Motors and Generators

Introduction
Asking students to draw diagrams of a motor or a generator and explain the principle of their operation are popular Leaving Certificate examination questions.
While the textbooks have clear diagrams of motors and generators, they are drawn by commercial artists using shading, perspective, etc. The average student finds it a little daunting to produce a simple but adequate diagram to score a good mark in the examination question. The drawings given here are simple line drawings, which the students can practise reproducing with pencil and paper. Having mastered the basic drawing it is then much easier for the student to progress to a good understanding of the principle of operation of the device. To help with this understanding drawings are also provided showing one complete rotation of the coil in the case of the D.C. motor and basic diagrams for the D.C. generator and alternator. The student should compare and contrast the basic diagrams, and note the use of Fleming's left and right hand rules.

Direct Current Motor

Basic Diagram

Fleming's Left-hand rule applies to motors. The thumb, fore-finger and second finger of the left hand when held at right angles to one another will give the arrangement of the directions of force, magnetic field and current, respectively. (FBI will help you remember). Apply Fleming's left hand rule to the side of the coil AB and CD. What conclusion is reached if Fleming's Left-hand rule is applied to side AC?

Coil vertical

Note that no current flows when the coil is vertical. There is no motor effect but the momentum of the coil continues to rotate it until the brushes again conduct current into the split ring commutator.
Coil rotated through 180o from start

Apply Fleming's left hand rule to AB and DC again and note that the coil continues to rotate in the same direction as before.

Direct Current Generator

Basic Diagram

Fleming's Right hand rule applies to generators. The thumb, fore-finger and second finger of the right hand when held at right angles to one another will now give the arrangement of the directions of force, magnetic field and current, respectively, for a generator. (FBI will help you remember again). Apply Fleming's right hand rule to the side of the coil AB and CD as before. What conclusion is reached if Fleming's right hand rule is applied to side AC?
Draw a diagram to show the coil in the vertical position. Note that no current flows when the coil is vertical. There is no generator effect because the coil is not cutting field lines.
Draw a diagram to show the coil rotated through 180o from the start. Apply Fleming's right hand rule to AB and DC again and note that the current continues to flow in the same direction as before. Thus this device generates a direct but not steady current. Draw a diagram to show output voltage versus time.

Alternating Current Generator (Alternator)

Basic Diagram

Fleming's Right hand rule applies to this generator also.
Draw a diagram to show the coil in the vertical position. Note that no current flows when the coil is vertical. There is no generator effect because the coil is not cutting field lines.Draw a diagram to show the coil rotated through 180o from the start. Apply Fleming's right hand rule to AB and DC again and note that the current flows in the opposite direction than before. Thus this device generates an alternating current. Draw a diagram to show output voltage versus time.