Operating Principles Of DC Machines - Electrical Machines - Electrical

DC Machine: Introduction

Rotating electrical machines are electromechanical devices that convert energy between electrical and mechanical forms. A DC machine can operate either as a DC generator (converting mechanical energy to direct electrical energy) or as a DC motor (converting direct electrical energy to mechanical energy). By reversing the direction of energy flow, the same basic machine construction can be used for motoring or generating.

Electrical Machines

Construction and Working

DC machines are often called commutating machines because they use a commutator to convert alternating induced voltages (in the rotating conductors) into an output that is unidirectional at the external terminals. DC machines are widely used where good speed regulation and a wide range of speeds are required. Typical applications include traction (trains, trams), variable-speed drives, and industrial processes such as electroplating and welding.

Working principle of a DC machine

A DC machine works on the basic electromagnetic interaction between current-carrying conductors and magnetic fields. When current flows in an armature conductor placed in a magnetic field, a mechanical torque is produced on the conductor (motor action). Conversely, when the armature conductors are moved through the magnetic field by a prime mover, an emf is induced in the conductors (generator action).

Main functions

• The DC generator converts mechanical power into DC electrical power.
• The DC motor converts DC electrical power into mechanical power.
• In a motor the induced emf opposing the applied voltage is called the back emf. In a generator the induced emf is usually termed the generated emf.

Construction of a DC Machine

The principal parts of a DC machine are the yoke, poles and pole cores, pole shoes, field windings, armature core and armature windings, commutator, brushes and bearings. Each part has a distinct mechanical or electromagnetic role.

Parts of D.C. Machine

Yoke (Frame)

• The yoke or frame provides mechanical support for poles and protects the machine from dust and moisture.
• The yoke also provides the return path for magnetic flux in the magnetic circuit.
• Yokes are normally made of cast iron, cast steel or rolled steel.

Pole and Pole Core

• Each pole is an electromagnet formed by a pole core and the field winding (field coil) placed on it.
• When the field winding is energised it produces the required magnetic flux for the machine.
• Pole cores are usually made of steel and are often constructed from annealed steel laminations to reduce eddy current losses.

Pole Shoe

• The pole shoe enlarges the pole area to spread the flux in the air-gap and to direct more flux across the armature surface.
• The pole shoe helps in the uniform distribution of flux in the air-gap.
• Pole shoes are made of cast iron/steel or stacked laminations to reduce eddy currents.

Field Windings (Field Coils)

• The field winding is wound on the pole core and produces the main flux when excited by a DC current.
• Field windings are usually made of copper conductor.

Armature Core

• The armature core carries slots in which the armature conductors are placed.
• The core provides a low-reluctance path for the flux produced by the field winding.
• The core is made of laminated iron to reduce eddy current losses.

Armature Winding

• The armature winding consists of conductors connected in a specified pattern (lap or wave winding) and placed in the armature slots.
• When the armature rotates in the magnetic field, an emf is induced in these conductors. In a generator the armature delivers current to the external circuit; in a motor external current flows into the armature.

Commutator

• The commutator collects current from the armature conductors and converts the alternating emf induced in the conductors into unidirectional voltage at the external terminals.
• It is built from a number of copper segments insulated from each other by mica.
• For motors the commutator ensures that the torque on the armature remains unidirectional.

Brushes

• Brushes are stationary contacts (usually carbon or graphite) that press against the commutator to transfer current between the rotating armature and the external circuit.
• Brushes wear with time and require periodic inspection and replacement.

Types of DC Machines

DC machines are classified by the method of field excitation. The two broad excitation classifications are separately excited and self-excited. In separately excited machines the field winding is supplied by an independent DC source. In self-excited machines the machine's own output (generator) or supply (motor) provides the field current.

The principal winding arrangements used are:

Types of DC Machines

1. Separately Excited

• Field winding is supplied from a separate DC source.

Separately Excited DC Machine

2. Shunt Wound (Shunt)

• Field winding is connected in parallel (shunt) with the armature. The shunt field draws a relatively small current because it is wound with many turns of fine wire.

Shunt Wound DC Machine

3. Series Wound (Series)

• Field winding is connected in series with the armature so that the field current equals the armature current. Series field windings have few turns of large cross-section wire to carry the large current.

4. Compound Wound (Compound)

• A compound machine has both series and shunt field windings on each pole. The series winding provides high starting torque and the shunt winding improves speed regulation.
• Compound connections can be of two basic types:

• Short shunt - the shunt field is connected in parallel with the armature only.
• Long shunt - the shunt field is connected in parallel with the series field and armature together.

EMF Equation of a DC Machine

When an armature conductor cuts magnetic flux, an emf is induced in proportion to the flux cut per unit time. The generated emf per conductor and the total emf produced by the armature follow directly from flux and speed.

Derivation (symbols defined after):

The flux cut by a conductor in one revolution = P Φ, where P is number of poles and Φ is flux per pole (Wb).

If the armature rotates at n revolutions per second, the time for one revolution = 1/n seconds.

Induced emf in one conductor = flux cut per revolution / time for one revolution.

