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# 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

- distance
- 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
- V
_{net}= V_{supply}– EMF_{back} - 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 V_{supply} = EMF_{back}

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