# Investigate the operation of a simple DC motor

Current supplied produces interacting magnetic field which keeps the motor spinning

The function of the components of motor are as follows:

• Armature: laminated (reduces eddy current) ferromagnetic cylinder, rotates on axle to give actual motion, concentrates magnetic field to increase
• Coil: Wrapped around armature, medium for current, better motion with more coils at right angles
• Split ring commutator: Changes direction of current to allow motion to continue
• Brushes: Allows current flow into split ring
• Magnets: Produce external magnetic field, curved magnets can improve torque (coil plane is always parallel)
• Axle: provides centre of rotation

Torque production

• Torque due to a force is defined as :
• distance d is the distance of OP and MN from the axis of rotation.
•  , where w is the width of the conductor loop
• so
• Force on a current carrying wire :
• now conductor NO and MP are parallel to the magnetic field so no force acts on then
• conductor MN and OP are always perpendicular to the magnetic field , so force on them is F = IlB where l is the length of MN and OP.
• So using and F = IlB , we get
• , for NM and OP
• If n loops are wounded, then the formula becomes , where
• is the angle between direction of area (of loop) and direction of magnetic field.

Back emf in electric motor:

• Energy must be conserved.
• Therefore back EMF must opposite to supplied. Otherwise there would be an unchecked increase.
• This reduces the efficiency of the motor.
• A smaller back EMF gives a greater current under load.
• Slower spin gives a lower back EMF
• Vnet = Vsupply – EMFback
• When starting there is a low back EMF and the high current could burn the motor out.
• Variable resistance is used until more back EMF is produces

A steady speed is produced when Vsupply = EMFback

Extract from Physics Stage 6 Syllabus © 2017 NSW Education Standards Authority (NESA)