DC machines deal with two namely DC motor and DC generator.
Generator:
It is a device that converts applied mechanical energy into electrical energy.
DC Generator:
It works on the principle of Faraday's law of electromagnetic induction.
In a DC generator, the output will be at the rated voltage with zero frequency.
Faraday's law:
Whenever a conductor cuts the flux or when flux cuts the conductor then an emf will be induced in the conductor.
Using Fleming's Right-Hand Rule can find the direction of the emf induced in the conductor.
Fleming's Right-Hand Rule:
If the forefinger points to the direction of the flux and the thumb indicate the direction of motion of the conductor then the middle finger indicates the direction of the emf/current.
In the case of a DC generator, the conductor is always a rotating type with the field being stationary but in a synchronous generator, a stationary conductor is preferred with the rotating field.
In motor as per Fleming's left-hand rule, the forefinger points to the direction of the flux and the thumb points to the direction of the force or the torque developed and it rotates in a counterclockwise direction so that the current flows towards the conductor as mentioned in the figure as X cross. In a motor the torque developed will be always in the same direction of rotation.
Here as shown in the figure the current flows from A to B and from C to D as per Fleming's left-hand rule the current goes towards and the direction is from A to B and comes outwards from C to D and then it flows through the commutator and the brushes where it is connected to the resistive load. When the rotor rotates the commutator also rotates and the brush will be stationary as the resistive load is stationary.
Here as shown in the figure the current flows from D to C and from B to A as per Fleming's left-hand rule
A simple loop generator consists of a single turn connected to a resistive load via a commutator and brushes. A commutator converts from AC to DC and it acts as a mechanical rectifier and the nature of the voltage across the load is pulsating DC. Hence it is called a mechanical rectifier.
Right-hand thumb rule/ Grip rule:
If the right thumb indicates the direction of current then the curled finger indicates the direction of the flux and vice versa.
DC Motor:
The motor converts the applied electrical energy into mechanical energy.
A DC motor rotates at the speed of N rpm and operates at the unity power factor always.
Lorrentz's force is the principle of the motor.
Whenever a current-carrying conductor is placed in a magnetic field then it will experience a force, which is converted into torque using the rotational system, therefore if the direction of the torque is in a clockwise direction then the motor also rotates in a clockwise direction. The direction can be easily found by Fleming's Left-hand rule.
Fleming's Left-hand rule:
If the forefinger points to the direction of the flux and the thumb indicate the direction of force or torque of the conductor then the middle finger indicates the direction of the emf/current.
Simple loop generator with commutator:
The pulsating DC can be converted into a pure DC by increasing the number of coils in the armature.