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

Brew12

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

Reef aquarium caretakers tend to obsess over flow. We use pumps to pull water through our sumps. We put in powerheads and wavemakers all in an effort to push water around. We use pumps with aerators to run skimmers. Even though we have many different applications almost all our aquarium motors work the same way. If you don't know how your motor works, I want to help with that.

upload_2016-10-26_12-23-28.png

First, let's make sure we are speaking the same language. All motors have two main parts. The Stator, or stationary part and the Rotor, or rotating part.
We also need to understand the two basic types of electricity. They are DC and AC as shown below. As you can see, AC alternates above and below the zero volt line (time) while DC stays above it.

upload_2016-10-26_12-45-18.png

There are many different types of motors and I will only go over a few of them but they all have one thing in common. They use magnetic forces to function. It is important to remember that electrical current creates magnetic fields.

upload_2016-10-26_12-19-34.png

If you need to move a large volume of water quickly, nothing will work better than an AC induction motor. These are the most common form of motor used around the world.

Alternating current is supplied to the stator. This causes current to be induced (hence the name) in the rotor. The magnetic fields between the stator and rotor interact and the pump spins. Odds are that you will only see induction motors used on the largest of aquarium systems.

Advantages: Cheap method to build very large motors
Disadvantages: Constant speed, stator can be damaged if the rotor stops

upload_2016-10-26_12-39-15.png
Moving on to something much more commonly found in the home aquarium, it's time to take a look at AC powerheads and return pumps. These are almost always AC Reluctance Motors. As with the induction motors, we supply AC power to the stator. Instead of inducing current in the rotor, these rotors are constructed with permanent magnets. As the current alternates, the magnetic fields between the rotor magnets and the stator current lock onto each other and the rotor spins.

Advantages: Very reliable, will not harm motor if rotor is stopped
Disadvantages: Low starting torque, constant speed, bi-directional

Before moving on it is worth discussing this a bit more starting with the low starting torque. When starting, there is very little force applied to the rotor. If you stick your finger on the impeller it is easy to keep it from starting to rotate. For this reason it is important to keep these motors and their impellers clean.

You will also notice that they are bi-directional. This is not true of all AC reluctance motors but it is true of the ones for home use. The direction your pump will turn is dictated by where the rotor has stopped in relation to the AC current when first turned on. Needless to say, we don't want our return pumps or powerheads running backwards. To fix this problem, manufacturers install an anti-reverse direction device of one design or another. If your pump starts turning the wrong way, it should hit this device and bounce off of it establishing rotation in the correct direction.

upload_2016-10-26_13-0-44.png

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.

upload_2016-10-26_13-23-10.png

Disadvantages: Requires separate controller
Advantages: This needs a list.
Variable speed by creating different output frequencies.
Directional starting, soft starting, and improved starting torque since the motor can be started at lower frequencies.
Rotor is not damaged if it is stopped with the motor energized.
Motor operates at a lower voltage than an AC motor making it safer when operated in salt water.
Can be quieter than AC motors due to both the lower voltage and by modifying the shape of the controller output.

I hope some of you found this information useful, especially if you were wondering what makes DC pumps special. The real answer is that their is very little difference in the motors, it is almost all in the controllers.

Discuss this Article Here
 
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Brew12

Brew12

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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!
 
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Brew12

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Not to mention an inductive motor can also run off of a modified square sine wave to control its speed. Really this is a good write up..... thanks!!
Yes it can... but the danger there is if some junk floating in the tank locked up the rotor at the very least you would trip your supply breaker. If used on small motors like this it would probably just burn up.

Glad you liked it!
 
Top Shelf Aquatics

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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!
Fixed it. :p
 

kschweer

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Excellent article! Some very interesting info.
 

melypr1985

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@Brew12 This is a great article!
 
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Brew12

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Slow week at the office? :p
It actually has been. I have most of the RFQ's I needed to get done out to the vendors for most of the work I am planning through the end of the year. Today, unfortunately, I won't be on much. Taking down a 160MVAR filter bank for maintenance in around an hour.
 

ksed

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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
 

Bruce Burnett

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Thanks I had never checked the output on the DC wave motors makes more sense now that you made me think. Off the subject if one want a very powerful pump for a very large system, AC induction high cycle motor with frequency speed controller. You can take 110 volt and put it through an inverter to power a 10 horsepower high cycle motor that is not much bigger than some of the external return pumps. They usually have 2 problem muchn higher RPM and produce a lot of heat they are commonly geared down to normal range of RPM.
 

pfoxgrover

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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
 

Bruce Burnett

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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.
 

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