AC and DC pumps.... not as different as you may think.

[ksed]

I notice many are complaining that the DC calcify much quicker. I always thought it was because they were not getting enough water to the back bearings for cooling?

I think you will find that a Danner Mag drive will put out more pressure than a DC pump as they have more torque compared to the average DC pump. To get the volume on most submergible DC pumps they have larger input and output fittings and they are more prone to calcium buildup. I believe when you get a comparable DC pump the only real advantage is speed control and they are quite.

 

[Bruce Burnett]

I notice many are complaining that the DC calcify much quicker. I always thought it was because they were not getting enough water to the back bearings for cooling?

Since they don't have as much torque they can not over come the build up and carbonates do build up faster on parts that run warmer. External pumps with carbon seal (direct drive not magnetic) will run until the carbon seal starts leaking and making a mess. If you keep high alk or dose kalkwasser you are more prone to build up in pumps and on heater. None of us like to pull pumps and clean them but it is part of the normal work.

 

[Brew12]

Great article Brew!
I found this a while back which I thought described AC/DC quite well . I do like how he describes it as a marketing term.
http://www.engineering.com/Designer...icleID/8131/What-is-a-DC-Brushless-Motor.aspx

That really is a very well written article. Thanks for posting it!

 

[Brew12]

Also why are some motor blocks like Eheim so large yet you get these DC pumps which are half the size and put out more flow at much lower wattage.
For example the DC pump in the picture is smaller and puts out twice the flow at similar wattage.
I believe you can have sine-wave waveform on DC pumps as well?

Thanks for you help

The controllers for DC pumps can't produce a true sine wave, but they can do wave shaping to produce a more sine wave like shape. I haven't tested it, but I'm sure that the shape of the waveform is the major difference between the ecosmart and quiet drives on Vortechs.

DC pumps tend to be more efficient for several reasons, but the motor itself is a small part. It has more to do with impeller design. The ability to do a soft start, or start the pump slowly, allows for more efficient impellers. The impeller design also benefits from the motor being able to spin faster than an AC pump. There is nothing preventing the DC controller from outputting 120hz, giving twice the speed for the same motor design. This is the major factor that allows a DC pump to be both smaller and more efficient.
The fact that it operates at a lower voltage and higher current also impacts design, but it is less of a factor than the impeller and speed capabilities.

Hope I explained that okay.

 

[Brew12]

Great Article!
Are all of the increases in efficiency over typical AC pumps like a Mag drive due to the impeller options available with a directional motor or are there other gains in efficiency due to other factors?
Thanks,
Paul

I think you will find that a Danner Mag drive will put out more pressure than a DC pump as they have more torque compared to the average DC pump. To get the volume on most submergible DC pumps they have larger input and output fittings and they are more prone to calcium buildup. I believe when you get a comparable DC pump the only real advantage is speed control and they are quite.

The big advantage is definitely the speed control. Since the motor speed is not limited to 60hz, the impeller design can be optimized for much faster speeds that are achieved with a DC pump. As for which one puts out more pressure, that tends to be a manufacturer decision and not necessarily impacted by the pump being AC or DC.

I notice many are complaining that the DC calcify much quicker. I always thought it was because they were not getting enough water to the back bearings for cooling?

Unfortunately, I just don't have the experience to comment on this. It is interesting though!

 

[ksed]

The controllers for DC pumps can't produce a true sine wave, but they can do wave shaping to produce a more sine wave like shape. I haven't tested it, but I'm sure that the shape of the waveform is the major difference between the ecosmart and quiet drives on Vortechs.

DC pumps tend to be more efficient for several reasons, but the motor itself is a small part. It has more to do with impeller design. The ability to do a soft start, or start the pump slowly, allows for more efficient impellers. The impeller design also benefits from the motor being able to spin faster than an AC pump. There is nothing preventing the DC controller from outputting 120hz, giving twice the speed for the same motor design. This is the major factor that allows a DC pump to be both smaller and more efficient.
The fact that it operates at a lower voltage and higher current also impacts design, but it is less of a factor than the impeller and speed capabilities.

Hope I explained that okay.

Thank you for your informative reply
One thing I would like to mention is when converting from a high voltage to a low voltage there are losses in efficiency.
There are some pumps on the market that are controllable (I don't like to use the word DC in this application)but run at a higher voltage eg Abyzz .
They make both low and high voltage, low voltage being at around upper80% in motor efficiency and the high voltage pumps being in the lower to mid 90's. From what I understand the loss in efficiency has to do with the lower voltage .

