DC MACHINE CONFIGURATION.

DC MACHINE CONFIGURATION.

A simple two-pole stator of a DC machine is shown in figure below. The field winding consists of an N turn concentrated winding of which each half is wrapped around a pole. The two parts of the winding are connected in series and attached to a DC power supply. The frame is in this case also

the yoke-part of the magnetic flux path. The use of a field winding gives us the ability to control the magnetic flux in the circuit in terms of amplitude and polarity. Permanent magnets are often used to replace the field winding which leads to a more compact and efficient machine. However, we loose in practical terms one degree of freedom, as we are now unable to control (in electrical terms) the flux magnitude during operation. We also loose the potential of operating the machine on an AC source, therefore universal machines (AC/DC) always have a field winding and no permanent magnets.
A very simple example of a DC rotor is given in figure, which shows the same single turn winding introduced for the synchronous machine. In this case, the slip ring/brush combination is replaced by a so-called commutator. This commutator consists, for this simple rotor, of two brushes and two commutator segments. Segment 1 and 2 are connected to coil side A and B respectively.

The brushes are connected to a direct current (DC) power supply. The purpose of the commutator is to reverse the current polarity in the rotor winding every half revolution in this case. For example, in the case shown, coil side A is connected via segment 1 and a brush to terminal 1. Likewise side B is connected via segment 2 and a brush to terminal 2. When we rotate the rotor by 180 degrees, side A will be electrically connected to terminal 2 and side B to terminal 1. This means that a current reversal in the winding will take place twice during one period. In reality the number of segments is greater than 2 and this has a marked effect on the torque ripple.