Thats the First Question I askVarios pumps from Reef Octopus. We sell a bunch of those pumps and very rarely ever see them come back to us with issues.
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Thats the First Question I askVarios pumps from Reef Octopus. We sell a bunch of those pumps and very rarely ever see them come back to us with issues.
In a 'fixed speed' pump driven off AC wall current it's a lot easier to predict the flow curve based on pump rpm and impeller diameter. DC pumps on the other hand might be a bit tougher to predict as an apples to apples comparison without hands on testing. We'd need BRS to build at test apparatus itself rather than just doing wattage power, rpm, and impeller measurements to compare pumps.... will be pretty expensive so I wouldn't hold my breath that the accounting department okay's it at BRS. But hey, I'd love it if they did.I'd like to see a brs episode on pump curves and their truth or fiction. Some vendors present them (sicce), and some don't (reef octopus), so is there truth in advertising or is it a gimmick? Specifically do DC pumps have the pressure rating that ac pumps are known for.
Power usage can be a very confusing issue with DC pumps. There can be a lot of efficiency gained in a DC pump through the design of the impeller and volute. Especially in higher pressure applications. This is one reason that it isn't uncommon to see DC pumps with a max head of 20ft where AC pumps of a similar size only sit around 12ft. The ability to "soft start" a DC pump allows for more efficient mechanical designs where an AC pump with the same design would sit and buzz as it does when a rotor jams.I will say it seems centrifugal pump design is a very mature type of engineering, so a lot of times I think people overstate the power consumption advantage of DC models. In reality it's really just adding controllability but the efficiency of the motor and impeller itself are likely not significantly different than an AC model of similar power. And if we look at AC pumps like eheim hobby or the Fluval SP series with low power factors by design -we see that their VA, or apparent power, is quite high compared to the watts radiated at the tank. So even though an SP4 might only radiate 65 watts in a typical setup, it might seem to smoke an 80 watt DC unit since it's VA is closer to 110. This might explain some of the low head pressure issues reported with a lot of the mid/low priced DC pump ranges, but it very well could be optimistic marketing numbers creating the effect as well. Would be cool to see some testing for sure!
This is pretty much it. The power conversion components have become both more reliable and much cheaper. An AC to DC to AC controller used to cost several hundred dollars even for a fractional horsepower motor. Now, you can buy printed circuit boards with the converters on them for under $5.Is it the DC electronics that have gotten better?
Cool info! Out of curiosity can you provide some reasons as to where the 'lot of efficiency' can be gained in a DC pump through impeller and volute design. I'm not an electric motor guy but I do have a bit more knowledge of impeller and volute mechanics than the driving impetus itself. Generally I would work from the pump's intended application in terms of desired flow/head pressure curve and especially the intended operating point of best efficiency. Knowing the motor rpm I can then determine impeller diameter needed as well as calculate the specific speed for that pump and arrive at a suitable impeller/volute design. This should give us a fairly stable range of centrifugal pump design configurations for the types of head heights and flow rates we work with in aquarium systems across various manufacturers. Do the DC pumps have a much higher rpm than the ~3400 we see in the typical 2 pole reluctance motor AC units? I could see this giving DC an advantage purely from the higher rotor rpm allowing for a more efficient impeller and volute. Is that what it is? Or am I totally missing something here? Thanks in advance.Power usage can be a very confusing issue with DC pumps. There can be a lot of efficiency gained in a DC pump through the design of the impeller and volute. Especially in higher pressure applications. This is one reason that it isn't uncommon to see DC pumps with a max head of 20ft where AC pumps of a similar size only sit around 12ft. The ability to "soft start" a DC pump allows for more efficient mechanical designs where an AC pump with the same design would sit and buzz as it does when a rotor jams.
Manufacturers can also be misleading. A DC controller can have 7% to over 20% in losses to heat that an AC pump doesn't have. Some manufacturers have listed power at the motor, not power at the wall. This way they can make the system look more efficient than it actually is. For those of you who have controllers that radiate heat like a toaster oven, you know what I'm talking about.
Another challenge is the impact of actual flow on power usage. DC pumps tend to be fairly linear with power use and flow. An AC motor will use less power when used in higher head loss situations or when the flow is throttled, but it isn't linear. For well designed versions of each, a DC pump will increase its efficiency advantage as flow restrictions (head or throttle valves) increase.
This is only applicable to return pump applications. It would be a challenge for a DC power head to be more efficient than an AC power head but that is a different discussion.
DC pumps can spin at a faster rate but the biggest advantage is in starting torque. DC pumps can start at a slow speed and ramp up over a second or two. This gives the rotor a chance to lock into the rotating magnetic field of the stator as speed increases. On an AC pump the magnetic field starts rotating at full speed immediately. If there is too much water resistance the rotor will vibrate back and forth instead of spinning. This isn't an issue with induction motors that we normally use to power pumps (industrial anyway) since the larger the difference in speed between the rotor and stator the more torque is developed. But, with reluctance motors, it does become a factor.Do the DC pumps have a much higher rpm than the ~3400 we see in the typical 2 pole reluctance motor AC units? I could see this giving DC an advantage purely from the higher rotor rpm allowing for a more efficient impeller and volute. Is that what it is? Or am I totally missing something here?
