Basic Motor Theory (7)


Energy Conversion As stated above, mechanical energy is changed into electrical energy by movement of conductor through a magnetic field. The converse of this is also true. If electrical energy is supplied to a conductor lying normal to a magnetic field, resulting in current flow in the conductor, a mechanical force and thus mechanical energy will be produced.
Producing Mechanical Force As in the generator, the motor has a definite relationship between the direction of the magnetic flux, the direction of motion of the conductor or force, and the direction of the applied voltage or current.
Since the motor is the reverse of the generator, Fleming’s left hand rule can be used. If the thumb and first two fingers of the left hand are extended at right angles to one another, the thumb will indicate the direction of motion, the forefinger will indicate the direction of the magnetic field, and the middle finger will indicate the direction of current. In either the motor or generator, if the directions of any two factors are known, the third can be easily determined.
Value of Mechanical Force The force exerted upon a current carrying conductor is dependent upon the density of the magnetic field, the length of conductor, and the value of current flowing in the conductor. Assuming that the conductor is located at right angles to the magnetic field, the force developed can be expressed as follows:
F = (B I) / 10
F = force in dynes
B = flux density in lines per square centimeter
= length of the conductor in centimeters
I = current in amperes.
At the same time torque is being produced, the conductors are moving in a magnetic field and generating a voltage. This voltage is in opposition to the voltage that causes current flow through the conductor and is referred to as a countervoltage or back EMF. The value of current flowing through the armature is dependent upon the difference between the applied voltage and the countervoltage.
Sample Calculations Generator
N = 60 turns
B = 40,000 lines per square inch
= 3.0 inches
v = 600 inches per second
E = voltage
E = 60 x 40,000 x 3 x 600 x 10-8 = 43.2 volts Motor
B = 6,000 lines per square centimeter
= 10 Centimeters
I = 50 amps
F = force
F = (6,000 x 10 x 50) / 10 = 300,000 dynes
Newtons = Pounds x 4.44823
Dynes = Newtons x 100,000

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