Induced emf in one conductor = (P Φ) / (1/n) = n P Φ volts.

Let Z be the total number of armature conductors and A the number of parallel paths in the armature winding. Conductors in series per parallel path = Z/A.

Therefore the total generated emf (E) at the armature terminals (neglecting drop in armature circuit) is:

E = n P Φ × (Z / A)

Using rpm N (revolutions per minute), n = N/60 and the formula becomes:

E = (P Φ Z N) / (60 A)

where

• E = generated emf (V)
• P = number of poles
• Φ = flux per pole (Wb)
• Z = total number of armature conductors
• A = number of parallel paths in armature (A = P for lap winding, A = 2 for wave winding when P > 2)
• N = speed in rpm

Electrical Relationships and Motor Operation

For a DC motor supplied with terminal voltage V, armature current I_a and armature resistance R_a, the following relation holds:

V = E_b + I_a R_a

where E_b is the back emf, given by the emf equation with the rotor speed and flux. In steady operation the mechanical power developed (air-gap power) equals electrical input minus electrical losses in the armature.

Torque developed

The electromagnetic torque developed in a DC machine is proportional to the product of flux per pole and armature current. In standard form:

T = (P Φ Z I_a) / (2 π A)

where T is torque (in SI units when units are consistent). This shows torque ∝ Φ I_a.

Armature Reaction and Commutation

Armature reaction is the effect of the magnetic field produced by the armature current on the distribution of the main field flux. Armature reaction distorts and may weaken the main flux under certain pole regions, causing:

• Shift of the neutral plane (the region of zero induced emf),
• Distortion of flux distribution which can lead to poor commutation and sparking at brushes.

Remedies for armature reaction and commutation problems include:

• Providing commutating (inter) poles connected in series with the armature to produce a compensating flux.
• Using compensating windings embedded in the pole faces to cancel armature reaction.
• Ensuring proper brush setting (shift in brush position) to match the new neutral plane under load.

Losses in a DC Machine

• Copper losses (I2R losses) in armature and field windings. Armature copper loss = I_a^2 R_a; field copper loss = I_f^2 R_f.
• Core (iron) losses in the magnetic circuit due to hysteresis and eddy currents; these are frequency and flux dependent.
• Mechanical losses including friction and windage.
• Brush contact losses due to voltage drop across brush-commutator contact.
• Stray load losses (additional unclassified losses), typically estimated from tests.

Overall machine efficiency is given by the ratio of output mechanical (or electrical, for a generator) power to input power after accounting for these losses.

DC Machine versus AC Machine

DC and AC machines differ in construction, control and typical applications. Key differences include:

• Supply type: DC machines use direct current supply for the field (and armature in motors), while AC machines operate from alternating current.
• Speed control: DC motors permit straightforward speed control by varying armature voltage or field flux; AC motors require more complex drives or variable-frequency supplies for speed variation.
• Commutation: DC machines require mechanical commutation (commutator and brushes); most AC machines use slip rings or are brushless for synchronous/induction types.
• Typical applications: DC machines are used where variable speed and good speed regulation are required; AC machines dominate bulk generation and many industrial drives due to the convenience of AC distribution.

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Advantages of DC Machines

• High starting torque, useful for traction and lifting.
• Good and easily obtainable speed control over a wide range by varying armature voltage or field current.
• Quick reversing by reversing armature or field connections.
• Smooth operation with low speed and torque ripple when properly designed.
• Relatively simple control for many variable-speed applications.

Applications of DC Machines

• Traction systems (historically and in some modern traction where DC drives are used).
• Battery charging and small standalone DC generation (historically cars with dynamos; now largely replaced by alternators and rectification).
• Electrochemical processes such as electroplating and electrolysis.
• Variable-speed drives for cranes, hoists, paper mills, rolling mills and other industrial applications where precise speed/torque control is required.
• DC motors are also used in servo drives and other control systems where smooth variable speed is necessary.

Starting and Speed Control of DC Motors

• Because a stationary DC motor has very low back emf, a large starting current will flow if the motor is connected directly to the supply. A starting resistor in the armature circuit or a suitable starter limits this initial current.
• Common methods of speed control:

• Armature voltage control - changing the applied voltage across the armature (using rheostats historically, now electronic converters or choppers).
• Field flux control - varying field current to change flux Φ; speed varies approximately inversely with flux for a given armature voltage.
• Ward-Leonard system - a motor-generator set that provides variable armature voltage (historical but important conceptually).
• Electronic DC drives - controlled rectifiers, choppers and power electronic converters allow efficient and precise control of armature voltage and current.

Summary

DC machines are fundamental electromechanical devices capable of both motoring and generating. Their construction includes the yoke, poles, pole shoes, field windings, armature core and windings, commutator and brushes. The emf equation relates generated voltage to flux, speed and armature geometry. Important practical considerations include armature reaction and commutation, various losses that determine efficiency, and methods of starting and speed control. Although AC systems dominate bulk power systems, DC machines remain important where variable speed, high starting torque and simple closed-loop control are needed.