In dc motor with shunt wound element:
-The armature voltage VA is the sum of the internal armature EMF VE and the volt drop due to the armature current IA flow through the armature resistance RA.
Armature Voltage V A
= V E
- The DC motor speed is directly proportional to the armature back EMF VE
and indirectly proportional to the field flux Φ, which in turn depends on the field excitation current IE. Thus, the rotational speed of the motor can be controlled by adjusting either the armature voltage, which controls VE, or the field current, which controls the Φ.
Motor Speed n ∝ V E /Φ
- The output torque T of the motor is proportional to the product of the armature current and the field flux.
Output Torque T ∝ IA
- The direction of the torque and direction of rotation of the DC motor can be reversed either by changing the polarity of Φ, called field reversal, or by changing the polarity of IA, called armature current reversal. These can be achieved by reversing the supply voltage connections to the field or to the armature.
Therefore four-quadrant operation in dc motor just applicable in separated exited motors.
The torque is directly proportional to the armature current, which in turn depends on the difference between the applied voltage V and the back e.m.f. E. We can therefore make the machine develop positive (motoring) or negative (generating) torque simply by controlling the extent to which the applied voltage is greater or less than the back e.m.f. An armature voltage controlled d.c. machine is therefore inherently capable of what is known as ‘four-quadrant’ operation, with reference to the numbered quadrants of the torque–speed plane shown in Figure below.
In the dual converter, one rectifier provides the positive current to the load and the other the negative current. Due to the instantaneous voltage differences between the output voltages of the converters, a circulating current flows through the bridges. The circulating current is normally limited by circulating reactor, LD, as shown in Figure below. The two converters are controlled in such a way that if α+ is the delay angle of the positive current converter, the delay angle of the negative current converter is α− = 180◦ − α+