The AC/DC Motor

AC and DC Power are different beasts. This has been covered here before and it’s covered to death in training courses for contractors. They seem like oil and water. If you put AC power into a DC application, it’ll end in tears. If you put DC power into an AC application, it’ll end equally in tears. Somewhere along the line, there may even be a ball of fire, some sparks, and an explosion if you mix AC and DC. That begs the question: HOW can one motor work happily with both AC and DC power? It seems a little counter intuitive.


The Expected Problem

In a motor using direct current, we expect the power to always move in the same direction. We use commutators to manipulate where that power flows so we can change the magnetic fields, cause changing attractions, and force a rotor to spin. What if we used AC? The field would be reversing rapidly, changing sixty times a second or more. One instance it’s forward, the next it’s backwards. Intuitively, you’d think this sort of power would create vibration. The rotor should lurch right and left a few degrees violently rattling about until something finally breaks.

This isn’t the case, so long as some accommodations are made for the different type of power. Like all things, we need to be clever about where we put our wires.


Accommodating AC Power in a DC Motor

For this to work, you’ll need to start with a DC Series Motor, a motor where the rotor and field windings are connected to each other. This is the key thing that’ll make it all work. With AC Power, when the polarity inverts, it doesn’t just invert on the rotor or the field winding. It inverts on both.  The result is essentially a double negative. The inversion is cancelled out and rotation will continue in the original direction.

It’s best to think of it like your car. Imagine if you put it in reverse, the engine also reversed and began to spin in the opposite direction? You’d end up going forwards. The same thing happens here. The reverser is effectively reversed itself because everything is all connected. In other motors with different wiring or with permanent magnets, this reversal wouldn’t take place and things wouldn’t end so well.

Things are moving now, regardless of the type of power coming in, but there’s another couple of things that make a Universal Motor special, and different from a regular Series Motor.

One of the greater challenges is Eddy Currents. These are electromagnetic fields which resist movement in AC Applications. This resistance requires more power to overcome, creates excess heat, and is so potent that it is often employed as a breaking mechanism for AC Motors. The way to overcome these currents is with laminations and ferrite cores which reduce these unwanted currents. This adds weight to the rotor and reduces it’s efficiency.

There’s also challenges around the brushes. The changing current can lead to sparking and excessive wear on the brushes. This is in addition to the losses already experienced from friction and wear and tear in high RPM usage. The only mitigation here is to use brushes designed to take the abuse and resist sparking.

Finally, there’s the outright loss of efficiency. The design is not optimized for either type of current. It doesn’t have the precision of a VFD operating for the perfect duty cycle. It doesn’t have permanent magnets like many small DC motors either. The end result is a 30% or more loss of efficiency compared to an AC or DC only application.


What Possible Application Could There Be?

These motors are actually incredibly common for a few simple reasons. They’re easy to manufacture, they reduce design costs, they develop a lot of torque under load, and in small applications power efficiency is not the number one selling point. These are the types of motors used in kitchen appliances, power tools, and other small places around the home and office. They’re used in power drills, vacuums, mixers, and pretty much all those day to day tools you couldn’t live without.

Most of these use cases don’t need to run on both AC and DC power, but the otherwise cheap manufacturing costs make the motors an ideal choice. Except for Power Tools. These fall into that one category where AC and DC Compatibility is a high point. Power tools are offered in both battery-powered and wall-powered applications. Manufacturers can basically use the same guts in both types of drills and reduce on design and production costs across the board. This is probably the approach taken in all the lower and mid-lower tier tools. Higher end tools will have nicer permanent magnets.


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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