DC Machines: Principles, Types, and Applications
DC Machines
Introduction
The most common application of DC machines is as motors. DC motors offer superior speed and torque control compared to AC motors. However, advancements in power electronics have led to the increasing popularity of AC motors due to their lower manufacturing and maintenance costs. Additionally, the use of DC generators has become nearly obsolete. AC systems offer more advantages for power generation, transmission, and distribution due to the simplicity and cost-effectiveness of using transformers for voltage conversion.
Constructive Aspects
A DC machine comprises a stationary part (stator) and a rotating part (rotor). It has two windings:
- Field Winding (Stator): Produces the magnetic field using coils wound around salient poles.
- Armature Winding (Rotor): Induces an electromotive force (EMF) or counter-EMF when the machine operates as a motor or generator.
DC machine windings are closed, meaning they form a continuous loop. These windings can be either lap-wound or wave-wound, depending on the coil connection arrangement. The coils are positioned to interact with poles of opposite polarity, maximizing the induced EMF. The commutator, a defining feature of DC machines, facilitates the mechanical conversion of the induced AC in the coils to DC output. Brushes, typically made of graphite or specialized electro- and metallo- materials, establish electrical contact with the commutator segments.
Working Principle
DC machines can function as both generators and motors. To illustrate EMF generation, consider a ring-shaped armature winding. When the winding rotates within the stator’s magnetic field, an EMF is induced in the conductors. To utilize this induced EMF, brushes (A and B) are placed on the commutator, dividing the winding into two parallel paths. The axis along which the brushes align is called the neutral line. This line is crucial as it marks the points where the EMF in the armature coils reverses direction. The brushes collect the EMFs induced in the coils, resulting in a DC output voltage. The magnitude of the induced EMF (E) is given by:
E = (n/60) * Z * Φ * P / C = Ke * n * Φ
Where:
- n = rotational speed
- Z = total number of conductors
- Φ = flux per pole
- P = number of poles
- C = number of parallel paths
- Ke = constant
Whether operating as a generator or motor, the current flowing through the armature conductors interacts with the magnetic field, producing torque. The direction of this torque depends on the machine’s operating mode. The torque (T) is given by:
T = (1/2π) * (P/C) * Z * Ii * Φ = KT * Ii * Φ
Where:
- Ii = armature current
- KT = constant
The electromagnetic power (Pa) is given by:
Pa = E * Ii = T * ω = T * 2π * (n/60)
Where:
- ω = angular velocity
Armature Reaction
When a DC machine runs under no-load conditions, the armature current is zero, and the magnetic field in the air gap is solely due to the field winding. However, when the armature circuit is loaded, current flows through the armature conductors, creating its own magnetic field. This armature magnetic field interacts with the stator field, resulting in a distorted air gap flux. This effect is known as armature reaction. Armature reaction can shift the magnetic neutral axis, leading to sparking at the brushes and reduced machine performance. To mitigate armature reaction, several techniques are employed, including:
- Brush Shifting: Shifting the brushes to align with the new magnetic neutral axis.
- Compensating Windings: Placing windings in the pole faces to counteract the armature MMF.
- Interpoles: Small auxiliary poles placed between the main poles to improve commutation.
Commutation
Commutation refers to the process of reversing the current direction in the armature coils as they pass under the brushes. Ideal commutation occurs without sparking at the brushes. During commutation, the coil undergoing current reversal is short-circuited by the brush. The time taken for the current reversal is called the commutation period (T). Poor commutation, often characterized by sparking, can lead to excessive brush wear and reduced machine efficiency. Factors affecting commutation include:
- Commutation Period: A shorter commutation period generally results in better commutation.
- Armature Reaction: Armature reaction can distort the commutating field, leading to sparking.
- Brush Material and Condition: The choice of brush material and its condition significantly impact commutation quality.
DC Generators: General Aspects
DC generators convert mechanical energy into electrical energy. They consist of a field winding (stator) and an armature winding (rotor) connected to a commutator. When the rotor spins, an EMF is induced in the armature winding. The induced voltage (E) is related to the terminal voltage (V), armature current (Ii), armature resistance (Ri), and brush contact voltage drop (Vesc) by:
E = V + Ri * Ii + Vesc
Types of DC Generators
DC generators can be classified based on how the field winding is excited:
- Separately Excited Generators: The field winding is powered by an external DC source.
- Self-Excited Generators: The field winding receives its current from the generator itself.
Self-excited generators are further categorized into:
- Series Wound Generators: The field winding is connected in series with the armature winding.
- Shunt Wound Generators: The field winding is connected in parallel with the armature winding.
- Compound Wound Generators: A combination of series and shunt field windings is used.
DC Generator Characteristics
Several characteristics define the performance of a DC generator:
- Open-Circuit Characteristic (E = f(Ie), I = 0, n = constant): This characteristic shows the relationship between the generated voltage and field current when the generator is not supplying any load.
- Load Characteristic (V = f(Ie), I = constant, n = constant): This characteristic depicts the relationship between the terminal voltage and field current for a constant load current.
- External Characteristic (V = f(I), Ie = constant, n = constant): This characteristic illustrates the relationship between the terminal voltage and load current for a constant field current.
- Regulation Characteristic (Ie = f(I), V = constant, n = constant): This characteristic shows the relationship between the field current and load current for a constant terminal voltage.
DC Motors: General
DC motors convert electrical energy into mechanical energy. The same fundamental equation for induced voltage in a generator applies to a motor:
E = V + Ri * Ii + Vesc
However, in a motor, the induced EMF (E) opposes the applied voltage (V). The difference between E and V determines the armature current and the direction of torque.
DC Motor Characteristics
DC motors are categorized similarly to generators based on field winding excitation:
- Separately Excited Motors
- Shunt Excited Motors
- Series Excited Motors
- Compound Excited Motors
The torque (T) and induced EMF (E) in a DC motor are given by:
T = KM * Φ * Ii
E = V – Ri * Ii
Where:
- KM = motor constant
Speed Regulation
The speed of a DC motor can be regulated by varying:
- Field Flux (Ie): Increasing the field current decreases the speed, and vice versa.
- Armature Voltage (V): Increasing the armature voltage increases the speed, and vice versa.
- Armature Resistance (Ri): Adding resistance in series with the armature decreases the speed.
The speed (n) of a DC motor is given by:
n = (V – Ri * Ii) / (Ke * Φ)
DC Motors with Independent Excitation and Ward-Leonard System
DC motors with independent excitation have their field winding supplied by a separate DC source, allowing for independent control of field flux and speed. The Ward-Leonard system is a classic example of speed control using separately excited DC machines. It consists of:
- An AC motor driving a separately excited DC generator.
- The DC generator supplying power to a separately excited DC motor.
By varying the generator’s field current, the DC motor’s armature voltage, and consequently its speed, can be controlled smoothly over a wide range. The Ward-Leonard system offers precise speed and torque control but is relatively complex and expensive.
DC Motor with Series Excitation
The flow of the machine depends on the armature current I = I i and consequently depends on the load. If there is saturation in the magnetic circuit, the flow is directly proportional to the current I i and the load characteristic can be obtained using the equations: T = K T * O * I i, V = E + R i * I i , E = K E * n * Ö;
If the motor is not saturated and the following proportionality Ö = KI II: T = K T * K i * I i2 => I i = root of T / (K T * K I)
Torque characteristics: n = V-(R i * R i) / (K E * Ö) = V / (K I K E * * I i)-R i / (K E * K I) =
1 / K E * root of (K t / K I) * V / root (T)-R i / K I K E *