig in to a technical question.
I’ll conclude this post with an observation that I’ve seen too many RFPs come in where the municipality/ regional district is asking for a variable speed design but as far as I can tell hasn’t performed sufficient due diligence to about why that is necessary. It is true that variable speed designs have some merit and may reduce power consumption, but is it also true that these designs work against the KISS principal, with a corresponding penalty in reliability and expense.
I used to poke at this a bit during the RFP process but I've never won a job where I did that, so I've stopped doing that.
Another great sunny day here, hope life is good wherever you are. Until next time...