Introduction
We know from the law of conservation of energy that energy cannot be created or destroyed — it only changes form. In a DC machine, not all input power is converted into useful output. Some portion is inevitably lost as heat, friction, or magnetic inefficiency. These are called losses in DC machine.
Understanding these losses is essential because they directly affect the efficiency of the machine. Whether you're designing a motor, selecting one for an application, or preparing for exams — knowing where power is lost and why helps you make better engineering decisions.
Table of Contents
- Classification of Losses
- Copper Losses (Electrical Losses)
- Iron Losses (Core Losses)
- Mechanical Losses
- Stray Losses
- Constant vs Variable Losses
- Efficiency of DC Machine
- How to Reduce Losses
- FAQs
Classification of Losses in DC Machine
Losses in a DC machine can be broadly classified into four categories:
- Copper Losses — due to current flowing through windings
- Iron Losses (Core Losses) — due to alternating magnetization of the core
- Mechanical Losses — due to friction and air resistance
- Stray Losses — miscellaneous losses difficult to calculate exactly
Let's examine each type in detail.
Copper Losses (Electrical Losses)
Copper losses occur due to the resistance of the conductors carrying current. When current flows through any wire, power is dissipated as heat according to the formula I²R. In a DC machine, copper losses occur in multiple windings.
Armature Copper Loss
This is the most significant copper loss in a DC machine:
Where:
- Ia = Armature current
- Ra = Armature winding resistance
This loss varies with load — as the machine delivers more power, armature current increases, and copper loss increases as the square of current.
Field Winding Copper Loss
For a DC shunt machine:
For a DC series machine, the field winding carries the full armature current:
In a shunt machine, field copper loss is nearly constant (since If = V/Rf is constant). In a series machine, it varies with load.
Brush Contact Loss
There is also a small voltage drop at the brush-commutator contact (typically 1-2V per brush). The power lost is:
Iron Losses (Core Losses)
Iron losses occur in the armature core because it rotates in a magnetic field. As different parts of the core pass under North and South poles alternately, the magnetic flux through the core keeps reversing. This causes two types of losses:
Hysteresis Loss
When the armature rotates, each portion of the core undergoes magnetic reversal — it gets magnetized in one direction under the North pole, then in the opposite direction under the South pole. This continuous reversal requires energy to realign the magnetic domains in the iron.
The energy consumed per cycle of magnetization is represented by the area of the B-H (hysteresis) loop of the core material.
Where:
- η = Steinmetz hysteresis coefficient (depends on core material)
- Bmax = Maximum flux density
- f = Frequency of magnetic reversal
- V = Volume of the core
How to reduce: Use silicon steel (CRGO steel) for the armature core, which has a narrow hysteresis loop and low hysteresis coefficient.
Eddy Current Loss
When the armature core rotates in the magnetic field, an EMF is induced in the core body itself (as per Faraday's law). This EMF drives circulating currents within the core called eddy currents. These currents serve no useful purpose and simply heat up the core.
Where:
- Ke = Eddy current coefficient
- t = Thickness of lamination
- f = Frequency of flux reversal
How to reduce: Build the armature core from thin laminations (0.3–0.5 mm) insulated from each other with varnish. This breaks the path for eddy currents, drastically reducing the loss.
Why Are Iron Losses Considered Constant?
In a DC machine running at constant speed and constant supply voltage, both the flux density and frequency of reversal remain nearly constant regardless of load. Therefore, iron losses are practically independent of load — they remain the same whether the machine is at no-load or full-load.
Mechanical Losses
These losses are due to the physical rotation of the machine and include:
- Bearing friction loss — friction in the shaft bearings
- Brush friction loss — friction between brushes and commutator surface
- Windage loss — air resistance (drag) on the rotating armature
Mechanical losses are relatively small compared to copper and iron losses. Like iron losses, they are approximately constant at a given speed — they don't change significantly with load.
How to reduce: Use high-quality bearings, proper lubrication, and aerodynamic armature design.
Stray Losses
Stray losses (also called miscellaneous losses) include all losses that are difficult to measure or calculate precisely:
- Short-circuit currents in coils undergoing commutation
- Distortion of magnetic flux due to armature reaction
- Eddy currents in conductors due to slot leakage flux
Stray losses are typically estimated as 1% of the full-load output for practical calculations.
Constant vs Variable Losses
This classification is important for understanding efficiency and for tests like Swinburne's test:
Key insight: Maximum efficiency occurs when variable losses equal constant losses (Wv = Wc). This is a fundamental result used in machine design.
Efficiency of DC Machine
The efficiency of a DC machine is defined as:
For a DC motor:
For a DC generator:
Typical efficiency of DC machines ranges from 85% to 95% depending on size and design.
How to Reduce Losses — Summary
FAQs
Which loss is the largest in a DC machine?
Armature copper loss (Ia²Ra) is typically the largest loss, especially at full load. It increases with the square of the load current, making it the dominant loss under heavy loading conditions.
Why are iron losses called constant losses?
Because they depend on flux density and speed — both of which remain nearly constant in a DC machine operating at rated voltage and speed. They don't change significantly whether the machine is lightly loaded or fully loaded.
What is the condition for maximum efficiency?
Maximum efficiency occurs when variable losses (primarily Ia²Ra) equal constant losses (iron + mechanical + shunt field). At this point, the total loss curve has its minimum relative to output power.
Why is the armature core laminated?
Lamination breaks the core into thin insulated sheets, which restricts the path of eddy currents. Since eddy current loss is proportional to the square of lamination thickness (t²), thinner laminations dramatically reduce this loss.
How are losses measured in practice?
Constant losses are measured using Swinburne's test (a no-load test). The machine is run at rated voltage with no load — the input power at no-load equals the constant losses (since variable losses are negligible at no-load).
Conclusion
Losses in a DC machine reduce its efficiency and generate unwanted heat. The four main categories — copper losses, iron losses, mechanical losses, and stray losses — each have specific causes and reduction methods. Understanding the distinction between constant and variable losses is crucial for efficiency calculations and machine testing.
For practical applications, remember: maximum efficiency occurs when variable losses equal constant losses, and this principle guides the design of machines for their expected loading conditions.