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 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
- 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)