 

[Brew12]

Thank you for your informative reply
One thing I would like to mention is when converting from a high voltage to a low voltage there are losses in efficiency.
There are some pumps on the market that are controllable (I don't like to use the word DC in this application)but run at a higher voltage eg Abyzz .
They make both low and high voltage, low voltage being at around upper80% in motor efficiency and the high voltage pumps being in the lower to mid 90's. From what I understand the loss in efficiency has to do with the lower voltage .

You are correct that higher voltages tend to be more efficient than lower voltages. In it's most simplified form, power = current times voltage. For the same power, if voltage is higher than current can be lower. Another equation, Power (in this case, losses)=current squared x resistance, shows that raising current has a very high impact on losses. This is why electrical distribution systems transmit power at very high voltages.
The fact that our hobby motors don't have long leads does mitigate this impact. I think the reality of it is that all electronics, and their applications, are not equal. A properly designed controller, using higher quality parts but operating at a lower voltage would be more efficient than a poorly designed low quality controller at a higher voltage. The fact that the higher output voltage almost dictates the use of a better design and higher quality components probably impacts their efficiency more than any other factor.

 

[Phil Cusimano]

@Brew12 Outstanding ... well written and articulated! Lot's of great information.

 

[Paul B]

Very Nice

 

[mcarroll]

I notice many are complaining that the DC calcify much quicker. I always thought it was because they were not getting enough water to the back bearings for cooling?

Sounds from the writeup like there's an implication of higher rotational speeds in "DC pumps", which I'd guess would further imply higher bearing temps/cooling requirements, no?

Dunno if I'm right, but if so then I think it stands to reason they'd calcify more....

 

[ksed]

Correct
The other option is to run them at a slightly reduced speed.

 

[billwill]

Hmm... not sure why the image of the waveforms showed up twice. I need to work on my forum skills! And I can't edit it to remove it!

Excellent write up!

 

[ksed]

@Brew12
Correct me if I am wrong. Is it the commutation from the controller (eg modified square wave) that makes it quite compared to an AC?
Also from my understanding, these DC pumps has anywhere from 4- 8 pole motor and the AC have 2 pole. What affect does having more poles have on the motor?

Thanks

 

[Brew12]

@Brew12
Correct me if I am wrong. Is it the commutation from the controller (eg modified square wave) that makes it quite compared to an AC?
Also from my understanding, these DC pumps has anywhere from 4- 8 pole motor and the AC have 2 pole. What affect does having more poles have on the motor?

Thanks

Motor speed = 120 x frequency/# of poles. A 2 pole motor at 60hz will spin at 3600rpm, a 4 pole motor at 1800 rpm and a 6 pole motor at 1200 rpm.

There are plenty of factors that can make it more quiet, but I think the biggest one is the lower voltage. Unfortunately, not too many aquarium pump manufacturers are willing to release their design data for me to make more than an educated guess.

 

[ksed]

So the #of poles doesn't really doesn't matter because you can change the frequency therefore you will also change the RPMs?
When talking about DC pumps

 

[ksed]

Basically this is what a VFD does for an AC motor.
Will this make an AC motor as efficient as DC?

 

[mcarroll]

I think a properly spec'd AC pump is already at least as efficient (due to the current transformation?) isn't it? All else being equal. Might even be likely to impart less heat from the sound of it.

 

[Brew12]

So the #of poles doesn't really doesn't matter because you can change the frequency therefore you will also change the RPMs?
When talking about DC pumps

Think of the number of poles as being a course adjust that can't be changed. Changing frequency becomes your fine adjust.

Basically this is what a VFD does for an AC motor.
Will this make an AC motor as efficient as DC?

That really is exactly what these DC pumps are, Variable Frequency Drives. Not many places use VFD's on reluctance motors but the technology is the same.

I think a properly spec'd AC pump is already at least as efficient (due to the current transformation?) isn't it? All else being equal. Might even be likely to impart less heat from the sound of it.

Yes, all other things being equal, the AC would be more efficient because it doesn't have the losses in the DC controller. However, because of the ability to utilize different designs with the pump impeller, these DC pumps can gain an overall edge in efficiency.

 

[mcarroll]

Counting heat transfer and everything, or just counting W/gph?

 

[Alfrareef]

Finally a subject I fully understand!!! I'm a electronics engineer specialized in IC VLSI and all this it's easier than SW chemistry and coral biology...
The article it's ok Brew.