Physics » Magnetism and Faraday's Law » Electrical Machines - Generators And Motors

Electric Motors

Electric Motors

The basic principles of operation for an electric motor are the same as that of a generator, except that a motor converts electrical energy into mechanical energy (motion).

Definition: Electric motor

An electric motor is a device that converts electrical energy into mechanical energy.

If one were to place a moving charged particle in a magnetic field, it would experience a force called the Lorentz force.

Definition: The Lorentz Force

The Lorentz force is the force experienced by a moving charged particle in an electric and magnetic field. The magnetic component is:

\[F=qvB\]

where \(F\) is the force (in newtons, N), \(q\) is the electric charge (in coulombs, C), \(v\) is the velocity of the charged particle (in \(\text{m·s$^{-1}$}\)) and \(B\) is the magnetic field strength (in teslas, T).

In this diagram a positive charge is shown moving between two opposite poles of magnets. The direction of the charge’s motion is indicated by the orange arrow. It will experience a Lorentz force which will be in the direction of the green arrow.

A current-carrying conductor, where the current is in the direction of the orange arrow, will also experience a magnetic force, the green arrow, due to the Lorentz force on the individual charges moving in the current flow.

If the direction of the current is reversed, for the same magentic field direction, then the direction of the magnetic force will also be reversed as indiced in this diagram.

We can if there are two parallel conductors with current in opposite direcions they will experience magnetic forces in opposite directions.

An electric motor works by using a source of emf to make a current flow in a loop of conductor such that the Lorentz force on opposite sides of the loop are in opposite directions which can cause the loop to rotate about a central axis.

Fact:

The force on a current-carrying conductor due to a magnetic field is called Ampere’s law.

The direction of the magnetic force is perpendicular to both the direction of the flow of current and the direction of the magnetic field and can be foundusing the Right Hand Rule as shown in the picture below. Use your right hand; your first finger points in the direction of the current, your second finger in the direction of the magnetic field and your thumb will then point in the direction of the force.

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Both motors and generators can be explained in terms of a coil that rotates in a magnetic field. In a generator the coil is attached to an external circuit that is turned, resulting in a changing flux that induces an emf. In a motor, a current-carrying coil in a magnetic field experiences a force on both sides of the coil, creating a twisting force (called a torque, pronounce like ‘talk’) which makes it turn.

If the current is AC, the two slip rings are required to create an AC motor. An AC motor is shown in the figure below.

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Layout of an alternating current motor.

If the current is DC, split-ring commutators are required to create a DC motor. This is shown in the figure below.

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Layout of a direct current motor.

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