A Brief insight to basic theory & operation of motor.

The discovery that led to the invention of the electric motor was Lenz’s law : A current carrying
conductor will experience a force when placed in a magnetic field. The conductor can be any metal—iron, copper, aluminum, and so on. The direction of the force is perpendicular to both the magnetic field and the current.

F = IBL sin θ
where
F = force on the conductor (in Newtons)
I = current through the conductor (in amperes)
B = magnetic flux density (in gauss)
L = length of the wire (in meters)
θ = angle between the magnetic field and current

An electric motor must harness this force in such a way as to cause a rotary motion. This can be done by forming the wire in a loop and placing it in the magnetic field. The loop (or coil) of wire is allowed to rotate about the axis shown and is called the armature winding. The armature is placed in a magnetic field called the field. The commutator and brushes supply current to the armature while allowing it to rotate.

Counter-EMF (CEMF) or Back EMF:

As the armature in the motor is rotating and thus cutting the surrounding magnetic field; a certain voltage is generated the armature according to Faraday’s law of electromagnetic induction.

It has the opposite polarity of the line voltage;hence, it is called the counter-EMF (CEMF). Its effect is to cancel out some of the line voltage. In other words, the actual voltage available to the armature is the line voltage minus the CEMF:
VA = Vln – CEMF
where
VA = actual voltage available to the armature
Vln = line voltage supplied to the motor
CEMF = voltage generated within the motor

Significance of Counter EMF:

Ia = (Vln –CEMF)/Ra.

Ia = armature current
Vln = line voltage to the motor
Ra = armature resistance
CEMF = voltage generated within the motor

Equation tells us that the armature current is a function of the applied voltage minus the CEMF. Because CEMF increases with motor speed, the faster the motor runs, the less current the motor will draw, and consequently its torque will diminish. This explains why most DC motors have a finite maximum speed; eventually, if the motor keeps going faster, the CEMF will nearly cancel out the line voltage, and the armature current will approach zero.

Speed regulation is the ability of a motor to maintain its speed when the load is applied. The basis of this self-regulation is the CEMF. When the motor’s load is increased, the speed tends to decrease, but the lower speed reduces the CEMF, which allows more current into the armature. The increased current results in increased torque, which prevents the motor from slowing further.

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