DC Machines

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Transcript of DC Machines

Dr. Adel Gastli

Electromechanical Systems & Actuators

DC MACHINESThese slides are the contributions of: Dr. A. Gastli, Dr. A. Al-Badi, and Dr. Amer Al-Hinai

DC MachinesLEARNING GOALS IntroductionApplication of DC Machine Advantages & Disadvantages of DC Machine

Construction of DC MachineField System Armature Commentator Brush

Principle of OperationFaradays Law Armature Voltage & Developed Torque

Classification of DC MachinePermanent Magnet Self-Excited Separately-Excited

DC Machine Representation Magnetization Curve (Saturation) DC Motor & Generator Equations Power Flow & Efficiency Torque-Speed CharacteristicsDr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators Starting of DC Machine 2

IntroductionMost of the electrical machine in service are AC type. DC machine are of considerable industrial importance. DC machine mainly used as DC motors and the DC generators are rarely used. DC motors provides a fine control of the speed which can not be attained by AC motors. DC motors can developed rated torque at all speeds from standstill to rated speed. Developed torque at standstill is many times greater than the torque developed by an AC motor of equal power and speed rating.Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 3

Application of DC Machines

The d.c. machine can operate as either a motor or a generator, at present its use as a generator is limited because of the widespread use of ac power. Large d.c. motors are used in machine tools, printing presses, fans, pumps, cranes, paper mill, traction, textile mills and so forth. Small d.c. machines (fractional horsepower rating) are used primarily as control device-such as tachogenerators for speed sensing and servomotors for position and tracking.Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 4

Application of DC MachinesDC Motor Paper Mills Oil Rigs

Steel Mills

Robots

Dr. Adel GastliMining

MCTE3210: Machine ToolsSystems & Actuators Electromechanical

Petrochemical

5

Advantages & Disadvantages Of D.C. MotorsAdvantages High starting torque Rapid acceleration and deceleration. Speed can be easily controlled over wide speed range. Used in tough gobs (traction motors, electric trains, electric cars,.) Built in wide range of sizes. Disadvantages Needs regular maintenance Cannot be used in explosive area High costDr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 6

IntroductionElectric Machine

Mechanical Input

Generator

Electrical Output

Electrical Input

Motor

Mechanical Output

Electromechanical Energy Conversion+ Electrical system v _ iIdeal Electric Machine T Mechanical system

Motor Energy Flow Generator

v i=T

Dr. Adel Gastli

MCTE3210: Electromechanical Systems & Actuators

7

Construction of DC Machine

Parts of a DC MachineArmature winding Armature core Leading pole tip Field coil Rotation Pole axis Shaft Pole core Trailing pole tip Pole face Field yoke

Dr. Adel Gastli

MCTE3210: Electromechanical Systems & Actuators

8

Construction of DC MachineShaft

Armature

Commutator

Stator

Field coil

Dr. Adel Gastli

MCTE3210: Electromechanical Systems & Actuators 2 Pole DC Machine

9

Construction of DC Machine: Field SystemThe field system is to produce uniform magnetic field within which the armature rotates. This consists of Yoke or frame: Acts as a mechanical support of the machine

2000HP DC Motor field SystemDr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 10

Construction of DC Machine: ArmatureThe rotor or the armature core, which carries the rotor or armature winding, is made of sheet-steel laminations. The laminations are stacked together to form a cylindrical structureSlots Slots for wedges Teeth

The armature coils that make the armature winding are located in the slots Non-conducting slot liners are wedged in between the coil and the slot walls for protection from abrasion, electrical insulation and mechanical supportCooling ducts for air circulation

Portion of an armature lamination of a dc machine showing slots and teethDr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 11

Construction of DC Machine: Armature

Armature of a DC MachineDr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 12

Construction of DC Machine: CommutatorCommutator: is a mechanical rectifier, which converts the alternating voltage generated in the armature winding into direct voltage across the brush. It is made of copper segments insulated from each other by mica and mounted on the shaft of the machine. The armature windings are connected to the commutator segments.

