- DC Machine Losses Definition: In a DC machine, losses refer to the input power that doesn’t convert into useful output power, reducing efficiency.
- Copper Losses: These occur in the windings due to resistance and are divided into armature loss, field winding loss, and brush contact resistance loss.
- Core Losses: These include hysteresis loss, due to the constant reversal of magnetization in the armature, and eddy current loss, caused by induced emf in the iron core.
- Mechanical Losses: Caused by friction in moving parts and air resistance, typically accounting for about 15% of total losses.
- Hysteresis Loss in DC Machine: This specific type of core loss occurs due to the reversal of magnetization in the armature core, consuming energy.
A DC generator converts mechanical input power into electrical output power, while a DC motor performs the reverse conversion. In either case, the output is lower than the input because some energy becomes heat, sound and air movement. The difference between input and useful output is the total loss. These losses reduce efficiency and raise the machine temperature. Engineers commonly group them as winding, core, mechanical and stray-load losses in a DC machine.
Copper Losses or Electrical Losses in DC Machine or Winding Loss
Winding loss occurs when current flows through a conductor with finite resistance. Each winding term follows I²R at its operating temperature. A wound-field DC machine has armature and field circuits, and some designs also have series, interpole or compensating windings.
The electrical-loss calculation therefore includes the applicable armature and field winding losses. Brush-contact loss is a separate term based on contact voltage drop and armature current. Winding losses vary with current, and their resistance rises as the conductors heat.
Armature Copper Loss in DC Machine
Armature copper loss = Ia2Ra
Where, Ia is armature current and Ra is armature resistance at the relevant winding temperature.
The share of total loss depends on machine design, load and temperature; it is not a fixed percentage.
Field Winding Copper Loss in DC Machine
Field winding copper loss = If2Rf
Where, If is field current and Rf is field resistance at the relevant winding temperature.
Field loss can remain approximately steady when field current is regulated, but it varies when excitation or a series-field current changes with load.
Brush Contact Resistance Loss in DC Machine
Brush-contact loss results from the voltage drop at the resistance between each brush and the commutator. It is normally calculated from the total brush-contact voltage drop multiplied by armature current. The drop depends on brush material, current density, contact condition and commutator surface.
Core Losses or Iron Losses in DC Machine or Magnetic Losses
The rotating armature core passes through alternating regions of the main magnetic field. Its magnetisation therefore changes as each section moves from one pole to the next. Core loss has two principal components: Hysteresis loss and Eddy current loss. These losses can be approximately steady only when speed, flux and temperature remain approximately steady.
Hysteresis Loss in DC Machine
The rotating core that carries the armature winding undergoes repeated magnetic reversal. A core section under a south pole moves under a north pole after half of the pole-pair cycle, so its magnetic flux direction reverses. The energy dissipated during each magnetisation loop appears as hysteresis loss. Its magnitude depends on the material, core volume, flux density and reversal frequency.
The Frequency of Magnetic Reversal
Where,
P = Number of poles
N = Speed in rpm
Steinmetz Formula
The Steinmetz formula provides an empirical estimate of hysteresis loss over the range for which its material constants were obtained.
Where,
η = Steinmetz hysteresis co-efficient
Bmax = Maximum flux Density in armature winding
F = Frequency of magnetic reversals
V = Volume of armature in m3.
Eddy Current Loss in DC Machine
Under Faraday’s law of electromagnetic induction, changing flux through the conductive armature core induces voltage. This voltage drives circulating currents within the core material. The currents dissipate power as heat and do not contribute to useful output. Eddy-current loss depends on flux density, reversal frequency, material resistivity and lamination thickness. Thin insulated laminations interrupt the current paths and reduce this loss in the magnetic field.
Mechanical Losses in DC Machine
Mechanical losses include bearing friction, brush friction and windage from the rotating parts moving through air. They depend on speed, bearing condition, brush pressure, ventilation and machine construction. Their share of full-load loss must be measured or calculated for the particular machine rather than assumed as 15%.
Stray Load Losses in DC Machine
Stray-load loss covers additional load-dependent effects that are not included in the main calculated terms. Sources can include commutation currents, local eddy currents and flux distortion caused by armature reaction. The value depends on machine design and operating point, so a test standard or manufacturer data should set the method used to determine it.
