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# Electric motors

Monday - 03/02/2020 09:47

Flick a switch and get instant power—how our ancestors would have loved electric motors! You can find them in everything from electric trains to remote-controlled cars—and you might be surprised how common they are.
How many electric motors are there in the room with you right now? There are probably two in your computer for starters, one spinning your hard drive around and another one powering the cooling fan.

If you're sitting in a bedroom, you'll find motors in hair dryers and many toys; in the bathroom, they're in extractor fans, and electric shavers; in the kitchen, motors are in just about every appliance from clothes washing machines and dishwashers to coffee grinders, microwaves, and electric can openers. Electric motors have proved themselves to be among the greatest inventions of all time. Let's pull some apart and find out how they work!

Electricity, magnetism, and movement

The basic idea of an electric motor is really simple: you put electricity into it at one end and an axle (metal rod) rotates at the other end giving you the power to drive a machine of some kind. How does this work in practice? Exactly how do your convert electricity into movement? To find the answer to that, we have to go back in time almost 200 years.

Suppose you take a length of ordinary wire, make it into a big loop, and lay it between the poles of a powerful, permanent horseshoe magnet. Now if you connect the two ends of the wire to a battery, the wire will jump up briefly. It's amazing when you see this for the first time. It's just like magic! But there's a perfectly scientific explanation. When an electric current starts to creep along a wire, it creates a magnetic field all around it.

If you place the wire near a permanent magnet, this temporary magnetic field interacts with the permanent magnet's field. You'll know that two magnets placed near one another either attract or repel. In the same way, the temporary magnetism around the wire attracts or repels the permanent magnetism from the magnet, and that's what causes the wire to jump.

How an electric motor works—in theory

The link between electricity, magnetism, and movement was originally discovered in 1820 by French physicist André-Marie Ampère (1775–1867) and it's the basic science behind an electric motor. But if we want to turn this amazing scientific discovery into a more practical bit of technology to power our electric mowers and toothbrushes, we've got to take it a little bit further. The inventors who did that were Englishmen Michael Faraday (1791–1867) and William Sturgeon (1783–1850) and American Joseph Henry (1797–1878). Here's how they arrived at their brilliant invention.

Suppose we bend our wire into a squarish, U-shaped loop so there are effectively two parallel wires running through the magnetic field. One of them takes the electric current away from us through the wire and the other one brings the current back again. Because the current flows in opposite directions in the wires, Fleming's Left-Hand Rule tells us the two wires will move in opposite directions. In other words, when we switch on the electricity, one of the wires will move upward and the other will move downward.

If the coil of wire could carry on moving like this, it would rotate continuously—and we'd be well on the way to making an electric motor. But that can't happen with our present setup: the wires will quickly tangle up. Not only that, but if the coil could rotate far enough, something else would happen.

Once the coil reached the vertical position, it would flip over, so the electric current would be flowing through it the opposite way. Now the forces on each side of the coil would reverse. Instead of rotating continuously in the same direction, it would move back in the direction it had just come! Imagine an electric train with a motor like this: it would keep shuffling back and forward on the spot without ever actually going anywhere.

How an electric motor works—in practice

There are two ways to overcome this problem. One is to use a kind of electric current that periodically reverses direction, which is known as an alternating current (AC). In the kind of small, battery-powered motors we use around the home, a better solution is to add a component called a commutator to the ends of the coil. (Don't worry about the meaningless technical name: this slightly old-fashioned word "commutation" is a bit like the word "commute". It simply means to change back and forth in the same way that commute means to travel back and forth.)

In its simplest form, the commutator is a metal ring divided into two separate halves and its job is to reverse the electric current in the coil each time the coil rotates through half a turn. One end of the coil is attached to each half of the commutator. The electric current from the battery connects to the motor's electric terminals.

These feed electric power into the commutator through a pair of loose connectors called brushes, made either from pieces of graphite or thin lengths of springy metal, which (as the name suggests) "brush" against the commutator. With the commutator in place, when electricity flows through the circuit, the coil will rotate continually in the same direction.

A simple, experimental motor such as this isn't capable of making much power. We can increase the turning force (or torque) that the motor can create in three ways: either we can have a more powerful permanent magnet, or we can increase the electric current flowing through the wire, or we can make the coil so it has many "turns" (loops) of very thin wire instead of one "turn" of thick wire. In practice, a motor also has the permanent magnet curved in a circular shape so it almost touches the coil of wire that rotates inside it. The closer together the magnet and the coil, the greater the force the motor can produce.

Although we've described a number of different parts, you can think of a motor as having just two essential components:

There's a permanent magnet (or magnets) around the edge of the motor case that remains static, so it's called the stator of a motor.
Inside the stator, there's the coil, mounted on an axle that spins around at high speed—and this is called the rotor. The rotor also includes the commutator.

Universal motors

DC motors like this are great for battery-powered toys (things like model trains, radio-controlled cars, or electric shavers), but you don't find them in many household appliances. Small appliances (things like coffee grinders or electric food blenders) tend to use what are called universal motors, which can be powered by either AC or DC.

Unlike a simple DC motor, a universal motor has an electromagnet, instead of a permanent magnet, and it takes its power from the DC or AC power you feed in:

When you feed in DC, the electromagnet works like a conventional permanent magnet and produces a magnetic field that's always pointing in the same direction. The commutator reverses the coil current every time the coil flips over, just like in a simple DC motor, so the coil always spins in the same direction.

When you feed in AC, however, the current flowing through the electromagnet and the current flowing through the coil both reverse, exactly in step, so the force on the coil is always in the same direction and the motor always spins either clockwise or counter-clockwise. What about the commutator?

The frequency of the current changes much faster than the motor rotates and, because the field and the current are always in step, it doesn't actually matter what position the commutator is in at any given moment.

Animation showing how a universal motor works with an AC supply

Other kinds of electric motors

In simple DC and universal motors, the rotor spins inside the stator. The rotor is a coil connected to the electric power supply and the stator is a permanent magnet or electromagnet. Large AC motors (used in things like factory machines) work in a slightly different way: they pass alternating current through opposing pairs of magnets to create a rotating magnetic field, which "induces" (creates) a magnetic field in the motor's rotor, causing it to spin around. You can read more about this in our article on AC induction motors.

If you take one of these induction motors and "unwrap" it, so the stator is effectively laid out into a long continuous track, the rotor can roll along it in a straight line. This ingenious design is known as a linear motor, and you'll find it in such things as factory machines and floating "maglev" (magnetic levitation) railroads.

Another interesting design is the brushless DC (BLDC) motor. The stator and rotor effectively swap over, with multiple iron coils static at the center and the permanent magnet rotating around them, and the commutator and brushes are replaced by an electronic circuit. You can read more in our main article on hub motors. Stepper motors, which turn around through precisely controlled angles, are a variation of brushless DC motors.

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