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.


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.


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.


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

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.


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.


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.

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About author
Steven Frick has spent much more time under the ocean than keeping a small piece of it. He got his start in the electrical field in the US Navy Nuclear Power program as an electricians mate. After 5 years of service on the submarine USS Henry M Jackson he finished his final 3+ years of service teaching electrical theory at the Knolls Atomic Power Laboratory. Currently, he runs the projects and maintenance for the power distribution system of one of largest electrical consuming heavy industrial companies in the Southeastern United States. He wrote his sites electrical safety policies and routinely acts as a consultant to other industrial facilities looking to improve their electrical safety programs. As someone who loves to both learn and teach, he has focused his attention on his newest hobby, reefing.

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