Day 3: Eddy Currents

• Back EMF in Motors

• Eddy Currents

• Nature of Eddy Currents

Back EMF in Motors• When a motor turns, the magnetic flux through the coil changes and

an EMF is generated

• The induced EMF acts to oppose the rotation of the armature because of Lenz’s Law. This is called Back EMF

• The greater the speed of the motor, the greater the EMF opposing its motion

• With no load on the motor, the EMF will increase until it equals the input voltage and equilibrium (constant speed) is achieved

• With a mechanical load is put on the motor, the speed will decrease and the back EMF will decrease

Back EMF in Motors• Example: An armature winding having a resistance of

5.0Ω, and connected to a 120 V source. When the motor reaches full speed under normal load, the back EMF is 108 V.

AV

R

VI 24

5

120

AV

I

IVV

4.25

12

5108120

The start-up current is:

The running current is (using Kirchhoff’s Law):

Eddy Currents• Currents induced in a conductor as a result of a

changing magnetic flux are called Eddy Currents

Eddy Currents• Eddy currents can be present in any conductor

that is moving across a magnetic field, or through which the magnetic flux is changing

Eddy Currents• Eddy currents are a source of wasted energy

which is dissipated as heat.

• In motors, windings are laminated (made of thin sheets of iron that are well insulated from one another). The total path length of the eddy currents is confined to each slab, increasing the total resistance, hence reducing the current, wasting less energy

eddyIP

Laminated Armatures