eceg439:-electrical machine ii · pdf file02/01/2014 · eceg439:electrical machine...

ECEg439:-Electrical Machine II

2.1.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

By Sintayehu Challa ECEg439:Electrical Machine II 33

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.

? A DC motor convert Electrical energy into rotary (or linear) mechanical energy at the output shaft.

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


Principle of DC machines

? 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.


Conversation of Energy

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EnergyMechanicalEnergyElectrical

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1. A Generator converts Mechanical Energy into Electrical Energy, Where as a Motor converts Electrical Energy into Mechanical Energy.

2. The Energy conversion from Electrical to mechanical or vice-versa take place via the magnetic field provided by the field system.

3. A single rotating dc machine can either be operated as a generator or as a motor.


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 generated current into the DC by a mechanical device is called a commutator.


Contd.

Fig. 1( a) DC generator:Induced AC emf is converted to DC voltage using commutator ;

Fig. 1(b) DC motor: input direct current is converted to alternating current in the armature at to produce a unidirectional torque.


DC Generator Operation



? The N-S poles of a dc machine produces constant magnetic field and the rotor coil turns in this field.

? The conductors in the rotor 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.

30 NS Vdc

Bv

v

a

b

1

2

Ir_dc

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



? In Figure below , 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.

30NS Vdc

Bv

v

a

b

1

2

Ir_dc


1. The induced voltage is connected to the generator terminals through the commutators (1 & 2) and brushes.

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

3. The positive terminal is connected to commutator segment 2 and to the conductors in slot b.

4. The negative terminal is connected to segment 1 and to the conductors in slot 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.? The simultaneously the commutator reverses its terminals, which

assures that the output voltage (Vdc) polarity is unchanged.

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 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.


DC Motor Operation


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


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

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

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 commutator changes the current direction, which maintains the counterclockwise rotation.


Vdc30 NS

Bv

v

a

b

1

2

Ir_dc

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

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

30 NS Vdc

a

b

1

2

B

v v

Ir_dc


Vdc30 NS

Bv

v

a

b

1

2

Ir_dc

Neutral Zone


Basic DC Motor Operation

? 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 below

• 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 (Neutral zone)

? 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.

? How can this be avoided?The answer would be to reverse either the magnetic field or the direction of current


Contd.


Contd.

• 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 two 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 utilize 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

|

Shaft

Brush

Coppersegment

InsulationRotor

Winding

N S

Ir_dcIr_dc/2

RotationIr_dc/2

Ir_dc

12

3

45

6

7

8

Polewinding


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

eceg439:-electrical machine ii · pdf file02/01/2014 · eceg439:electrical machine...

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