Chapter 14: Electromagnetism

Version 0.51

Electromagnetism

An electrical current in a wire produces a magnetic field

Ampere-Turns

The strength of the magnetic field in a coil depends on

Magnitude of the current

Number of turns

Known a the magnetomotive force (mmf)

MMF Formula

MMF = I x N

MMF – magnetomotive force in Amp-turns

I – current in ampreres

N – number of turns in a coil

Amp - turn Examples

Field Intensity

Defined as H

Mmf per unit length

Amp-turns of mmf ÷ length of coil

Unit is the amp-turns per meter.

MMF Examples

Magnetic Polarity

Depends

Flow of the current

Direction of the coil winding

Reversing either changes the polarity of the coil

Coil Magnetic Polarity

Induced Current

When magnetic flux moves across a conductor it forces free electrons in a coil to move, creating electrical current

Induction

Conductor must be perpendicular to the magnetic flux

Magnetic Induction

Faraday’s Law

Amount of induced voltage depends on:

Amount of magnetic flux

Number of turns

Rate of change in the magnetic flux

Faster the flux changes the greater the induced voltage

Faraday’s Law Formula

Relay

Electromechanical device

Operates by induction

Use coil to operate a set of contacts

NC or NO

Relay Diagram

Relay Contacts

Typical Relays

Typical Relays

Relay Applications

Relay Specifications

Contactors

Motor Starter