head height vs watts? AC vs DC pumps

[DBR_Reef]

I think I'm correct in thinking that a AC pump will decrease it's wattage as head height increases, similar to these curves: http://www.dolphinpumps.com/diamond-amp-master-pumps

And I think a DC pump will increase it's wattage as head height increases, based on many of the controllers displaying watts, and needing more of them to get higher head heights.

I just don't understand why this is the case, and if it holds true for all pumps.
Thanks in advance for any input.

 

[gcarroll]

DC pumps wattage increases as you turn up the controller. AC pumps wattage decreases ad head pressure is added.

 

[Greybeard]

More head height on an AC pump will slow the motor (greater load), causing it to draw less amperage.

current (I) = voltage (E) / resistance (R)

R goes up, I goes down.

The exact same thing applies to a DC motor. You increase head height WITHOUT changing motor speed, and you'll use less current.

Watts is something else again, that's a measurement of power.

Watts = Volts X Amps

As you turn up the speed on your DC pump, you're increasing volts, thus increasing watts.

Sorry, I've been told I tend to go overboard in answering questions

 

[DBR_Reef]

More head height on an AC pump will slow the motor (greater load), causing it to draw less amperage.

current (I) = voltage (E) / resistance (R)

R goes up, I goes down.

The exact same thing applies to a DC motor. You increase head height WITHOUT changing motor speed, and you'll use less current.

Watts is something else again, that's a measurement of power.

Watts = Volts X Amps

As you turn up the speed on your DC pump, you're increasing volts, thus increasing watts.

Sorry, I've been told I tend to go overboard in answering questions

Thank you, I do understand the basics of electricity, I was just using watts as a proxy for amps as that is what most manufactures report, which was my mistake.

So you are saying that AC pumps just vary the current based on the resistance on the motor caused by load, and obviously hold volts constant. And DC pumps vary the volts?

And since both pumps are really AC pumps, if you hold the voltage the same on both pumps, increasing the head height will decrease current.

I just want to check to make sure my understand is correct- the DC pumps usually list a voltage that they run at, so I did not realize that was what was varied.

But thank you for your very clear explanation- I probably could have gotten there, but I started to confuse myself

 

[Greybeard]

So you are saying that AC pumps just vary the current based on the resistance on the motor caused by load, and obviously hold volts constant. And DC pumps vary the volts?

Yup.

And since both pumps are really AC pumps, if you hold the voltage the same on both pumps, increasing the head height will decrease current.

They're not both AC... but yes, to the rest of that, if you hold voltage constant, AC or DC, and increase load (head height), you'll decrease current (slightly!)

One point often left out in the AC vs DC pump conversation is the inefficiency of the DC power supply bricks we use. You've got a DC pump that is, at the speed you want to run it, say, 15% more efficient that an AC pump of the same output... but you've got it plugged into a cheap DC wall wart that runs at 70% efficiency... you're not gaining anything.

 

[hart24601]

Yup.



They're not both AC... but yes, to the rest of that, if you hold voltage constant, AC or DC, and increase load (head height), you'll decrease current (slightly!)

One point often left out in the AC vs DC pump conversation is the inefficiency of the DC power supply bricks we use. You've got a DC pump that is, at the speed you want to run it, say, 15% more efficient that an AC pump of the same output... but you've got it plugged into a cheap DC wall wart that runs at 70% efficiency... you're not gaining anything.

I was under the impression our DC pumps are really AC pumps, the DC box just changes the amount of AC going to the pump?

https://www.reef2reef.com/ams/ac-and-dc-pumps-not-as-different-as-you-may-think.86/

 

[Greybeard]

I was under the impression our DC pumps are really AC pumps, the DC box just changes the amount of AC going to the pump?

Nope, they're really DC motors. The controller varies the voltage. On the cheap ones, at least, they lower voltage by increasing resistance... meaning they do not actually use less voltage when you run them at lower speeds, they're just shunting off some through a resistor (hopefully, a linear regulator).

