eceg439:-electrical machine ii - · pdf fileby sintayehu challa eceg439:electrical machine ii...

ECEg439:-Electrical Machine II

General Arrangement of DC Machine

By Sintayehu Challa

By Sintayehu Challa ECEg439:Electrical Machine II

Objectives

? To instill an understanding of the underlying electromagnetic effects permitting electric machine operation and introduce basic DC machine types

• To describe the construction of these machines

• To examine the main types of DC machine


Electromagnetic Basics - repeat!

By Sintayehu Challa ECEg439:Electrical Machine II 44

DC Machines

? The direct current (dc) machine can be used as a motor or as a generator.

DC Generator convert Mechanical Energy Input at their shafts into to Electrical Energy in the form of Voltage or Current. Where as DC motor do the reverse

? The major advantages of dc machines are the easy speed and torque regulation.

? However, their application is limited to mills, mines and trains. As examples, trolleys and underground subway cars may use dc motors.

? In the past, automobiles were equipped with dc dynamos to charge their batteries.


Principle of DC machines

? A DC motor is a device for converting DC electrical energy into rotary (or linear) mechanical energy. This process can be reversed, as in a DC generator, to convert mechanical to electrical power.

? The working principle of the DC generator is Faraday’s Law, which states that emf and electric current if the circuit is closed, is produced when a conductor cuts through magnetic force lines.

? The opposite of the law applies for the DC motor. Motion is produced when a current carrying wire is put in a magnetic field.


commutation• In DC machines the current in each wire of the

armature is actually alternating, and hence a device is required to convert the alternating current generated in the DC generator by electromagnetic induction into direct current, or at the armature of a DC motor to convert the input direct current into alternating current at appropriate times, as illustrated in Fig. 1.

• Fig. 1( a): DC generator: induced AC emf is converted to DC voltage; (b) DC motor: input direct current is converted to alternating current in the armature at appropriate times to produce a unidirectional torque.


Contd.


Basic DC Motor

? Consider the illustration below With the current flowing the wire as shown, and the magnetic field in the direction indicated, it is clear there is a force on the conductors acting as shown

• If the wire is free to rotate around the ends (terminals) then the wire would rotate - beginnings of motor action.


Contd.? Consider the situation shown on the last

slide, The currents in the wire are, taking a cross- section, in opposite directions

? The magnetic field across this cross-section clearly illustrates the areas where the magnetic flux is increased and decreased due to the magnetic flux from the wires

? This illustration demonstrates the ‘elastic band’ nature of lines of magnetic flux, which will always act in a way to try to shorten themselves


Contd.


Contd.

? Consider the situation if the wire rotates through 900


Contd.? The forces on the conductor remain acting in the

same directions , With the pivot point at the wire ends (terminals) there is now zero torque acting on the wire forcing it to rotate

? Consider the situation after a further 900 rotation of the wire (assume the wire has sufficient momentum to rotate)

? The torque acting on the conductors now rotates the wire in the opposite sense! No use as a motor!!

? How can this be avoided?


Contd.


Contd.? The answer would be to reverse either the

magnetic field or the direction of current• It is easier to reverse the current flow in the

wire - this is managed by employing a commutator

• A commutator ensures that the current is reversed in the armature (turning conductor) every half rotation thereby allowing the forces on the wire to aid rotation

• Take the previous example and add a commutator to the supply end of the wires.


Contd.


Contd.• By including a basic commutator it is possible

to obtain the forces on the conductors in the same sense for a 3600 rotation.

• However there are still periods of zero torque for the simple 2 piece commutator considered

• This would lead to a very uneven drive and could, dependent on the load, seriously effect either the motor, the load or both

• To compensate for this effect utilise a commutator with more segments


Contd.


Improved Commutator

? Having a commutator with more segments means that there are no zero torque parts of the rotation cycle This significantly improves the drive of the motor


Example of many segment Commutator & Motor

? A common application of a DC motor is a battery powered hand drill. The commutator has many segments and delivers relatively smooth output torque


Commutator Action

? As the commutator passes a brush, the direction of current flow reverses, ensuring constant drive torque in the direction of rotation


DC Generator Operation



? The N-S poles produce a dc magnetic field and the rotor coil turns in this field.

? A turbine or other machine drives the rotor.

? The conductors in the slots cut the magnetic flux lines, which induce voltage in the rotor coils.

? The coil has two sides: one is placed in slot a, the other in slot b.

