Electrical Machines - DC Machines Complete Study Notes

Introduction

• 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

DC MOTOR

It works on the principle that when an electric current carrying conductor is placed in a magnetic field, mechanical force experienced on the conductor, the direction is given by Fleming left hand rule and hence conductor moves in the direction of force.

TYPES OF DC MOTOR:

1. Separately excited DC Motor : There is no connection between armature & field current.

2. Self excited: There are different connections of self-excitation of DC motor 

2.1. Series motor : There is same flow of current in armature & field winding.

2.2. Shunt motor : There is common voltage for field and armature winding.

STARTING OF DC MOTORS:

At starting, the speed of DC motor is zero and hence the induced EMF which is proportional to speed of motor is also zero.

Due to which,

V_t = I_ar_a  for shunt & separately excited motor.

V_t = I_a (r_a + r_se) for series and compounded motor

As a result the current  I_a is very high due to small values of r_a and (r_a + r_se).

Such heavy current may cause:

• Sparking at commutator
• Damage to armature winding and isolation.
• High starting torque and acceleration
• Large dips in supply voltage.

So this current must be limited by inserting a resistance in the armature circuit. But this resistance must be cut off as rotor accelerates else-

• Operating speed of motor will be reduced.
• Losses will be higher and efficiency will be less.

To avoid the above dangers while starting a DC motor, it is necessary to limit the starting current. So, a DC motor is started by using a starter. There are various types of dc motor starters, such as 3-point starter, 4point starter, no-load release coil starter, thyristor controller starter etc. The basic concept behind every DC motor starter is adding external resistance to the armature winding during starting.

Hence, 3-point starters and 4-point starters are used for starting shunt wound motors and compound wound motors.

Three-Point Starter:

When the connected dc motor is to be started, the lever is turned gradually to the right. When the lever touches point 1, the field winding gets directly connected across the supply, and the armature winding gets connected with resistances R1 to R5 in series. During starting, full resistance is added in series with the armature winding. Then, as the lever is moved further, the resistance is gradually is cut out from the armature circuit. Now, as the lever reaches to position 6, all the resistance is cut out from the armature circuit and armature gets directly connected across the supply.

It can be seen that, when the arm is moved from the position 1 to the last position, the starter resistance gets added in series with the field winding. But, as the value of starter resistance is very small as compared to the shunt resistance, the decrease in shunt field current may be negligible. However, to overcome this drawback a brass or copper arc may be employed within a 3 point starter which makes a connection between the moving arm and the field winding.

Four-Point Starter:

The main difference between a 3-point starter and a 4-point starter is that the no voltage coil (electromagnet E) is not connected in series with the field coil. The field winding gets directly connected to the supply, as the lever moves touching the brass arc (the arc below the resistance studs). The no voltage coil (or Hold-on coil) relates to a current limiting resistance R.

ELECTRIC BRAKING

Delay in stopping a motor may result in heavy damaged to the equipment or to the manufactured products and even the loss of human life.

Requirements of braking:

• Braking should be quick and reliable in action
• Braking torque must be controllable
• Failure of any part of braking system must result in application of brakes
• Provision of some suitable means to dissipate kinetic energy of the moving parts of the motor and its driven machine or machines.

Disadvantages of Electric Braking:

• Shock caused to the motor and the equipment
• Heavy in-rush of current at the time of braking.
• This method is used to get either a quick reversal or get a rapid stop.

Applications:

 This method of braking can be applied to brake with a separate source of dc excitation during braking

• Direct current motors
• Synchronous motors
• Induction motors

TYPES OF BRAKING :

• Regenerative Braking:

• Plugging:

• Rheostatic or Dynamic Braking:

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