Electrical - Wind Energy: Renewable Energy and the Environment - page 131

z

F

B

I

x

B

B

I

I

F

B

y

FIGURE 7.2 Forces on the sides of a current-carrying loop in an external magnetic field. The resultant of the
set of forces gives a torque, T , which makes the loop rotate, a motor.

If charged particles are placed in external electric fields, and if moving charged particles are
placed in external magnetic fields, there is a force on the charged particles. The amount of force
depends on the strength of the electric and magnetic fields, the amount of charge, and the velocity
of the charge.

(7.8)

FEvB q ( r )

This equation is the basis for understanding the conversion of electric energy to mechanical energy,
a motor, and the conversion of mechanical energy to electric energy, a generator.
Motor: A loop of wire has moving charges (current) in it due to a connection to an electric plug.
The loop is in an external magnetic field; therefore, there is a force on the charges and a torque on
the wire, a motor (Figure 7.2). The torque on the loop is given by the current in the wire, I , the area
of the loop, A , and the strength of the magnetic field, B . Now in the motor there is a coil of wire,
many loops. Check the links to see how an electric motor works.

T I ( A × B ) (7.9)

Generator: A loop of wire is moved (rotated) by an external force ( Figure 7.3 ) . The shaft power,
P T q , which in this case comes from the wind turbine rotor, either directly or through a gearbox.
The charges (electrons in the wire) are moving in an external magnetic field, and there is a force on
the charges, a generator. Of course, there is not just one loop of wire, but many loops in a coil. If
there is just one coil, it is a single-phase generator. If there are three coils of wire, then it is three-
phase generator.
The external magnetic field can be produced by permanent magnets or electromagnets. A current
in a coil produces a magnetic field, and with an iron core in the coil, the magnetic field is stronger.
The number of these coils is referred to as poles. So the current from the utility grid or a part of
the generator current is used to produce the magnetic fields. Check the links to see how an electric
generator works.