30NS Vdc

Bv

v

a

b

1

2

Ir_dc

(a) Rotor current flow from segment 1 to 2 (slot a to b)

30 NS Vdc

a

b

1

2vv

B

Ir_dc

(b) Rotor current flow from segment 2 to 1 (slot b to a)



? In Figure , the conductors in slot a are cutting the field lines entering into the rotor from the north pole,

? The conductors in slot b are cutting the field lines exiting from the rotor to the south pole.

? The cutting of the field lines generates voltage in the conductors.

? The voltages generated in the two sides of the coil are added.

30NS Vdc

Bv

v

a

b

1

2

Ir_dc


30 NS Vdc

a

b

1

2vv

B

Ir_dc




? The induced voltage is connected to the generator terminals through the commutatorand brushes.

? In Figure , the induced voltage in b is positive, and in a is negative.

? The positive terminal is connected to commutatorsegment 2 and to the conductors in slot b.

? The negative terminal is connected to segment 1and to the conductors in slot a.

30NS Vdc

Bv

v

a

b

1

2

Ir_dc(a) Rotor current flow from segment 1 to 2 (slot a to b)

30 NS Vdc

a

b

1

2vv

B

Ir_dc(b) Rotor current flow from segment 2 to 1 (slot b to a)



? When the coil passes the neutral zone: ? Conductors in slot a

are then moving toward the south pole and cut flux lines exiting from the rotor

? Conductors in slot b cut the flux lines entering the in slot b.

? This changes the polarity of the induced voltage in the coil.

? The voltage induced in a is now positive, and in b is negative.

30NS Vdc

Bv

v

a

b

1

2

Ir_dc


30 NS Vdc

a

b

1

2vv

B

Ir_dc(b) Rotor current flow from segment 2 to 1 (slot b to a)

By Sintayehu Challa ECEg439:Electrical Machine IIBy Sintayehu Challa ECEg554:Electrical Machines 2626


? The simultaneously the commutator reverses its terminals, which assures that the output voltage (Vdc) polarity is unchanged.

? In Figure B ? the positive terminal

is connected to commutator segment 1 and to the conductors in slot a.

? The negative terminal is connected to segment 2 and to the conductors in slot b.

30NS Vdc

Bv

v

a

b

1

2

Ir_dc


30 NS Vdc

a

b

1

2vv

B

Ir_dc



DC Motor Operation

By Sintayehu Challa ECEg439:Electrical Machine IIBy Sintayehu Challa ECEg554:Electrical Machines 2828

DC Motor Operation

? In a dc motor, the stator poles are supplied by dc excitation current, which produces a dc magnetic field.

? The rotor is supplied by dc current through the brushes, commutator and coils.

? The interaction of the magnetic field and rotor current generates a force that drives the motor

|

Shaft

Brush

Coppersegment

InsulationRotor

Winding

N S

Ir_dcIr_dc/2

RotationIr_dc /2

Ir_dc

12

3

45

6

7

8

Polewinding


DC Motor Operation

? The magnetic field lines enter into the rotor from the north pole (N) and exit toward the south pole (S).

? The poles generate a magnetic field that is perpendicular to the current carrying conductors.

? The interaction between the field and the current produces a Lorentz force,

? The force is perpendicular to both the magnetic field and conductor


Vdc30 NS

Bv

v

a

b

1

2

Ir_dc


30 NS Vdc

a

b

1

2

B

v v

Ir_dc


DC Motor Operation

? The generated force turns the rotor until the coil reaches the neutral point between the poles.

? At this point, the magnetic field becomes practically zero together with the force.

? However, inertia drives the motor beyond the neutral zone where the direction of the magnetic field reverses.

? To avoid the reversal of the force direction, the commutatorchanges the current direction, which maintains the counterclockwise rotation. .


Vdc30 NS

Bv

v

a

b

1

2

Ir_dc


30 NS Vdc

a

b

1

2

B

v v

Ir_dc


DC Motor Operation

? Before reaching the neutral zone, the current enters in segment 1 and exits from segment 2,

? Therefore, current enters the coil end at slot a and exits from slot b during this stage.

? After passing the neutral zone, the current enters segment 2and exits from segment 1,

? This reverses the current direction through the rotor coil, when the coil passes the neutral zone.

? The result of this current reversal is the maintenance of the rotation.


Vdc30 NS

Bv

v

a

b

1

2

Ir_dc


30 NS Vdc

a

b

1

2

B

v v

Ir_dc

eceg439:-electrical machine ii - · pdf fileby sintayehu challa eceg439:electrical machine ii...

Documents