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Old October 18th, 2018, 10:18 AM   #1
g.mccormick
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OT: Thoughts on powering off VFD few times a day every day.

I am design/building a booster pump for my house. I found seemingly good deal on a Grundfos pump on ebay and I already have a AD GS1 VFD in the garage from a previous ebay find.
So a pressure transducer, some other parts, and some PLC work will get me a constant water pressure control system for my house. I have a well with submersible pump. This new booster pump will be fed from the existing well pump.

So on to my question. When the demand is 0, i will command the vfd to stop mode. Even in stop mode, the VFD with fan will be taking excess power (probably not much I'll admit). So I was thinking of putting contactor ahead of VFD and shutting off the contactor(power to vfd) if the system has been in idle for some amount of time. This would mean that during day time when everyone is gone or at night when we are all sleeping, the vfd would be powered down.

Is there really any reason that turning on/off the vfd a few times a day will shorten its life?
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Old October 18th, 2018, 10:30 AM   #2
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Yes, it will shorten its life. But I might do it anyway in a residential situation. VFDs are noisy little fokkers, much more noticeable when not surrounded by a bunch of other noisy machinery. So not just to save a little bit of energy, but to keep from listening to motor whine and fan noise.
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Old October 18th, 2018, 10:48 AM   #3
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The motor noise will not be there if vfd is in stop mode, and I'm putting it in the garage.
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Old October 18th, 2018, 11:19 AM   #4
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I don't know about the GS1 but most drives can have a sensor wired directly and use a PID algorithm to deliver constant pressure (no PLC required). They usually have settings to let you configure what to do if the sensor signal is lost, minimum pump speed, etc. Then your drive will run as needed, and go to a stop condition when the pressure setpoint is reached at minimum motor speed.
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Old October 18th, 2018, 11:50 AM   #5
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Whenever a VFD is initially powered up, the capacitors on the DC bus charge themselves up instantly and would pull such high current that they will damage themselves or other components. So to prevent that, every VFD has what is called a “pre-charge circuit”, usually a series current limiting resistor and a relay or contactor (depending on size) that shunts power around it after a second or so. Those circuits are good for around 1,000 cycles, after which the resistor either burns out or the relay contacts weld closed, both of which result in the demise of the VFD shortly thereafter (if not immediately). So powering it up 3x per day will consume that life cycle in roughly 1 year before you kill your VFD. It’s not a 100% sure thing, some people get lucky.

“Do you feel lucky? Well do ya, punk?” Dirty Harry...

Last edited by jraef; October 18th, 2018 at 11:53 AM.
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Old October 18th, 2018, 01:29 PM   #6
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OkiePC.
The GS1 does not have PID built in. I hava Productivity P1K plc that will do the ocntrol.

Jraef.
Thank you. I will just leave on the VFD and if we go out of town for vacation or otherwise, i will shutoff the breaker. Thanks for the info. I knew a little about precharge circuits, but never thought about their lifespan.
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Old October 18th, 2018, 01:32 PM   #7
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Sorry duplicate post

Last edited by g.mccormick; October 18th, 2018 at 01:51 PM. Reason: duplicate
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Old October 20th, 2018, 10:09 AM   #8
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As this GS1 has no start circuits i dont see a problem, even a live circuit will have wear.

I would use a start circuit , so the pressureswitch will put a start relais ON, this will activate the PLC and PLC will take over this startrelais.
The PLC will start a big contactor (as there is a PLC no need for big one.
after few seconds start the VFD and run it.
after stop you can deactivate the takeover relais and every thing is power less.
accept for a 24 V power supply to the sensor.)
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