Losses in DC Machine

Energy cannot be created or destroyed, only transferred from one form to another. In a DC machine, mechanical energy converts into electrical energy. However, not all input power becomes output power; some is lost in various forms. These losses cause the machine to heat up and reduce its efficiency. There are four main categories of energy loss in a DC machine.

Copper Losses or Electrical Losses in DC Machine or Winding Loss

The copper losses are the winding losses taking place during the current flowing through the winding. These losses occur due to the resistance in the winding. In a DC machine, there are only two windings, armature and field winding.
Copper losses are divided into three parts: armature loss, field winding loss, and brush contact resistance loss. These losses are proportional to the square of the current flowing through the windings.

Armature Copper Loss in DC Machine

Armature copper loss = I_a²R_a
Where, I_a is armature current and R_a is armature resistance.
These losses are about 30% of the total full load losses.

Field Winding Copper Loss in DC Machine

Field winding copper loss = I_f²R_f
Where, I_f is field current and R_f is field resistance.
These losses are about 25% theoretically, but practically it is constant.

Brush Contact Resistance Loss in DC Machine

Brush contact loss occurs due to the resistance between the brush and commutator surfaces. This loss is part of the variable losses and contributes to both types of copper losses. It is considered when calculating the total copper losses.

Core Losses or Iron Losses in DC Machine or Magnetic Losses

As iron core of the armature is rotating in magnetic field, some losses occurs in the core which is called core losses. Normally, machines are operated with constant speed, so these losses are almost constant. These losses are categorized in two form; Hysteresis loss and Eddy current loss.

Hysteresis Loss in DC Machine

Hysteresis losses occur in the armature winding due to reversal of magnetization of the core. When the armature core is exposed to the magnetic field, it undergoes a complete rotation of magnetic reversal. The part of the armature under the S-pole moves under the N-pole after half a revolution, reversing the magnetic lines to change the magnetism in the core. This constant magnetic reversal consumes energy, called hysteresis loss. The amount of this loss depends on the iron’s quality and volume.

The Frequency of Magnetic Reversal

Where,
P = Number of poles
N = Speed in rpm

Steinmetz Formula

The Steinmetz formula is for the calculation of hysteresis loss.

Where,
η = Steinmetz hysteresis co-efficient
B_max = Maximum flux Density in armature winding
F = Frequency of magnetic reversals
V = Volume of armature in m³.

Eddy Current Loss in DC Machine

According to Faraday’s law of electromagnetic induction, when an iron core rotates in the magnetic field, an emf is also induced in the core. Similarly, when armature rotates in the magnetic field, the small amount of emf induced in the core which allows the flow of charge in the body due to the conductivity of the core. This current is useless for the machine. This loss of current is called eddy current. This loss is almost constant for the DC machines. It could be minimized by selecting the laminated core.

Mechanical Losses in DC Machine

The losses associated with mechanical friction of the machine are called mechanical losses. These losses occur due to friction in the moving parts of the machine like bearing, brushes etc, and windage losses occur due to the air inside the rotating coil of the machine. These losses are usually very small about 15% of full load loss.

Stray Load Losses in DC Machine

Other losses, known as stray-load losses, also occur. These miscellaneous losses result from short-circuit currents in the coil during commutation and flux distortion due to the armature. These losses are hard to identify but are generally considered to be about 1% of the total load power output.