ps basics of a dc motor
DESCRIPTION
PS Basics of a DC MotorTRANSCRIPT
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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 ???