Electric Motors

Principle of Electric Motors

When a current flows through a conductor placed in a magnetic field, a force acts on the conductor. This happens because the magnetic field interacts with the moving charges (electrons) in the conductor.

This force is called the motor effect. It is the fundamental principle behind electric motors, where electrical energy is converted into mechanical energy (motion).

The size of the force depends on:

• The strength of the magnetic field
• The size of the current flowing through the conductor
• The length of the conductor within the magnetic field

The force acts perpendicular to both the direction of the magnetic field and the current.

For instance, if a straight wire carrying current is placed between the poles of a magnet, the wire will experience a force that pushes it sideways.

Fleming's Left-Hand Rule

To find the direction of the force on a current-carrying conductor in a magnetic field, we use Fleming's left-hand rule. This rule helps predict the direction of motion in electric motors.

The rule states:

• Hold your left hand so that the First finger points in the direction of the magnetic Field (from North to South)
• Your seCond finger points in the direction of the Current (from positive to negative)
• Your thuMb will then point in the direction of the Motion or force on the conductor

This mnemonic is often remembered as FBI: First finger = Field, seCond finger = Current, thuMb = Motion.

For example, if the magnetic field is pointing upwards and the current flows to the right, the force on the wire will be directed out of the page (towards you).

Components of an Electric Motor

A simple electric motor consists of several key parts:

• Coil: A loop of wire that carries the current. When placed in a magnetic field, forces act on opposite sides of the coil.
• Commutator: A split ring that reverses the direction of current every half turn, ensuring the coil keeps rotating in the same direction.
• Brushes: Conductive contacts that press against the commutator, allowing current to flow into the coil.
• Magnetic field: Usually provided by permanent magnets, creating a uniform field in which the coil rotates.

The coil is free to rotate between the poles of the magnet. The motor effect causes forces on opposite sides of the coil in opposite directions, creating a turning effect (torque).

Working of a Simple Electric Motor

When current flows through the coil, the motor effect causes a force on each side of the coil:

• On one side, the force pushes the coil up
• On the opposite side, the force pushes the coil down

These forces create a turning effect, causing the coil to rotate.

After half a turn, the commutator reverses the direction of the current in the coil. This reversal changes the direction of the forces, so the coil continues to turn in the same direction rather than stopping or reversing.

This continuous rotation is how electric motors convert electrical energy into mechanical rotation.

For example, if the coil is rotating clockwise, the commutator switches the current so the forces keep pushing it clockwise rather than letting it slow down or spin back.

Example: Direction of Force Using Fleming's Left-Hand Rule

A wire carrying current to the right is placed in a magnetic field directed into the page. Using Fleming's left-hand rule, find the direction of the force on the wire.

Solution:

• First finger (Field) points into the page
• Second finger (Current) points to the right
• Thumb (Force) points upwards

So, the wire experiences a force upwards.