Hi,
>> Now, we have a brush DC motor. We show that in a circuit as RLE load, am I right?
>> R-> internal resistance, L -> inductance due to armature coils, E -> back emf due to rotation.
R= Resistance of the windings, L= Inductance of the windings, E= - Emf generated due to the angular rotation
>> Assume that I've powered the motor. At that instant, current is at its max because of zero speed of armature.
The current is not a max at that instant, due to the inductance of the winding, it quickly rises to a maximum.
The current is creating the initial magnetic field within the motor.
>> During the speed up, back emf occurs and opposes the main supply voltage.
>> The back EMF is proportional to the speed of rotation. At this time current begns do decrease.
>> First question; when current decreases what happens to inductor, due to dI/dt change?
Nothing happens to the inductor, its a function of the windings and the motor core.
>> Does inductor forms another back emf that opposes the first created back emf? Because it tries to maintain current?
No.
>> Later at the instant we cut the power off, some says that very large back emf occurs and damages the switch if we have not any flyback diode.
>> Another question; what's the origin of that huge back emf? In my mind, if we cut the power off, main supply voltage becomes zero and only voltage occurs due to still rotating armature. And this cannot be larger than main supply voltage.
Lenzs Law states that EMF = -L * di/dt, as the inductance is a function of the winding and core, its the rate of change of current with respect to time that generates this back EMF.
So if the motor is running at say full load and you switch OFF the supply voltage, in say, 10mSec, assuming the inductance to be 1 H and the current was say, 10 amps, then you would get 1 * 10/0.01 = - 1000 volts.
Its important to remember this back emf will generate a current in the opposite sense to the original applied voltage and it will cause regenerative breaking of the motor.
The flyback diode provides a path for the current due to the back EMF and effectively clamps it to a safe value.
>> If you say that, this huge emf forms because of the inductor (dI/dt), why there's no emf like this at the begining of the >> procedure (when powering the system)
A back EMF is generated whenever a current flowing in a magnetic circuit is suddenly switched off.