AC motors don't work that way. You have to manipulate frequency in order to adjust speed of an AC motor. Rossmont has a line of wavemakers that can use a variable frequency drive controller... it's a $200 add on, but to the best of my knowledge, it's the first real variable speed AC pump system in the aquarium hobby. I've got two

Industry uses variable frequency AC drives (VFD) all the time, mainly because brushless AC motors simply last longer.

 

[hart24601]

Huh, I guess was looking at this part of the article I linked:

"And finally, we have made it to the DC pump section. These utilize special Brushless DC motors. These motors are also built with permanent magnets on the rotor. For these to work, we apply AC current to the Stator. As the current alternates, the magnetic fields between the rotor magnets and stator current lock onto each other and the rotor spins.

No, the fact that we apply AC wasn't a typo. To make these work motors work we have to alternate the positive and negative DC voltages between the motor windings. If you applied the correct voltage AC directly to the motor, it would work just fine.

Brushless DC motors and almost identical to AC reluctance motors. The speed they rotate is constant, depending on the supplied frequency. It can be calculated with the following equation. Speed = 120 x (frequency)/number of motor poles. Yes, it is because of this that AC return pumps have higher flow ratings in the US (60hz) than they do in Europe (50hz).

So why do we call it a DC motor if it actually uses AC? The magic is in the controller. All brushless DC motors need to have some form of controller. This controller converts the AC to DC. Using onboard electronics, this DC can be converted to any frequency AC that is desired. This is what allows DC pumps to be variable speed. Here is a sample of a switched DC wave and a stepped DC wave so you can compare them to a true AC sine wave."

 

[Greybeard]

Huh. Obviously, I didn't read the whole article.

I know that the cheap Chinese DC pumps that I've bought were, in fact, DC motors... I put a meter on the output of one I had and tested it. Obviously, Dolphin isn't doing that. That's a good thing, really, but I seriously doubt that it would be that way for the vast majority of DC pumps on the market.

I'm running a Current USA DC return pump. I'd almost guarantee that it's a typical DC motor... I may stick my meter on it later and check.

 

[gcarroll]

I was under the impression our DC pumps are really AC pumps, the DC box just changes the amount of AC going to the pump?

https://www.reef2reef.com/ams/ac-and-dc-pumps-not-as-different-as-you-may-think.86/

This is true from all the pump manufacturers that I have talked to. Hopefully @ksed can chime in on this.

 

[ksed]

Huh. Obviously, I didn't read the whole article.

I know that the cheap Chinese DC pumps that I've bought were, in fact, DC motors... I put a meter on the output of one I had and tested it. Obviously, Dolphin isn't doing that. That's a good thing, really, but I seriously doubt that it would be that way for the vast majority of DC pumps on the market.

I'm running a Current USA DC return pump. I'd almost guarantee that it's a typical DC motor... I may stick my meter on it later and check.

No they are not DC motors. The only true DC motors are the ones driven with brushes.
What you have is brushless DC motors. Which are really modified AC motors. They are wounded with different windings.
Here is a link explaining the difference.
http://www.engineering.com/Designer...icleID/8131/What-is-a-DC-Brushless-Motor.aspx
Perhaps @Brew12 can correct me if I am wrong.

 

[Brew12]

Perhaps @Brew12 can correct me if I am wrong.

Thanks for the invite! I love these conversations!

More head height on an AC pump will slow the motor (greater load), causing it to draw less amperage.

current (I) = voltage (E) / resistance (R)

R goes up, I goes down.

This isn't quite what happens. In Ohms Law (I=E/R) the resistance refers to the electrical resistance of the motor, not the opposition to flow. The only way to change resistance would be to mechanically alter the motor. Ohms law also only applies to DC. In AC we also have impedance that goes along with resistance. In the case of an AC return pump as the rotor slows down due to increased back pressure the relative speed between the rotor and stator increases. As the relative speed increases the counter generator action in the stator increases and this reduces current, which reduces power.

