Curious about Electric Motors?

Electric Motors work by following a rule called Fleming's Left Hand Rule. But before that rule, when an electric current flows through a conductor, it produces magnetic flux around the conductor.

Picture 1.

Right-hand grip rule, a rule concerned with current through a conductor. It says, Imagine  the wire to be held firmly in the right hand with the thumb pointing along the wire in the direction of the current. The direction of the fingers will give the direction of the magnetic flux. Like in the illustration above.

Note these symbols

Picture 2.

they mean...

Picture 3.

When a conductor currying current is placed in a magnetic field created by some other sources than its self, it experiences a force. The fields of the conductor carrying current tend to repel those of the permanent magnet and this produce a turning couple. See this in the picture below.

Picture 4. Magnetic field pattern of permanent magnet and coil of wire (conductor).

Picture 5. Simple direct current motor. Blocks labelled N and S are permanent magnets with a conductor carrying current (coil) placed between them.

Picture 6. A sketch to show Fleming's Left-hand rule

Fleming's Left-hand Rule states that , 'if the thumb (T) and the first two fingers of usually left hand are mutually at right angles with the first finger pointing in the direction of the field (B) and the second finger pointing in the direction of the current (I)  then the thumb (T) predicts the direction of the force or motion.

Picture 7. The coil of wire as shown in picture 5 above. Observe the direction of the flux when current is flowing through it.

By applying  Fleming's left-hand rule, the side where flux is anticlockwise will experience an upward force and the side where flux is clockwise will experience a downward force. These two forces cause the coil to rotate with its momentum supporting it.

The magnitude of the force acting on the conductor depends on:

1. The strength of magnetic field (B)
2. The length of the conductor cutting the magnetic flux (L),
3. The magnitude of current(I) flowing in the conductor
4. The angle between the magnetic field and the current

Picture 8. The length of the conductor as mentioned in the list above, that means the number of turns of the coil should be suitable enough for the motor to be powerful.

 

Picture 9. Electronic DC motors

Picture 10. Electromagnet

For better results, a number of coils should be wound on a soft iron armature made up of soft iron discs with slots. The coils are wound in slots. The armature, when magnetized, adds its magnetic flux to that of the coils. Also the commutator is multi-segmented. Electronic DC motors are often constructed this way.

Picture 11.

Picture 12. Permanent magnets inside

In larger motors, the magnetic field in which the armature rotates is produced by an electromagnet. Now let's get the difference, the coils of the electromagnet are called field coils and the coils of the armature are called armature coils. An example of an electromagnet is shown in picture 10.

OHM'S LAW

Ohm's law states that for a given current, the Potential difference (P.d) between two points is directly proportional to the resistance of the circuit between those points.

V = IR

Calculating Resistances in a Circuit

If current is to flow through a circuit, it must go against resistance with majority of it taking paths of least resistance and if resistance is great (in a certain path), current will not flow.
Circuits are designed with resistances both in parallel and in series.

Series Connection of Resistances
The figure below shows three resistors having resistances of R1, R2, R3, connected in series across the supply voltage labelled Vs.

Note. When resistors are connected in series the total current flowing through the circuit is the same (current supplied (Is)) while voltage drop depends on the value of the resistance of the individual resistor and total voltage of the circuit is the sum of the voltage drops of the individual resistors.
Vs = V1+V2+V3
From ohm's law
V=IR
Substitute V for IR in the equation Vs = V1+V2+V3
IsRT=IsR1+IsR2+IsR3 (Is being current supplied and RT being total resistance)
Divide both sides by Is
IsRT/Is=Is(R1+R2+R3)/Is (Is dies on both sides)
RT=R1+R2+R3

Parallel Connection of Resistances
For resistors in parallel connection, the supply voltage is the same while current flowing depends on the value of the resistance of the individual resistor. The total supply current is the sum of all the currents flowing  through the circuit.

In the figure above three resistors having resistances of R1, R2 and R3 respectively, are connected across supply voltage Vs.
From total current
Is = I1+I2+I3
And from Ohm's law
I = V/R
Substituting I for V/R in the equation Is = I1+I2+I3
Vs/RT = Vs/R1+Vs/R2+Vs/R3 (Vs being supply voltage and RT being total resistance)
Vs/RT *Vs = Vs(1/R1+1/R2+1/R3)/Vs
1/RT = 1/R1+1/R2+1/R3
If there are two resistors connected in parallel, the total resistance RT is given by product/sum (Product over Sum)
1/RT= 1/R1+1/R2
1/RT = R1+R2/R1*R2
RT = R1*R2 /R1+R2

Current, Potential difference and Resistance

In an electric circuit, there are three things to measure; Current, Potential difference and Resistance.

Current (I)
An electric current in a wire is a drift of electrons, but according to the convention adopted in 1800, current is regarded as the flow of positive electricity.
Current can also be defined as charge flowing in a circuit per unit time. Current (I) = Charge (Q)/time (t).
Current is measured in Amperes.
An Ampere is the Current which, if flowing in two straight parallel and infinitely long wires, placed one meter apart in a vacuum with a negligible cross sectional area, will produce on each of the wires a Force of 0.0000002 N per meter length of the wire.

Potential difference

Potential difference between 2 points x and y, in a circuit, is the work done when moving a unit of charge from x to y.

The SI unit is the Volt. A volt is the potential difference between 2 points of a circuit carrying a constant current of one ampere when the power dissipated between these points is one Watt.

The circuit consists of resistors R1 and R2 each having a potential difference, in volts, V1 and V2 depending on the value of their resistances. The arrow shows the path of convention current.

Resistance

This is the opposition to the flow of Current through a conductor. The SI unit of Resistance is the Ohm. An Ohm is the resistance in which current of one ampere flowing for one second generates one joule of thermal energy.