As already explained, when the motor armature continues to rotate due to motor action, the armature conductors cut the magnetic flux and, therefore, emfs and induced in them. The direction of this induced emf, known as Back emf, is such that it opposes the applied voltage.
Since the back emf is induced due to the generator action, the magnitude of it is, therefore, given by the same expression as that for the generated emf in a generator
i.e. Back emf, Eb = ΦZNP/A*60
Since the back emf is induced due to the generator action, the magnitude of it is, therefore, given by the same expression as that for the generated emf in a generator
i.e. Back emf, Eb = ΦZNP/A*60
where Φ = flux per pole in webers
Z = the total number of armature conductors
N = the rotational speed of armature in rpm
P = the number of poles
A = the number of parallel paths in the armature
The equivalent circuit of a motor in shown in fig. The armature circuit is equivalent to a source of emf, Eb in series with a resistance, Ra put across a dc supply main of V volts. It is evident from fig. that the applied voltage V must be large enough to balance both the voltage drop in armature resistance and the back emf at all times i.e.
V = Eb + IaRa ........... ....................... ..........(a)
Where V = applied voltage across the armature
Eb = the induced emf in the armature by generator action
Ia = the armature current
Ra = the armature resistance
The expression (a) may be rewritten as Ia = (V – Eb) / Ra
to give armature current in terms of applied voltage V, induced emf Eb and armature resistance Ra
As obvious from expressions from above equations the induced emf in the armature of a motor, Eb depends among other factors upon the armature speed and armature speed and armature current depend upon the back emf Eb for a constant applied voltage and armature resistance. If the armature speed is high, "back emf" Eb will be large and, therefore, the armature current small. If the armature speed is low, then back emf Eb will be less and armature current Ia more resulting in the development of large torque.
The presence of 'back emf' makes the dc motor a self-regulating machine i.e. it makes the dc motor to draw as much armature current as is just sufficient to develop the required load torque. This is explained below:
When the motor is operating at no load, small torque is required to overcome the friction and windage losses, therefore, back emf is nearly equal to the applied voltage and armature current is small. When the motor is loaded, the driving torque of the motor is not sufficient to counter the increased retarding torque due to load and the effect is to cause the armature to slow down. With the decrease in the speed of armature nack emf falls. The reduced back emf allows a larger current to flow through the armature. The increase in armature current results in higher electromagnetic driving torque. The motor continues to slow down till the electromagnetic torque developed matches the load torque and the steady sate condition are attained. The reverse phenomenon occurs when the mechanical load in the motor falls.
When the load on the motor falls, the electromagnetic torque developed in momentarily in excess of the load requirement and, therefore, the motor armature accelerates. With the increase in armature speed, back emf increases causing armature current to decrease. The decrease in armature current causes a decrease in electromagnetic torque and the steady-state conditions are attained when the electromagnetic torque developed matches the load torque.
Thus it is evident that back emf acts like a governor i.e. it makes a motor self-regulating so that it draws as much current as just required.
The presence of 'back emf' makes the dc motor a self-regulating machine i.e. it makes the dc motor to draw as much armature current as is just sufficient to develop the required load torque. This is explained below:
When the motor is operating at no load, small torque is required to overcome the friction and windage losses, therefore, back emf is nearly equal to the applied voltage and armature current is small. When the motor is loaded, the driving torque of the motor is not sufficient to counter the increased retarding torque due to load and the effect is to cause the armature to slow down. With the decrease in the speed of armature nack emf falls. The reduced back emf allows a larger current to flow through the armature. The increase in armature current results in higher electromagnetic driving torque. The motor continues to slow down till the electromagnetic torque developed matches the load torque and the steady sate condition are attained. The reverse phenomenon occurs when the mechanical load in the motor falls.
When the load on the motor falls, the electromagnetic torque developed in momentarily in excess of the load requirement and, therefore, the motor armature accelerates. With the increase in armature speed, back emf increases causing armature current to decrease. The decrease in armature current causes a decrease in electromagnetic torque and the steady-state conditions are attained when the electromagnetic torque developed matches the load torque.
Thus it is evident that back emf acts like a governor i.e. it makes a motor self-regulating so that it draws as much current as just required.
Reference
Theory & Performance of Electrical Machines
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