AC vs DC Motors

We’ve covered the basic concept of electric motors. Windings, magnets, and magnetic fields make a rotor spin, with some power applied. That begs the question, what kind of power? We have two basic forms of power, AC and DC, which are only related by physics. They both involve electrons and electro magnetism, but they don’t behave in an even remotely similar way. As a result, AC and DC motors are more different than they are alike.

 

AC Motors

The big things driving AC motors are frequency and induction. We talked about VFDs already. Well, that approach only works on AC Motors. By changing the frequency, you can alter the speed and torque of those motors. Induction is how a transformer works, by  using one coil to create a current in another coil. This causes transformers to hum, because the electrical fields create tiny bits of movement. The electrical fields create movement, because the field itself is moving.

The motion in these electrical fields comes from AC Power. Alternating Current doesn’t hold still, it… alternates. What do I mean? Take a look below.

AC-DC-01

The AC Power is oscillating. It moves forward, then it moves backward. This electrical motion creates a moving field. We don’t need to change the direction of the field with commutators and windings, because it already changes direction. In the end, this means that we can make a motor where only the rotor spins, and the rotor only needs to touch bearings to hold it up. We don’t need electrical contact.

This is also where we get to the two big designs. You have Induction and Synchronous Motors. An induction motor relies on inducing a current in the rotor. This causes some lag in speed and a noticeable loss of precise control. Think of it like controlling the speed of your car, by telling someone in another continent to make your car go faster.  A synchronous motor by contrast uses permanent magnets. It’s rotational speed is always tied directly to the input frequency. If it isn’t, you’ll be needing a new motor because something’s gone wrong.

These motors tend to be powerful, relatively cheap, and reliable. That’s part of why VFD’s and AC Motors at-large are so common. You also don’t have to invest in hardware rectifiers to get DC Power. You don’t have to maintain the motors by replacing worn brushes. You set them up and get onto the next task.

The advantages are significant enough that even in applications where DC would be easier to implement, AC motors are still used. For example, electric cars. Vehicles like Teslas use AC Traction Motors, similar to what’s present on trains, despite only having DC Power immediately on-hand. They have to sacrifice some range by inverting the DC power to run their AC Motors. The control and hardware lifespan make it worth while.

 

DC Motors

We based our earlier article on electric motors on a DC Motor. The problem with DC is that you can’t induce a moving current. DC Simply doesn’t move. It’s field is stationary. This tends to complicate matters when we want to make things move because we need to be very direct. We need to directly send power to the inner windings on the rotor. That we can control and we can make a field that’s able to move. This is all old hat, we’ve talked about it before.

Let’s look at the downside of this design. The Brushes. The rotor and commutators move, so we can’t exactly solder wires to them and think it’ll work. The part that touches the commutator is called a brush. It’s a conductive contact, usually spring-loaded, that gets ground into the commutators. It allows power to flow, but at some point it’s going to be ground down to nothing and stop working. Just think about the brakes on your car, they wear down eventually.

The upside is that this does allow us to create incredibly compact and efficient designs. Inducing current in AC motors causes you to lose a bit of power in the process. Here? Our biggest enemy is waste heat and wear. We’re able to generate a lot of speed and torque in a small package.

These types of motors are found all over your home and office. Coffee machines, mixers, power tools, the fans in your computer, the robotic skeleton Bob down the hall is gonna prank and scare you with on Halloween, all use compact, but powerful, DC Motors. The load in these applications is low enough that worn brushes aren’t too big of a factor.

 

Brush Less DC

The alternative to a standard DC Motor is a Brush Less DC Motor. We put expensive, permanent magnets on the rotor, then control the magnet field through the field windings. That the field windings don’t move, is a life safer. They just sit there, so we can easily switch power on and off between them using relays, mosfets, and other solid-state means. The rotor turns and all is not quite right in the world.

The drawback of a Brush Less Motor is those expensive magnets. They’re cheap enough in a small application. A little toy car or something might only have a few dollars worth of magnet in it. In a high performance application however, your wallet really wants to cry. These applications require bigger, rarer magnets. You need things like Neodymium. You need them by the tens to hundreds of pounds. It gets to be like going to the moon. Sure, you can do it, but it’s going to be expensive.

 


The Wrap Up

What did you think? Did we get something wrong? Got something for us to cover next time? Let us know in the comments below.

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