The losses in a DC machine can be divided as,
Where,
- Ia is armature current
- It is the field current (For a series motor, Ia = If = I)
- Ra is armature resistance
- Rf is field resistance
- N is the rotating speed of DC machine
Armature copper losses in DC Machine:
- Armature losses can occur due to several factors, including resistance losses, eddy current losses, and brush friction losses.
- Resistance losses occur when the electrical current flowing through the armature encounters resistance, which causes some of the electrical energy to be converted into heat.
- Eddy current losses occur when the magnetic field induces currents in the armature, which also generates heat.
- These losses are about 30%-40% of the total full load losses.
- It vary significantly with the load current.
Field copper losses:
- Field copper losses refer to the energy losses that occur in the copper windings of the field coils in a motor or generator.
- These losses occur due to the resistance of the copper wire, which causes some of the electrical energy to be converted into heat.
- These losses are about 25% theoretically, but practically it is constant.
Iron Loss or Core Loss:
- These losses occur in the armature of a d.c. machine and are due to the rotation of armature in the magnetic field of the poles.
- It depends on the frequency (Speed) and voltage and does not depend on load or load current.
(i) Hysteresis loss in DC Machine:
- Hysteresis loss occurs in the armature of the DC machine since any given part of the armature is subjected to magnetic field reversals as it passes under successive poles.
- The amount of hysteresis loss in a magnetic material depends on factors such as the frequency and amplitude of the magnetic field, as well as the properties of the material itself.
- To reduce Hysteresis loss in a DC machine, the armature core is made of such materials which have a low value of Steinmetz hysteresis co-efficient or high permeability e.g., silicon steel.
(ii) Eddy current loss in DC Machine:
- In addition to the voltages induced in the armature conductors, there are also voltages induced in the armature core.
- These voltages produce circulating currents in the armature core which causes eddy current loss.
Mechanical losses in DC Machine:
- These losses are due to friction and windage.
- Friction loss e.g., bearing friction, brush friction, etc.
- Windage loss i.e., air friction of rotating armature.
- These losses depend upon the speed of the machine. But for a given speed, they are practically constant.
FAQs on Losses of DC Machine:
1. What are the constant losses in a DC machine?
- Constant losses in a DC machine, also known as core losses or iron losses, remain relatively unchanged under different operating conditions. These losses consist of two main components:
- Hysteresis Loss: This loss occurs due to the continuous magnetization and demagnetization of the core material with each cycle of operation.
- Eddy Current Loss: Caused by the circulation of currents within the core material, leading to energy dissipation as heat.
- Both hysteresis and eddy current losses depend on the frequency and the properties of the core material. To reduce these losses, manufacturers use laminated cores and high-quality materials.
2. How to reduce eddy current loss in a DC machine?
- Eddy current losses in a DC machine can be reduced by the following methods:
- Using Laminated Core: A laminated core minimizes the path for eddy currents, thereby reducing their magnitude. Each lamination is insulated from the others, preventing large circulating currents.
- Selecting High-Resistivity Materials: Using materials with higher electrical resistivity reduces the strength of eddy currents.
- Increasing the Frequency of the Magnetic Field: By keeping the operating frequency low, eddy current losses can be minimized.
- These steps effectively reduce heat generation and enhance the efficiency of the DC machine.
3. How to minimize losses in a DC machine?
Minimizing losses in a DC machine involves reducing both constant and variable losses:
- Use of High-Quality Materials: Select materials with low resistance for windings to reduce copper losses, and use laminated cores to minimize eddy current losses.
- Efficient Design: A well-designed armature and field winding ensure that energy conversion is as efficient as possible, lowering losses.
- Proper Cooling: Effective cooling mechanisms help maintain optimal machine performance and reduce thermal losses.
- Regular Maintenance: Timely maintenance of brushes, bearings, and windings prevents excess friction and wear, further minimizing losses.
4. What is stray loss in a DC machine?
Stray losses in a DC machine refer to the energy losses that are difficult to quantify precisely. These losses occur due to:
- Leakage Flux: Part of the magnetic flux that does not contribute to useful work.
- Friction and Windage Losses: Losses caused by friction between mechanical parts such as brushes and commutators.
- Leakage Currents: Unwanted currents flowing through insulation or across gaps.
- Stray losses are usually small but should be minimized to improve overall machine efficiency.
- Iron losses and mechanical losses together are called stray losses.
Important MCQs on the loss of DC Machines:
1. What is the primary cause of armature copper losses in a DC machine?
a) Brush friction
b) Eddy current losses
c) Resistance losses
d) Mechanical friction
Answer: c) Resistance losses
2. How much of the total full load losses in a DC machine are typically attributed to armature losses?
a) 10%-20%
b) 20%-30%
c) 30%-40%
d) 50%-60%
Answer: c) 30%-40%
3. Field copper losses in a DC machine are primarily caused by:
a) The resistance of the copper windings
b) Eddy current losses in the field coils
c) Friction between brushes and commutator
d) Hysteresis in the core material
Answer: a) The resistance of the copper windings
4. Which of the following factors does not affect iron loss or core loss in a DC machine?
a) Load current
b) Frequency (speed)
c) Voltage
d) Magnetic field reversal
Answer: a) Load current
5. Hysteresis loss in a DC machine depends on all of the following except:
a) Frequency and amplitude of the magnetic field
b) Magnetic properties of the armature material
c) Load current
d) The material's Steinmetz hysteresis coefficient
Answer: c) Load current
6. How can hysteresis loss be minimized in a DC machine?
a) Using materials with high resistivity
b) Using materials with low Steinmetz hysteresis coefficient
c) Increasing the load current
d) Reducing the machine's speed
Answer: b) Using materials with low Steinmetz hysteresis coefficient
7. Eddy current losses in the armature of a DC machine are caused by:
a) Friction between the brushes and the commutator
b) Circulating currents in the armature core due to induced voltages
c) Resistance of the copper windings
d) The speed of the armature
Answer: b) Circulating currents in the armature core due to induced voltages
8. What is the primary source of mechanical losses in a DC machine?
a) Eddy current losses
b) Friction and windage losses
c) Core hysteresis losses
d) Copper losses
Answer: b) Friction and windage losses
9. How do mechanical losses in a DC machine vary with speed?
a) They decrease with increased speed
b) They are constant for a given speed
c) They increase with decreased speed
d) They vary significantly with load current
Answer: b) They are constant for a given speed
10. Which material is commonly used to reduce hysteresis loss in a DC machine's armature?
a) Copper
b) Silicon steel
c) Aluminum
d) Iron
Answer: b) Silicon steel