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29 Cards in this Set
- Front
- Back
- 3rd side (hint)
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What is electromagnetic induction? |
The induction of an EMF in a conductor by relative movement of the conductor and a magnetic field. |
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What determines the strength of a magnetic field? |
The strength of the current If it possess a core I.e soft iron The number of coils |
3x C |
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State two methods of electromagnetic induction and an application for each. |
Static induction-transformers Dynamic induction-motors/generators |
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State Faradays law |
When there is relative motion between a conductor and a magnetic field, an emf is induced in the conductor. The magnitude of the emf is dependant on the rate of change of the magnetic flux. |
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State Lenz’s law |
The direction of an induced emf is always to oppose the motion of the change that produced it. |
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State the expression for induced emf in a conductor |
E ∝ -dθ / dt E= E.M.F θ =magnetic flux t=time |
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How are Flemings rules applied to dc machines? |
Show direction of motion/magnetic flux/direction of current induced Left hand rule used for motors Right hand rule used for generators |
👆👇 |
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Describe the three methods of inducing an emf in a conductor. |
Moving the conductor through the magnetic field Changing the magnetic field across the conductor Mutual induction |
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What waveform is produced from a simple ac generator? |
Back (Definition) |
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What waveform is produced by a simple dc generator? |
Back (Definition) |
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State the three basic requirements of a dc machine |
Commutator Conduction coil Magnet |
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Sketch a rotor assembly belonging to a d.c generator |
Back (Definition) |
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Sketch and label a typical brush holder |
Back (Definition) |
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Why are the amateur and pole pieces in a dc generator laminated? |
It reduces any eddy currents being produced in these components. |
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Sketch a labelled diagram of a 4 pole d.c generator |
Back (Definition) |
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What is the two fold purpose of a commutator? |
To collect the electrical power from the armature Rectify the power(unidirectional) produced by the armature |
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How does the commutator in a dc generator ensure the current in the load is unidirectional? |
The commutator is made up of segments that are insulated from each other. |
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What is the effect of adding more coils and segments to a dc generator? |
The output voltage has more regular peaks and a higher average voltage |
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Draw the schematic diagram of a lap wound armature 4 pole 12 coil |
Back (Definition) |
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Compare the outputs of two similar lap/wave wound armatures. |
Lap wound - produces a low voltage, high current output Wave wound- produces a high voltage, low current output Due to P= I V , power output is the same for both armatures |
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State the characteristics of a lap wound armature |
Same number of parallel paths as poles and brushes End of each coils is connected to the adjacent commutator segment Whole winding forms a continuous closed circuit |
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State the characteristics of a wave wound armature |
Only ever two parallel paths Only two brushes are required(more can be fitted) Coil ends are connected to segments opposite or set number apart Whole armature winding forms a continuous closed circuit |
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What are the two main causes of sparking at the brushes |
Faulty collection I.e incorrect brush pressure/ worn,damaged brushes Incorrect commutation(caused by design) |
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Why is reactive sparking undesirable? |
Causes damage to commutator and brushgear Decreases efficiency It can be a fire hazard Interference for electrical/radio equipment Additional maintenance required |
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Why does the magnetic neutral axis (MNA) move when the dc generator is loaded? |
This is due to the conductors in the armature producing there own magnetic field which causes distortion in the main field flux |
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What is the purpose of the compensation windings and where are they located? |
The compensation windings are wound through slots located in the main pole shoes. They produce a field to compensate for the armature winding distortion They are in series with inter poles and the armature winding, this means they produces a magnetic field proportional to the field generated by the windings in the armature |
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Briefly describe armature reaction |
Distorts the MNA in the direction of rotation Since the MNA is no longer inline with the GEOMETRIC NEUTRAL AXIS(GNA), this can cause sparking. The effect of the reaction is proportional to the current flowing in the armature windings, therefore as the current increases sparking is seen to increase. |
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What is the effect of moving the brushes from the GNA in the direction of rotation? |
This will allow the distorted MNA to align with the GNA once it has started rotating. Sparking may happen on start up prior to the MNA and GNA coming into alignment |
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List three ways of minimising armature reaction |
Inter poles- located in between the main poles, produce a magnetic field in order to counter the armature distortion Compensation windings- located in the main poles, distort the field prior to reaching the armature windings High resistance brushes- change the time constant allowing quicker current decay |
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