DC Machines -2 Study notes For Electrical Engineering

Types of DC Machines

DC machines come in various types, each designed to cater to specific applications and operating conditions. The main types include DC generators and motors, which can further be classified as shunt, series, compound, and separately excited machines. Understanding the characteristics and applications of these different types is crucial in electrical engineering.

The types of DC machine depends upon the excitation of the DC machine.
The production of magnetic flux in the machine by circulating current in the field winding is called excitation.
DC Machines can be classified according to the electrical connections of the armature winding and the field windings.

There are two methods of excitation namely, separate excitation and self-excitation.

In separate excitation, the field coils are energised by a separate DC source. The terminals of the winding can be connected across the input voltage terminals or fed from a separate voltage source.
In self-excitation, the current flowing through the field winding is supplied by the machine itself. The field winding can be connected either in series or in parallel with the armature winding

Separately Excited DC Machine

As the name implies, the field coils are energized by a separate DC source. The armature and field winding are electrically separate from each other.

(a) Separately excited DC generator, and (b) Separately excited DC motor

Here, Ia = IL , and Ra = Armature resistance.

For Generator

Ea = V + IaRa

For Motor

Armature power = Pa = EaIa, Output = VIL, and Armature copper loss = I2a Ra

Shunt Wound DC Machine

The armature and field winding are connected in parallel.
A machine in which the field coils are connected in parallel with the armature is called a shunt machine.
The armature voltage and field voltage are the same.

(a) Shunt wound DC generator, and (b) Shunt wound DC motor

Characteristics Equations:

where, Pa = EaIa = Armature power (developed power), I2sh Rsh= shunt field Cu loss, I2a Rsh = Armature Cu loss, and VIL= Power delivered.

Series Wound DC Machine

A DC machine in which the field coils are connected in series with the armature is called a series machine.

The field winding carries the same current as the armature winding.

(a) DC series generator (b) DC series motor

A series wound motor is also called a universal motor. It is universal in the sense that it will run equally well using either an ac or a dc voltage source.

Characteristics Equations:

where, Pa = EaIa = Armature power (developed power), VIL = Power delivered, and I2se Rse= Series field Cu loss.

Compound Wound DC Machine

A DC machine, having both shunt and series fields is called a compound machine.
In a compound machine, the series field winding is connected in series with the armature, and the shunt field winding is connected in parallel.

Short-shunt compound DC Machine:

Here, Figure (a) is Short-shunt compound DC generator, and (b) is Short-shunt compound DC motor.

For generator

For motor

Similarly in motor,

where, Pa = Power developed, VIL = Power delivered, I2se Rse= Series field Cu loss, and I2sh Rsh= Shunt field Cu loss.

Two types of arrangements are possible in compound motors:

Cumulative compounding: If the magnetic fluxes produced by both series and shunt field windings are in the same direction (i.e., additive), the machine is called a cumulative compound.
Differential compounding: If the two fluxes are in opposition, the machine is a differential compound.

In both these types, the connection can be either a short shunt or a long shunt.

Long-shunt compound DC Machine

A long-shunt compound DC machine is a type of direct current (DC) machine with a field winding configuration where the shunt field winding is connected in parallel with both the armature winding and the series field winding. This arrangement provides the machine with desirable characteristics, such as good voltage regulation and increased stability, making it suitable for various industrial applications.

(a) Long-shunt DC generator (b) Long-shunt DC motor

For generator

Similarly, in motor

where, Pa = Developed power, and VIL = Delivered power.

Power Flow In DC Machines

Power flow in DC machines refers to the transfer and conversion of electrical power within the machine. It encompasses the generation, transmission, and utilization of electrical energy in the form of voltage and current. Understanding the power flow characteristics is essential for analyzing the performance and efficiency of DC machines.

Here the case given to understand is for DC Motor

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FAQs about DC Machines -2 Study notes For Electrical Engineering

What is armature reaction in a DC machine and how does it affect machine performance?

Armature reaction refers to the distortion of the magnetic field caused by the armature current. It affects machine performance by altering the flux distribution, leading to changes in terminal voltage, commutation, and overall machine behaviour.

What is commutation in a DC machine and why is it important?

Commutation is the process of transferring the current from one armature coil to another as the rotor rotates. It is crucial for maintaining a continuous, unidirectional flow of current, ensuring smooth operation and minimizing sparking and brush wear.

What are the different types of losses in a DC machine and how are they calculated?

The losses in a DC machine include copper losses (I^2R), iron losses (hysteresis and eddy current losses), and mechanical losses (friction and windage losses). These losses can be calculated using appropriate formulas and measurements.

What are the various starting methods for DC motors?

The common starting methods for DC motors include direct-on-line starting, reduced voltage starting (using autotransformer or resistance), and dynamic braking. These methods control the initial high starting current and provide smooth acceleration.

What are the advanced speed control methods for DC motors?

Advanced speed control methods for DC motors include armature voltage control, field flux control, and chopper control. These methods allow precise speed regulation, torque control, and efficient operation in various applications.