CommutatorDr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 13

Construction of DC Machine: Brush

The purpose of the brush is to ensure electrical connections between the rotating commutator and stationary external load circuit. It is made of carbon and rest on the commutator.

Commutator and BrushesDr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 14

Construction of DC Machine: Armature Winding

Top coil sides

Top coil sides

Bottom coil sides

Commutator

1

2

3 Brush

1

2 Brush

Elements of Lap Winding

Elements of Wave Winding

Dr. Adel Gastli

MCTE3210: Electromechanical Systems & Actuators

15

Construction of DC Machine: Armature Winding

End connection

Conductors

Turn

Coil

Winding

Dr. Adel Gastli

MCTE3210: Electromechanical Systems & Actuators

16

Construction of DC Machine: Armature Winding Lap Winding a b c d e a b c d1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

e

Nf g h

S

N

Sf g h

19 20 21 1 +

2

3

4

5

6 -

7

8

9 10 11 12 13 14 15 16 17 18 19 + -

+

+

+

+

+

Ia + Icoil // paths

-

-

-

-

a=b= pbrushes poles17

+Dr. Adel Gastli

MCTE3210: Electromechanical Systems & Actuators

Construction of DC Machine: Armature Windinga b c d e

Wave Winding4 5 6 7 8 9 10

a

b

c

d

e

1

2 3

11 12 13 14 15 16 17 18 19 20 21

Ni h g f j k

S

N

Sf g h j k

17 18 19 20 21 1 -

2

3

4

5

6 +

7

8

9 10 11 12 13 14 15 16 17 18 19 +

i

+

+

+

-

+

Ia + Icoil

+

a=2Nb. of // paths18

Dr. Adel Gastli

-

-

MCTE3210: Electromechanical Systems & Actuators -

Principle of OperationThe Faraday Disk and Faradays LawAn emf is induced in a circuit placed in a magnetic field if either: the magnetic flux linking the circuit is time varying or there is a relative motion between the circuit and the magnetic field such that the conductors comprising the circuit cut a cross the magnetic flux lines. 1st form of the law is the basis of transformers. 2nd form is the basic principle of operation of electric generators.Magnet + N S Brush Copper disk Conducting shaft _ V

Dr. Adel Gastli

MCTE3210: Electromechanical Systems & Actuators

19

Principle of OperationThe right-hand rule and generator actionVelocity, u

VVoltmeter

Flux density, B

Conductor rails uemf, e

= B.A = B.l.s

d dB .l.s = dt dt ds ds e = B.l. ,u = dt dt e=Faradays law or flux cutting ruleDr. Adel Gastli

B

Moving conductor

e

l

e=BluMCTE3210: Electromechanical Systems & Actuators 20

Principle of Operation

Without CommutatorN

l1

S Field pole

Slip rings

N-turn coil

v

brushes

v t

External circuit

Dr. Adel Gastli

MCTE3210: Electromechanical Systems & Actuators

21

Principle of Operation

With CommutatorN

S

vb Commutator segments a

coil

brushes

t v

Dr. Adel Gastli

MCTE3210: Electromechanical Systems & Actuators

22

Single-Phase Full wave Rectifier

Dr. Adel Gastli

MCTE3210: Electromechanical Systems & Actuators

23

Multi-Pole Machines

If p is the number of poles, then p/2 cycles of variation of the flux are encountered every complete mechanical rotation.

360 o md One pole pitch = 180 ed = po

N

B() Pole pitch

ed

p = md 2

N

N S

2

3

S

4

ed md

S

S

N

2

ed : electrical degrees or angular measure in cycles md : mechanical degrees or angular measure in space

Dr. Adel Gastli

MCTE3210: Electromechanical Systems & Actuators

24

Principle of Operation: Armature VoltageEmf conductor = p. . N m Flux / Re v. p. = = 60 time / Re v. (60 / N m )

Emf Total =

Emf conductor Number of conductor / path p. . N m Z p. .Z . N m Emf Total = / = 60 a 60 a

where p = number of poles Z = total number of armature conductors a = number of parallel paths, 2 for wave and p for lab. = flux per pole (Weber) Nm = speed of the motor in the revolutions per minute