My article last week was heavy on theory, but now that we have the theory down we can talk about some cool practical applications. All of the most interesting electrical stuff comes from combining the properties of electricity with the properties of magnetism. Electromagnets, motors, sensors and more all rely on the fundamental interactions between electricity and magnetism.
One of the most difficult things to understand are AC induction motors. DC motors are actually pretty straightforward. Even with the basic understanding a 12 year old has, I was able to repair a few DC motors when I was a kid. I would salvage them from electronics and use them to drive fans for projects like a model hovercraft and a model submarine. I tended to wear them out by driving too much power through them (and by submerging them), so I would often have to open them and repair them.
In a DC motor, the current is always flowing in the same direction. The motor itself is composed of two main pieces: the stator and the rotor. The stator is the outer shell of the motor and in DC motors, it houses fixed permanent magnets that encircle the rotor that spins inside. The rotor is composed of a shaft with electromagnets that face outwards to the permanent magnets on the stator. When current is passed through the electromagnets on the rotor, they repel the permanent magnets and rotate. Eventually though, the rotation will stop because the poles of the electromagnets and permanent magnets match up. To keep the motor spinning, we have to continuously adjust which coils are engaged. We do this with brushes that contact the rotor shaft as it spins. When it reaches a certain angle, the brushes contact a different surface and the electromagnets on the rotor are engaged one after another to keep rotation going. DC motors are inherently self-starting, meaning that the initial repulsion created will “kick” the motor into motion. That’s basically all you need to know about DC motors. Simple stuff.
AC induction motors are far more complicated, but more elegant and reliable. As we know, AC power is a constantly changing current. Standard household power is single-phase AC, meaning that there is one sine wave of voltage/current going up and down. Induction motors are actually easier to understand initially if we consider industrial 3-phase AC power. This type of power is composed of three separate AC waveforms on top of each other. Each wave is 120 degrees apart from the others.
So if phase-1 peaks at time zero, phase-2 will peak 120 degrees later and phase-3 will peak 120 degrees after that. The stator of an AC induction motor is composed not of permanent magnets, but electromagnetic coils. In the simplest form, there will be three separate coils each connected to one of the three phases of the 3-phase power. The coils are physically placed 120 degrees apart from each other in the stator. Three of these complete the 360 degrees of a circle. As each coil is energized, it creates a magnetic field, and because they are arranged in a circle that is identical to the phase shift in the original power, what we have done is created a rotating magnetic field with no moving parts!
Ok, so do we put in permanent magnets on the rotor and call it a day? No need. The rotor is actually designed as something called a squirrel cage. It essentially looks like two disks held together by metal slats on the outside. Remember that because the AC power is always fluctuating, the strength of each magnetic field is fluctuating. When we place the squirrel cage inside this rotating magnetic field, the changing magnetic field induces current in the slats. This current isn’t going anywhere, it’s just trapped in the slats but like any current, it too creates a magnetic field. This magnetic field opposes the one created by the electromagnets on the stator and a force is delivered to the rotor causing it to spin. The mathematics and physics of these forces are really cool and worth checking out on your own. It’s called the Lorentz Force.
A 3-phase induction motor is inherently self-starting because regardless of the initial position of the rotor the outer magnetic field will be rotating. The power in your house is not 3-phase though, so how do those induction motors work? The principle is pretty much the same, you just have only one magnetic field to work with instead of three. Single-phase induction motors always have some way to shift the phase of the power coming in to make it off balance during startup so the motor can start. Without this the rotor would just vibrate in place. There are several ways to do this, but the most common is a starting capacitor and a secondary stator winding. This special circuit can be disengaged by a centrifugal switch once the motor spins up.
Because of their simplicity and lack of wear parts (brushes wear out on DC motors) AC induction motors are often chosen even when AC power isn’t immediately available, like in the case of modern electric cars. Their batteries provide DC power only but because AC induction motors are so advantageous, engineers actually go through the trouble of inverting the DC power to AC power. All modern diesel-electric locomotives use AC traction motors as well.
Once again, this is a difficult concept to compress, but hopefully I’ve done a bit to demystify AC motors. These kinds of motors were one of the most confusing things to me for such a long time. This was an interesting article to write, and I’m unsure what my topic will be next week. I’m thinking maybe something about inductive heating, but if any readers have any questions or suggestions, let me know!