Losses in DC Machines

In all machines, the whole of input energy does not get convertered into output energy. A portion of the input energy gets lost in the machine. DC motor is not exceptional in this case.

I²R loss in the armature winding

Every winding consist of resistance, when the current flow in the armature winding some parts of the energy is get lost (I_a²R_a loss) in the form of heat due to resistance in it, where I_a is the armature current and R_a is the resistance of the armature circuit. when load on the machine changes, as a result armature current too change. Thus I_a²R_a is called the variable loss as this loss varies with the variation of load on the motor.

Core loss or Iron loss in the armature

Core loss or iron loss consist of both hysteresis loss and eddy current loss. The core of any electrical device or machine is made of magnetic material like iron and is subjected to variation in magnetic flux. When a DC machine is given with supply, the armature rotates and comes under North poles and South poles alternately. Hysteresis loss occurs due to the alternate magnetization of the magnetic material. Hysteresis loss depends upon

• Flux density
• Frequency of variation of flux
• Volume of core material

On the other hand, eddy current loss is due to the presence of circulating current in the core material. Emf will induced in the armature core when it rotate in the magnetic field. This EMF causes a circulating current in the winding of the armature and dissipated in the form of heat. Eddy current loss depends upon

• Flux density
• Frequency of alternation of flux
• Thickness of laminations used for winding
• Volume of core material

Loss in the field windings

When the current flow in the field winding, some parts of energy is dissipated in the form of heat as in armature winding. Field winding loss = VI_f, where V is the applied voltage and I_f is the field current.

Friction and windage losses

Due to rotation of the armature, air-friction loss which is also called windage loss, take place. Frictional loss occurs due to brush and commutator rubbing and loss due to bearing friction.