Single phase variable speed controls for water pumps

[fastline12]

We are working on a geothermal system where we will need a rather efficient way of moving variable amounts of water. Due to the high duty cycle of this system, the thought came to mind to use a constant pressure, variable speed well pump system. It was my understanding that they were just using 3PH pump motors and a typical VFD configuration but in talking with some OEMs, it seems that they are able to speed control both a 2 wire and 3 wire, 1ph pump motor. In the 3 wire, they are sending current to the aux winding as well.

I realize the type of drive needed would probably need a basic PID loop to handle the pressure/speed commands but I am wondering if this is truly a different drive or if any 3PH VFD with PID functionality could be configured to do this? I am real soft on going to a specialty type drive and would rather use a more universal drive and set up the parameters specific for this application.

Can someone explain how they are effectively speed controlling a 1ph motor? Any link relative to the subject would be welcome as well.

 

[Skogsgurra]

One HP VFDs with single phase mains supply are cheap as dirt. Then use any standard three phase motor with it. VFDs with PID built-in are very common.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[fastline12]

Thanks. I rather agree with you that a 3ph motor just makes more sense because the control side is MUCH more standardized. The issue at hand really is that 1HP, 3PH 4" cased well pump motors are not very common and very pricey compared to their 1ph counter parts. Be cause the system will provide drinking water as well as climate controls, it is imperative that the reliability and serviceability be considered.


It is possible that if a 3PH motor is used with a VFD, we can easily protect the motor windings from overload thus making repair an option.

 

[jraef]

They do make VFDs for 1 phase motors, but the motors must be either PSC (Permanent Split Capacitor) or Shaded Pole. Shaded pole are typically unsuitable for pumps, but you may be able to use a PSC motor design on a submersible and then a 1ph VFD. The big mfr of those is Invertek from the UK. They are sold under many different brand names however, such as Anacon and Bardac in the US. The basic issue is, you will likely spend more money on a 1 phase motor and VFD than a 3 phase motor of the same HP and 3 phase VFD that will accept a 1 phase input. But if, as you seem to think, the 3 phase motor version is unavailable, then that would be your only choice. I find that hard to believe though.

Motor protection for a 3 phase motor or single phase motor controlled by a VFD is going to be no different, not sure what you were thinking there.

99% of all modern VFDs now provide basic PI or PID control capability.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[fastline12]

All I can tell you is the cap start/cap run 3wire 1ph motors being used right now do not need to be changed to run the 1ph pump drives that a couple companies offer right now.

I realize that the 3ph motors are available, the issue is just that most of the pump companies in our area do not stock them in the 1-1.5HP size range so they would have to be ordered should one go down. Even a couple days down could be a big deal. I guess just buying a back up could be an option.

I think I do like the idea of using 3ph better. Even if the pump is a bit harder to get, the drive, which probably would fail before the pump, would be MUCH easier to service and get going again.

Jraef, I know you have tons of experience with VFDs, what might you reach for on this application? Most likely go for a 2HP drive and use a single pressure transducer for control and another mechanical pressure switch for high pressure emergency shut down. Obviously I would need one that will allow 1ph inputs but of course derated.

 

[waross]

That's a good idea. If the reason for the high limit is to prevent over-pressures in the event that something fails, a horsepower rated high pressure switch in the supply lines will provide safety no matter what has failed in the control circuit.

Bill
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"Why not the best?"
Jimmy Carter

 

[LionelHutz]

If you want this to be efficient then I believe the 3-phase motor + VFD solution will be more efficient than the 1-phase motor + VFD. Overall, a VFD likely will reduce the efficiency of this application unless you are wasting the excess water being pumped.

You should also check the speed limits of the pump you use. Many submersible pumps won't like running at a low speed with low flow. Most use the flow to cool the motor and some use the pressure to support the rotating parts vertically.

 

[Compositepro]

Fastline, you should read previous threads on this site about energy savings with VFD's on pumps. You are not likely to save enough energy to payback the extra cost. While a throttled pump is less efficient than an unthrottled pump, it does use less power. You pay for power- not efficiency. The mistake people make is they think a one hp motor always uses one hp so the only variable is efficiency.

 

[ScottyUK]

That really depends on the type of pump and the process it is being used in, doesn't it? A low head application such as a circulator may well offer a net saving, where a high head application such as a boiler feedwater pump almost certainly won't. There isn't a definitive answer - it's totally dependent on the pump and the application.

The OP mentioned geothermal, which immediately suggests a high head application.


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