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DC Motors and DC Generators with stationary permanent magnetic fields have very similar action principles. |
If you supply a (DC) voltage to the rotating coil above at a & b Brushes, you will cause the rotating coil on the shaft member to turn. Thus you have a permanent field type (DC) motor, like a toy motor.
Here we are converting electrical (DC) energy to rotational mechanical energy.
If you physically rotate the rotational member (armature) by a hand crank or some form of prime mover (a motor, gas engine, turbine engine or water wheel from a mill etc.) this same piece of machinery becomes a (DC) generator and we receive electricity to power various electrical devices such as light bulbs, electric motors, arc welders, power tools or household appliances.
Here are some basic principles that cause the generator effect to take place. In all three sketches, a conductor (wire) is connected to a voltmeter and the magnetic field of a magnet in the vicinity of the conductor.
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If the magnet is moved to the left, the meter pointer will deflect to the right, showing that some voltage is being induced. |
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If the magnet is moved to the right, the meter pointer will deflect to the left, showing that some voltage is being induced in the opposite direction. |
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If the magnet is held absolutely still, there is no meter deflection whatsoever. |
If the magnet were to remain stationary and the conductor moved the same results would occur.
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If the conductor is wound into a coil the generated voltage and current are greater. |
The voltage generated by a specific generator is as follows:
Eg = K O S
K = a design constant
O = lines of magnetic flux per pole
S = speed of the armature
From this formula you can see that having a higher O which indicates a stronger magnet with more lines of magnetic flux per pole, a higher output voltage and current will be produced at the same speed.
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Now that you have learned what makes a (DC) generator and a (DC) motor tick, we can learn first hand what happens when you power some lights from the hand crank generator/motor.
Trial 1. First crank the generator/motor with one 1-amp bulb turned on. Concentrate on how hard you have to crank and notice the magnitude of the voltage and current meters. Crank at a comfortable speed.
Voltmeter Reading: _______ Volts Ammeter Reading ______ Amps
Cranking Effort: ___ Low ___ Medium ___ High
Trial 2. Then crank the generator/motor with one 2-amp bulb turned on. Again concentrate on how hard you have to crank and notice the magnitudes indicated on the voltage and current meters. Remain cranking at the same speed as before.
Voltmeter Reading: _______ Volts Ammeter Reading ______ Amps
Cranking Effort: ___ Low ___ Medium ___ High
Trial 3. Then crank the generator/motor with two 1-amp bulbs in parallel turned on. Again notice how hard you have to crank to power these bulbs and the voltage and current readings. Remain cranking at the same speed as before.
Voltmeter Reading: _______ Volts Ammeter Reading ______ Amps
Cranking Effort: ___ Low ___ Medium ___ High
Trial 4. Then crank the generator/motor with two 1-amp bulbs in series turned on. Again notice how hard you have to crank to power these bulbs and the voltage and current readings. Remain cranking at the same speed as before.
Voltmeter Reading: _______ Volts Ammeter Reading ______ Amps
Cranking Effort: ___ Low ___ Medium ___ High
Conclusions
What you are experiencing from this experiment is that it requires more mechanical effort as you power more light bulbs, thus supplying increasing light loads with electrical energy.
Trials 2 & 3 require the highest cranking effort (the most work) because they present the heaviest load (one 2-amp bulb or two 1-amp bulbs in parallel).
Trial 1 requires the second highest cranking effort (one 1-amp bulb).
Trial 4 requires the lowest cranking effort (two 1-amp bulbs in series.
Real Life Application
You may have observed with your automobile that belt slippage is more likely with more lights and electrical devices on.
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