ps basics of a dc motor

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DC MOTORDC MOTOR

BASICSBASICS

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Purpose of an “DC Motor”

ElectricalEnergy

MechanicalEnergy

The purpose of a DC Motor is to Convert Electrical Energy into Mechanical Energy

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Ohm’s Law

+

-

RE

I

E = I * RI = E / RR = E / I

E = VoltageI = CurrentR = Resistance

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Bar Magnet

N S

Lines of Flux

Flux Density = # of Flux Lines / per unit area

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Magnetic Poles

S NS N

Unlike Poles Attract

Like Poles Repel

N SS N

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Current Carrying Conductor

Lines of Flux

Conductor

Current Flow

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Electromagnetic Coil

Lines of Flux

Flux DensityDependant on:

• Current• # of Coils• Core Material

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Electo-magnetic Coils

Direction of Current determines Magnetic Polarity

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DC Motor Rotation Considerations

• Speed

• Torque

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Speed

Rpm - revolutions per minute

Rotation of the shaft

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Torque Torque is the product ofForce x Lever Arm Length (Radius)

Clockwise and Counter-Clockwise efforts are distinguished by differences in sign, + or -

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DC Motor

1) Armature

- coils of wire on the shaft

2) Field (Shunt Field)

- coils of wire built into stationary frame

A DC motor consists of two electromagnetic fields

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DC MOTOR CONSTRUCTION

Commutator & Commutator & Brush AssemblyBrush Assembly

Commutator & Commutator & Brush AssemblyBrush Assembly

Armature Armature AssemblyAssembly

Armature Armature AssemblyAssembly

Distinct Armature & Field Circuits Distinct Armature & Field Circuits

are mechanically separatedare mechanically separated

Field Poles Field Poles AssembliesAssemblies

Field Poles Field Poles AssembliesAssemblies NOTE: The Armature & NOTE: The Armature &

Field Circuits are Field Circuits are mechanically fixed at mechanically fixed at

90° at all times90° at all times

NOTE: The Armature & NOTE: The Armature & Field Circuits are Field Circuits are

mechanically fixed at mechanically fixed at 90° at all times90° at all times

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Force Effect of Magnetic Fields

Cancellation

Reinforcement

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Force Couple

N S+

F

F

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Mechanical Effects of Magnetic Fields

Rotation is a function of two fields pushing or pulling each other.

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Rotation

–

+

NFIELD

SFIELD

STRONGEST FIELDPUSHES CONDUCTOR DOWN

(REPULSION)

STRONGEST FIELDPUSHES CONDUCTOR UP

(REPULSION)

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Principle of a DC Motor• A DC motor has two independent

electromagnetic fields• Controlled independent of each other

• Either field can influence the performance of the motor• Speed (rpm)• Torque (ft lb)

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Motor Armature, Commutator and Field Wiring Arrangement

F2Brush

F1

Brush

Main FieldMain Field

Commutator Bars

A1

A2

ArmatureCoils

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Typical DC Motor

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DC Motor

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Commutator

DC brushes transfer electrical current

from DC Drive to DC Armature

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Brush Rig Assembly

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DC Motor Commutator/Armature

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Motor General Equation

EETT = K = KMMN + IN + IAARRAA

ET = Armature (Terminal) VoltageKM = Motor Constant = Motor Field Flux DensityN = Motor SpeedIA = Armature CurrentRA = Armature Resistance

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DC Motor Speed

N = EN = ETT

KKMM

Motor Speed Varies by:

ET = Armature (Terminal) VoltageKM = Motor Constant = Motor Field Flux DensityN = Motor Speed

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DC Motor Torque

T = KT = KTTIIAA

DC Motor Torque varies by:

T = Motor TorqueKT = Motor Constant

(# poles, armature conductors) = Motor Field Flux DensityIA = Armature Current

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DC Motor Horsepower

DC Motor Horsepower Can be Determined By:

HP = T x N 5252

HP = Motor HorsepowerT = Motor TorqueN = Speed

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Speed Power CurveArmature Voltage Contro l

Constant F ie ld Current

F ie ld Current Contro l

Constant ArmatureVoltage

Constant Power

Speed (% of Base Speed)

Po

we

r (%

of

Ra

ted

)100

100

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Speed Torque Curve

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Types of DC Motors• Shunt Wound

• Straight Shunt

• Compound Wound• Stabilized Shunt

• Permanent Magnet

• Series Wound

Most DC Motors are:

Note: Straight Shunt must be used with reversing/regen

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DC Motor Review• Speed is primarily determined by

Armature Voltage

• Torque is determined by

Armature Current

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DC Motor Review• Rotation is the result of the interaction of

two independent electromagnetic fields.• Armature windings• Field windings

• Extended motor speeds above base speed are achieved by reducing the field voltage (current)• Field Weakening

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Questions ???