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Direct Current (DC) Machines

Dr. Mostafa Soliman

DC Machine

Part no. 1

Basic principles and physical construction

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Outline

Fundamentals of DC Machines.

Commutation in DC Machines.

Construction of the DC Machines.

DC Generator/Motor Equivalent Circuits.

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Introduction DC power systems are not very common in engineering practice. However,

DC motors still have many practical applications, such automobile, aircraft,

and portable electronics, in speed control applications

An advantage of DC motors is that it is easy to control their speed in a wide

diapason.

DC generators are quite rare.

Most DC machines are similar to AC machines: i.e. they have AC voltages

and current within them. DC machines have DC outputs just because they

have a mechanism converting AC voltages to DC voltages at their terminals.

This mechanism is called a commutator; therefore, DC machines are also

called commutating machines.

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Direct Current (DC) Machines

Fundamentals Generator action:An emf (voltage) is induced in a

conductor if it moves through a magnetic field.

Motor action:A force is induced in a conductor that

has a current going through it and placed in a

magnetic field.

Any DC machine can act either as a generator or as

a motor.

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DC MotorElectricalEnergy

Mechanical

Energy

Field

DC Generator

Mechanical

Energy

Field

Electrical

Energy

Direct Current (DC) Machines

Fundamentals

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DC Generator Fundamentals

Both vand B are vectors.

is the angle between the direction in which theconductor is moving and the flux is acting.

( v B) indicates the direction of the current

flows in the conductor, or the polarity of the emf.

If the direction of the magnetic field is fixed, thepolarity of the emf will be determined by the

direction of motion, i.e. the direction ofv.

e = induced voltage, v = velocity

of the conductor, B = flux density

and l is the length of the

conductor.

Blve

lBveforcemotiveelectroemf

sin

)()(

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Generated Voltage in a Loop

(a coil of one turn)

For emf to be induced, the

conductors must cut the flux

lines as they move.

Otherwise, ( v B) = 0.

eloop = eab + ebc + ecd + eda

eloop = Blv + 0 + Blv + 0

eloop = 2 B l v

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Generated Voltage in a Loop

(a coil of one turn)

Note: Induced voltages are always AC.

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Commutation

Every time the voltage of the

loop switches direction, the

contacts also switch

connections, and the outputvoltage at the contacts is

always built up in the same

direction.

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DC Machines

Real DC machine Construction

Stator: Stationary part of the machine.

The stator carries a field winding

(concentric) that is used to produce the

required magnetic field by DC

excitation. Often known as the field.

Rotor: The rotor is the rotating part of

the machine. The rotor carries a

distributed winding where the emf is

induced in. Also known as the armature.

N S

Stator with

with polesBrush

Rotor

Field

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DC Machines

DC machine Construction

The picture shows the

stator of a DC machine

with 4 poles.

The iron core is supported

by a cast iron frame.

Stator constructionDC machine Construction

The picture shows the

stator of a DC machine

with 4 poles.

The iron core is supported

by a cast iron frame.

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DC MachinesDC machine Construction

The rotor iron core is mounted on

the shaft.

Coils are placed in the slots.

The end of the coils are bent and

tied together to assure mechanical

strength.

Note the commutator mounted on

the shaft. It consists of several

copper segments, separated by

insulation.

Rotor construction

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DC MachinesDC Machine Construction

The adjoining picture shows the

commutator of a large DC machine.

The segments are made out of

copper and mica insulation is placed

between the segments.

The end of each segment has a flag

attached. The coil endings arewelded to these flags.

An insulated ring is placed on the

coil ends to assure proper

mechanical strength.

RingInsulator

Copper

Flag

Commutator segment

brushe

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DC Generator Equivalent circuit

The magnetic field produced by the stator poles induces a voltage

in the rotor (or armature) coils when the generator rotates.

This induced voltage is represented by a voltage source.

The stator coil has resistance, which is connected in series with it.

The pole flux is produced by the DC excitation/field current, which

is magnetically coupled to the rotor.

The field circuit has resistance and a source.

The voltage drop on the brushes represented by a battery.

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Equivalent circuit of a separately excited dc generator.

DC Generator Equivalent circuit

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a

tbrushaa

R

VVEI

nk nEa

When the generator is loaded, the load current producesa voltage drop on the rotor winding resistance.

In addition, there is a more or less constant 13 V voltagedrop on the brushes.

These two voltage drops reduce the terminal voltage of

the generator. The terminal voltage is;

brushaataVRIVE

ff

f

f

f IKR

VI

DC Generator Equivalent circuit

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DC Motor Fundamentals

L is a vector in the direction of the flow ofthe current.

(LB) direction indicates the direction of

force.

F B i F = induced force, B = flux density, I is the current passing

in the conductor and l is the length of the conductor

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DC Motor Equivalent circuit

Equivalent circuit of a separately excited dc motor

Equivalent circuit is similar to the generator only the currentdirections are different.

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DC Motor Equivalent circuit

The operation equations are:

Armature voltage equation

brushaaat VRIEV

The induced voltage (counter emf or back emf) and

motor speed vs. angular speed

fa IKE n 2

aabrushtaf RIVVEIK