Intro to Motors

Intro to Motors (Video 1/4 in Motors Series)

Highlights

  •     Two types of magnets cause motors to spin: electromagnets create the magnetic field and permanent magnets chase the poles.
  •     Electromagnetic fields are created by passing current through a wire.
  •     Some motors are designed to handle DC current, while others are only for AC current. Connecting to the wrong source can damage the motor circuitry.
  •     Using too high of a voltage or current may destroy your motor circuitry and could lead to fires.

Notes

Many electromechanical systems include an electric motor. These motors transform electrical power (often from batteries) into mechanical power (like cranking a shaft in an engine). In many instances, the mechanical power is used to turn a shaft, which can in turn move things in a straight line much like a piston; however, electrical power can be immediately transformed into a linear mechanical power like a rail gun (shown in the photograph below). In essence, an electric motor is a device that plugs into a circuit and physically moves a system component. Example systems that include electric motors are windscreen wipers, vacuum cleaners, cell phone vibration motors, automatic garage doors, camera shutters.
Railgun_usnavy_2008

One way mechanical motion can be created using electricity is by using magnets as middleman. As you know, a magnet has two poles: a North and a South. If similar poles are in close proximity, the magnets repel each other; if dissimilar poles are close, then the magnets attract each other. This mechanism can be exploited (engineered) to create mechanical motion. However, after two magnets repel each other, once they are sufficiently far apart, they stop moving. Similarly, when two magnets attract each other, once they touch, they also stop moving. This is not what we want. What we want from an electric motor is a continuous motion. In order to accomplish this continuous motion, we must keep the magnets in consistent struggle. We need magnets to chase a carrot on a stick.

If we had two slightly misaligned magnets on top of each other, we could rotate one just fast enough to make the other one follow. However, how can we move the first magnet in the first place? This “solution” brings us back to the starting point: we were trying to get mechanical motion in the first place! In order to accomplish the movement of the first magnet, we will use a virtual magnet, a magnet that isn’t technically there but its magnetic field is there. And since we’re trying to use electricity to create a mechanical movement, we’ll use electromagnets. In order to move the virtual magnet, we will turn on and off the electromagnets by switching currents.

virtual-magnet

Before we figure out the virtual magnet, let’s figure out how an electromagnet really works. Specifically we need to understand how switching the direction of the current can change the orientation of the electromagnet. A wire that carries a current creates a circular magnetic field.

straight-wire-magnet

However, a permanent bar magnet’s magnetic field looks more like two eddies.

permanent-magnet

This magnetic field can be emulated by a coiled wire by twisting the wire so that all points of the wire is producing a magnetic field in the same direction within the coil. If the coil has more windings, the magnetic field it generates will be more powerful. 20 loops generates twice the magnetic field strength as 10 loops for example.

coil-electromagnet

Finally, the magnetic field within the coil can be reversed if the current is reversed.

coil-electromagnet-reverse

Returning to the idea of the electric motor, we will now use this virtual magnet, which we created with an electric current and attach it so that the motor can rotate. We will do this by changing the direction of the magnetic field so that the rotor, or moving part of the motor, has a magnetic field that is out of alignment with the stator, or fixed part of the motor. By changing the direction of electricity in the wire, also called commutation, in such a way that the magnetic field in the rotor can never catch the magnetic field of the stator the motor will keep spinning.

SimpleMotor-Animation

The key difference between many types of electric motor are how this commutation is done. Examining the different ways commutation is done is presented in the Motor Types video. Some motors use mechanical switching to change the direction of the current. Some use electric current that naturally changes direction, such as AC and DC Power. Understanding the different ways people have used changes in current direction to create changing magnetic fields and, as a result create, different electric motors will give you a better understanding of motor basics.

One final thing about electric motors; electric motors were invented and used in practical applications well before the physics and science on how they work was well understood. Because this is a conceptually complex topic, check out some of the links below, and keep inventing (the physicist will eventually catch up).

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Additional Information

To learn more, visit one of these recommended website below, or simply search the internet for the terms introduced in this lesson!

HyperPhysics — Permanent bar magnet
HyperPhysics — Magnetic field of electric current
HyperPhysics — Electromagnet coil
YouTube — Electromagnets are explained using special relativity
YouTube — Permanent magnets are explained using quantum mechanics
YouTube — Very strong magnets