Electrical Machines : DC Generators

In this article, you will find the study notes on DC Machines.

• DC machine is a highly versatile and flexible machine.
• It can satisfy the demands of load requiring high starting, accelerating and retarding torques.
• If the conversion is from mechanical to electrical energy, the machine is called as Generator.
• If the conversion is from electrical to mechanical energy, the machine is called as Motor.

To understand, design and use these machines the following laws must be studied.

• Electric circuit laws - Kirchoff′s Laws
• Magnetic circuit law - Ampere′s Law
• Law of electromagnetic induction - Faraday′s Law
• Law of electromagnetic interaction -BiotSavart′s Law

Most of the present day machines have one or two electric circuits linking a common magnetic circuit. In subsequent discussions the knowledge of electric and magnetic circuit laws is assumed. The attention is focused on the Faraday’s law and Biot Savart’s law in the present study of the electrical machines.

Application of Faraday's law according to electro mechanical energy conversion results in the generation of both transformer and rotational emf to be present in the coil moving under a changing field. This principle is utilized in the induction machines and a.c. commutator machines. The direction of the induced emf is decided next. This can be obtained by the application of the Lenz’s law and the law of interaction.

Law of induction-Generator action 

Commutation 

• The currents induced in armature conductors of a d.c. generator are alternating. These currents flow in one direction when armature conductors are under N-pole and in the opposite direction when they are under S-pole.
• As conductors pass out of the influence of a N-pole and enter that of S-pole, the current in them is reversed. This reversal of current takes place along magnetic neutral axis or brush axis i.e. when the brush spans and hence short circuits that particular coil undergoing reversal of current through it.
• This process by which current in the short-circuited coil is reversed while it crosses the M.N.A. is called commutation. The brief period during which coil remains short-circuited is known as commutation period Tc.
• If the current reversal i.e.the change from +I to zero and then to −I is completed by the end of short circuit or commutation period, then the commutation is ideal. If current reversal is not complete by that time, then sparking is produced between the brush and the commutator which results in progressive damage to both.
• The brush width is equal to the width of one commutator segment and one mica insulation.

Types of DC Machines:

• The types of DC machine depends upon the excitation of 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

Types of DC Machines:

• The types of DC machine depends upon the excitation of 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, I_a = I_L , and R_a = Armature resistance.

• For Generator

E_a = V + I_aR_a  

 or 

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, P_a = E_aI_a = Armature power (developed power),  I²_sh R_sh= shunt field Cu loss, I²_a R_sh = Armature Cu loss, and VI_L= 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, P_a = E_aI_a = Armature power (developed power), VI_L = Power delivered, and  I²_se R_se= 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

Long-shunt compound DC Machine:

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

• For generator

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