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Mechanical Design

The Engineer's Guide to AC vs DC Motors

While electric motors serve vastly different purposes, their core function remains the same—to convert electrical energy into mechanical energy. Numerous articles highlight the characteristics of each unique motor on the market, but they can be simplified into two overarching categories: AC (alternating current) motors and DC (direct current) motors.

Each type has its own pros and cons, but this article is intended to give you a clear picture of how either an AC or DC motor is best used. Let’s explore the ways in which they each convert electrical energy, the differences in their construction, and the best use cases for each type.

Types of Motors

Power

The glaring difference between the two types of motors is the power off which they run.

With AC electricity, or alternating current, the voltage reverses every half cycle, which in turn changes the direction of the current. This is done by alternating the polarity at each end of a wire. So, if you take the US standard of 120V/60Hz supply, there are approximately 120 half cycles per second.

With DC electricity, or direct current, the flow of current must remain in a single direction (positive to negative), so voltage must remain constant, in order to keep a steady flow of current. You can think of DC electricity as a battery with clearly marked negative and positive terminals.

AC Electricity for Motors

Inside a wire using AC electricity, electrons don’t travel at a consistent rate in a single direction as they would with DC—they simply wiggle back and forth and pass along energy (think of Newton’s Cradle).

Alternating current is used for power distribution systems (power to your home/office), for the simple reason that AC is much more efficient when it’s transmitted through wires over a long distance, or for appliances requiring heavy voltage. Why is it better for these applications? Because the back-and-forth “wiggling” of electrons creates an electrical field, so a transformer can be used to kick up the voltage and still keep current relatively low.

By keeping current low, we reduce resistance through the wire, which equates to a higher efficiency. This correlation between current and voltage can be found in the Power Law: Power = Current * Voltage.

So what does all of that have to do with an AC motor? Well, basically, AC motors are great for high power appliances/machines that require little precision, like a blender or your washing machine. They’re objects you need to start, and they may ramp up or down in speed, but the difference between 400 RPM and 420 RPM probably isn’t critical.

DC Electricity for Motors

DC motors, on the other hand, are all about precision and stability, because the DC power supplying these motors comes in at a constant voltage. The electrons inside the wire can only travel in a single direction, and they do so at a normally steady pace.

Again, this is similar to the way a triple A battery will constantly give your electronics a supply of 1.5V (excluding losses) until they die. This is better when you have delicate circuitry/circuit boards or electronics that need a steady supply of energy to function properly, like a laptop.

But wait—if my home is supplying AC power, but my product requires DC power, what do I do?! Most electronics will have an AC to DC converter. That’s what that block is on the power cord for your laptop.

In fact, you can thank the converter on a sewing machine for the inspiration behind the band AC/DC’s name. With a DC motor, you can use a speed controller to adjust the voltage up or down, which will speed up or slow down the motor, but also maintain speed by monitoring the RPM of the motor and fine-tuning the voltage to ensure the motor remains steady.

Construction

While the externals of both AC and DC motors may not be noticeably different, there are some fundamental differences internally that make them each unique. While you’re read the following information, it’s important to keep in mind that the difference is driven by their input/output: AC motors take in an alternating voltage for the sake of efficiency and power, while DC motors maintain a constant voltage for stability.

AC Motor Build

AC motors are very simple because the alternating current does all the work. By sending the current through a stationary wire that surrounds a shaft, you’re creating a varying magnetic field, which in turn will rotate the shaft.

Remember, AC electricity goes from “positive voltage” to 0 to “negative voltage” at double your frequency (Hz) every second. This means that with the U.S. standard of 60Hz, the current changes direction 120 times a second.

 

AC Motor Diagram.
AC Motor Diagram. Photo from ElectronicsTutorials.

AC motors’ simplicity makes them long-lasting and significantly reduces the chances of mechanical error. The mix between efficiency and long shelf life makes them popular for applications where you don’t want too much energy loss and don’t want to be constantly replacing a motor (think washing machines). Although I depend on my washing machine enough that I’d probably pay every year to get a new one if it broke down—don’t tell LG!

DC Motor Build

The job of a DC motor is to supply a stable and precise output, which makes its construction a little more complex. Returning to the battery example we used earlier, we want the battery to supply a consistent 1.5V to our circuit board, not the craziness of switching from +1.5 to -1.5V hundreds of times a second that AC power gives us. In that same way way, in order for a DC motor to convert the consistent voltage supplied to it, we need a motor construction that will precisely convert that voltage into mechanical energy.

To do so, we need to first implement some mechanical functionality to create a rotation that moves the motor. Again, this was easy with AC motors, because AC power naturally swings back and forth, which varies the magnetic field. With DC, the magnetic field will remain the same.

So to counter, we have several rotating coils at the center of a DC motor, which connect to a “commutator.” The commutator makes contact with stationary “brushes” of opposite polarities at the exact moment that it needs to change the direction of current to rotate the shaft.

 

coils and commutator at center of DC motor

 

It may be obvious, but the major downside here is loss of efficiency, due to the friction caused by the contact between the commutator and brushes. The loss in efficiency leaves in the form of heat and sometimes sparks if you overload the motor.

Function

If you're sick of reading all the technical details and really just care about what motor works best for your needs, the long answer is, it’s really going to depend on your product and its functionality. The short answer is, here’s a basic list of functional uses for each type of motor:

Advantages of AC Motors

  • Transfers energy more efficiently
  • Longer life and less chance of failure
  • Produces less heat
  • Better for high power applications (washers, refrigerators, machinery)
  • Can tie directly into a home or office outlet, without a converter

Advantages of DC Motors

  • Low electromagnetic interference (great for sensitive electronic devices)
  • Stability for circuit boards and sensitive electronics
  • You can run your product on battery power
  • Better speed control

Main Takeaways

In reality, there will likely be several factors in your design that will drive the type of motor you choose. If you’re using circuit boards, batteries, and require fine tuning on your speed controls, a DC motor is the way to go. If you’re going for high power, efficiency, and long lasting, an AC motor will do the trick.

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