The exact same thing applies to a DC motor. You increase head height WITHOUT changing motor speed, and you'll use less current.

This happens with a true DC motor, but it doesn't happen with a what hobbyists call DC motors. Since the DC motor has a controller, the controller dictates the power to the motor. The motor will achieve the speed and subsequent flow rate that match the provided power.

As you turn up the speed on your DC pump, you're increasing volts, thus increasing watts.

There are several ways to increase speed to a DC pump other than just increase voltage. You can either increase the frequency of the AC waveform you send to it (normally a square wave or filtered square wave). You can use PWM control. Or you can control which point in the AC sine wave the thyristors fire to control the power output to the motor.

Nope, they're really DC motors. The controller varies the voltage. On the cheap ones, at least, they lower voltage by increasing resistance... meaning they do not actually use less voltage when you run them at lower speeds, they're just shunting off some through a resistor (hopefully, a linear regulator).

They really aren't DC motors. A DC motor needs to have some form of commutation which would require brushes between the rotor and the stator.
If you took a brushless DC motor, and applied DC to it, it would not turn more than 1/2 of a rotation.

I know that the cheap Chinese DC pumps that I've bought were, in fact, DC motors... I put a meter on the output of one I had and tested it.

I don't think you were getting the result that you think you were. I'm not sure what kind of meter you were using but if it wasn't listed as "True RMS" you won't get an accurate reading. Even then, the best way to see what is really going on is with an oscilloscope. This way you can see exactly how the waveform is being manipulated to create relative motion.

One point often left out in the AC vs DC pump conversation is the inefficiency of the DC power supply bricks we use. You've got a DC pump that is, at the speed you want to run it, say, 15% more efficient that an AC pump of the same output... but you've got it plugged into a cheap DC wall wart that runs at 70% efficiency... you're not gaining anything.

Now this comment is something I completely agree with and, from a practical point of view is more important than any of the theory discussed above. Manufacturers love to talk pump efficiency with DC pumps. They don't like to mention total system efficiency. The heat generated in a DC controller is all electrical loses. It is common to have 10% to 30% total loses in a small motor controller like these. Yes, the pumps are more efficient (largely due to impeller design) but when you add the controller? Not so much. And the further you run them from 100% speed the less efficient they get as a system.



Hope that helps, and let me know if I can explain something better.

 

[ksed]

@Brew12 exactly what I was going to say but could not find the right words.

 

[gcarroll]

Thanks for chiming in @ksed and @Brew12

 

[Brew12]

Thanks for chiming in @ksed and @Brew12

My pleasure.. but I need to bash the fool that said this..

In the case of an AC return pump as the rotor slows down due to increased back pressure the relative speed between the rotor and stator increases. As the relative speed increases the counter generator action in the stator increases and this reduces current, which reduces power.

While that is true for some pump designs it is not true for all. Some AC pumps always turn the same speed regardless of flow or back pressure. In this case the impeller is unable to push the water so it slips around the blades. It can either be due to blade design, blade flexibility, or volute design. This takes less real power than pushing water up a pipe therefore less power is used.

So many types of AC motors and they all work a little differently.

 

[DBR_Reef]

Thanks for all the input guys! I knew I could count on you to answer the question and then some!

 

[Stigigemla]

All pumps that are having a magnet connect to the impeller are AC pumps. In DC pumps the AC is made from DC by electronics connected to the pump.
The electronics can be in the pump or in the controlling unit.
Most often the AC generator includes feedback from the current in the windings so the AC generated is optimized to the rotation in the pump. In such a pump the rotation strenght is influenced by the control voltage and the (mecanichal) resistance to the impeller rotation. And then the rotation speed will change if the load is changed.
But it is possible to control the rotation speed direct via the control voltage. But in that case it is a little more complicated electronics. (Money)