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- Jan 5, 2014
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A couple of other notes on capacitor lifespan. As mksj said, under maximum operating conditions, the average life for capacitors is fairly short, 2000-5000H is often specified although there are 10000H+ caps too. Yup, less a 3 months to just over a year. By design though most electronics that contain capacitors run many many times longer than that. Temperature has a lot to do with it along with the actual capacitor current and voltage during operation. When a VFD is powered up but "off", there is little to no capacitor current so it runs very cool. Examples: If the designer uses 400V capacitors instead of 350V capacitors for the same application, the lifetime goes up. If a capacitor averages 60C operating instead of 80C, lifetime goes up. If the RMS current drawn from the capacitor is 1A instead of 2A, the lifetime goes up. The lifetime rating also doesn't mean the capacitor suddenly stops working at that point either. Usually, the capacitance is <80% rated or internal resistance >2X rated. Those changed values can cascade and lead to hard failure of the product at some point though.
The design deratings combine to allow for real lifetimes of many years and even decades. I'm about to retire well ahead of most of the large, industrial capacitor containing products I designed/shipped three decades ago. I imagine though that some of the early ones are approaching the tipping point soon.
That said, mainly for safety reasons, I generally turn off the power to my VFDs when not in use. I turn them on often enough to not worry about turn-on surge and I rely on the soft-starting circuits within the VFD to "worry" about that.
The design deratings combine to allow for real lifetimes of many years and even decades. I'm about to retire well ahead of most of the large, industrial capacitor containing products I designed/shipped three decades ago. I imagine though that some of the early ones are approaching the tipping point soon.
That said, mainly for safety reasons, I generally turn off the power to my VFDs when not in use. I turn them on often enough to not worry about turn-on surge and I rely on the soft-starting circuits within the VFD to "worry" about that.
