# Electric motors

• Electric motors use force produced by current carrying conductors to create torque
• Instead of single conductors, they use wires wound into coils

### Permanent magnet DC motors

• Current flows from power supply into the stationary brushes
• It then flows from the brushes into the split rings (commutator) – they are electrically isolated from each other and rotate with the armature
• From the split rings, the current flows through the coil windings (the armature)
• The current carrying coil experiences a torque when it is exposed to the magnetic field of the permanent magnet, called the stator
• For the armature to continue to rotator in one direction, the direction of the current in the coil must be reversed every half turn
• This is achieved via the split rings

### Force on a loop

• The figure shows a loop of wire with a conventional current I.
• The loop rotates about the axis O-O.
• If we let L(m) equal the length of sides AB and CD, which are both perpendicular to the flux B, then the force on each of these sides of the coil is,

F = ILB

• Note that there is no force in the other sides of the coil as they are parallel to B.

### Force on a coil

• In motors and speakers a coil is used to multiply the effects of a loop. If a coil is made up of N loops of wire then the total force on one side of the coil is,

F = NILB

### Torque on a rotating coil

Torque that each force applies about O-O

τ = rF

Since there are two forces,

τtotal = 2rF

In a DC motor, N coils are used to multiply the torque and force

τtotal = 2rFN

Alternatively,

τtotal = INBA

### Torque in a rotating coil

τtotal = INBA cos α , where α is the angle the armature makes to the horizontal

τtotal= INBA sin β , where β is the angle between the force and the armature

### Industrial Motors

Since the torque fluctuations are quite high, industrial motors try to reduce the torque ripple and increase the efficiency of the motor:

• Increasing number of coils
• Coils are wound around a soft iron core to maximise magnetic field
• They have 4 or 8 poles instead of 2
• Both the stator and rotor use electromagnets rather than permanent magnets
• They use low friction bearings
##### 2 pole and 4 pole motors
###### 2 pole motors:
• Have 2 poles on the armature and 2 poles on the stator
• Current changes every half turn
• Jerky movement as there are large torque fluctuations
###### 4 pole motors:
• Have 4 poles on the stator and 4 on the armature
• Current changes direction every quarter turn
• Smooths out torque fluctuations

### Speakers

• Speakers create sound by using magnetic forces created by a magnet and a coil to vibrate the cone
• The cone is attached to the coil
• When a current is passed through the coil, it experiences a force
• AC is used to push the coil in both directions and vibrate it and thus the air.