Ok thanks a bunch! The rpm would definitely make sense. I have to say though, in terms of volute or impeller design I don't see how starting torque gives DC pumps the ability to gain a lot of efficiency compared to AC models as you state. Perhaps at the extreme ends of the flow curve, but it's still my understanding that there is a best efficiency range for a given volute and impeller design regardless of what technology is turning the rotor. I do agree that at very low desired flow rates the DC pump has a significant efficiency advantage. In that case, if someone had to run their AC pump throttled to only 10 or 20 percent of it's rated output I'd say they probably have the wrong tool for the job. There is more leeway in the selection of a DC pump due to it's throttling capacity and that's an advantage for sure.DC pumps can spin at a faster rate but the biggest advantage is in starting torque. DC pumps can start at a slow speed and ramp up over a second or two. This gives the rotor a chance to lock into the rotating magnetic field of the stator as speed increases. On an AC pump the magnetic field starts rotating at full speed immediately. If there is too much water resistance the rotor will vibrate back and forth instead of spinning. This isn't an issue with induction motors that we normally use to power pumps (industrial anyway) since the larger the difference in speed between the rotor and stator the more torque is developed. But, with reluctance motors, it does become a factor.
I was going off the comparative manufacturer model numbers meaning 9.0ADV/9.0 SDC. Thank you for noting the 10.0 ADV pump I have not seen it yet on vendor lists. Seeing the 10.0 I'm strongly leaning towards it being 'unlocked' in a software way because it otherwise stacks up the same as the SDC 9.0 in terms of head height. And if indeed our hypothesis that the ADV line is just a SDC block with an onboard set of control circuitry, then it stands to reason they'd be able to neuter one version of the same pump through software. Normally, mechanical flow reduction is achieved by putting an inlet restrictor in the intake aperture of a pump, reducing impeller vane count, or molding a restrictor ring into the throat of the outlet. The thing is it's normally flow that is lost with mechanical throttling rather than a lot of head pressure being take away, assuming the impeller disk is the same diameter between the pumps. We'd have to open up a 10.0 and a 9.0 ADV and compare to know for sure.There might be a mix up with your specs or Sicce specs. Have a look at the 9.0-10.0.
http://www.sicceus.com/sdc_adv.html
The design differences become more clear when you look at the impellers.I have to say though, in terms of volute or impeller design I don't see how starting torque gives DC pumps the ability to gain a lot of efficiency compared to AC models as you state.
AC pumps have curved impellers as well. They are not restricted to DC only.The design differences become more clear when you look at the impellers.
This is a typical design for an AC pump. The straight blades allow more water to bypass at slower speeds reducing water resistance when starting. The compare it to the curved blades of a cheap DC motor like the Jacod. Without a soft start feature the impeller for the DC pump wouldn't reliably start.
True, but its normally only the higher end pumps on the AC side that use them. There are other design considerations that can overcome the starting issues, they just cost money.AC pumps have curved impellers as well. They are not restricted to DC only.
As far as I can tell that design is actually dictated by bi-directional starts in a non-directionally controlled reluctance motor block rather than due to the lack of starting torque in any of the AC blocks. The inefficiency of an open radial flow impeller is quite poor compared to the open face, directional vane jebao example you provided. But that's not what's in every AC pump - this open face directional vane impeller starts reliably every time for me and it's an AC block... the more advanced AC units and the external asynchronous motors have directional start built into them these days.The design differences become more clear when you look at the impellers.
This is a typical design for an AC pump. The straight blades allow more water to bypass at slower speeds reducing water resistance when starting. The compare it to the curved blades of a cheap DC motor like the Jacod. Without a soft start feature the impeller for the DC pump wouldn't reliably start.
AC pumps with directional impellers (along with almost all AC power heads) will use a bump start to control direction of rotation so directional control has been around a long time. It doesn't bring value unless you have other modifications to take advantage of the direction of rotation. My understanding is that many manufacturers chose to forgo the bump start and let it be bidirecti0nal on traditional AC return pumps to improve reliability since they couldn't take advantage of an improved impeller design anyway.As far as I can tell that design is actually dictated by bi-directional starts in a non-directionally controlled reluctance motor block rather than due to the lack of starting torque in any of the AC blocks. The inefficiency of an open radial flow impeller is quite poor compared to the open face, directional vane jebao example you provided. But that's not what's in every AC pump - this open face directional vane impeller starts reliably every time for me and it's an AC block... the more advanced AC units and the external asynchronous motors have directional start built into them these days.
Ok yep, I definitely hear you on the traditional style of bidirectional 'dumb' reluctance blocks and concede their inherent lack of efficiency. But I do have to disagree on the point about not being able to take advantage of a directional impeller in a bump start or electronic start design. (Btw the pump whose impeller I posted a pic of is definitely a 2 pole reluctance motor and does not have a bump start- it's electronic). If we go with your assertion that there's no advantage to using a directional impeller on a fixed speed AC pump - assuming you can bump it or electronically start it in 1 direction, why does nearly every pump that turns in 1 direction use such an impeller? And I'll take it one step further and argue that a DC pump impeller and volute, while optimized, are less efficient in their peak efficiency point than a comparable fixed speed impeller and volute precisely because they have to be somewhat of a compromise across the whole range of the pump's operating rpm.AC pumps with directional impellers (along with almost all AC power heads) will use a bump start to control direction of rotation so directional control has been around a long time. It doesn't bring value unless you have other modifications to take advantage of the direction of rotation. My understanding is that many manufacturers chose to forgo the bump start and let it be bidirecti0nal on traditional AC return pumps to improve reliability since they couldn't take advantage of an improved impeller design anyway.
I suspect that some of the newer electronically controlled directional start AC pump have a control circuit built into them and are actually a constant speed DC pump. Not that any of them are actually DC since a DC pump would need conductive brushes connecting the power supply to the rotor. Take the Syncra ADV and SDC. My guess is that they are the exact same motor block and the only difference is the cord that connects them. If the cord supports a speed reference, it's an SDC. If it only supports constant speed, it's an ADV.