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Ohm’s Law
+
-
RE
I
E = I * RI = E / RR = E / I
E = VoltageI = CurrentR = Resistance
Bar Magnet
N S
Lines of Flux
Flux Density = # of Flux Lines / per unit area
Magnetic Poles
S NS N
Unlike Poles Attract
Like Poles Repel
N SS N
Current Carrying Conductor
Lines of Flux
Conductor
Current Flow
Electromagnetic Coil
Lines of Flux
Flux DensityDependant on:
• Current• # of Coils• Core Material
Electro-magnetic Coils
Direction of Current determines Magnetic Polarity
DC Motor Rotation Considerations
• Speed
• Torque
Speed
Rpm - revolutions per minute
Rotation of the shaft
Torque Torque is the product ofForce x Lever Arm Length (Radius)
Clockwise and Counter-Clockwise efforts are distinguished by differences in sign, + or -
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
Force Effect of Magnetic Fields
Cancellation
Reinforcement
Mechanical Effects of Magnetic Fields
Rotation is a function of two fields pushing or pulling each other.
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)
Motor Armature, Commutator and Field Wiring Arrangement
F2Brush
F1
Brush
Main FieldMain Field
Commutator Bars
A1
A2
ArmatureCoils
Motor General Equation
EETT = K = KMMN + IN + IAARRAA
ET = Armature (Terminal) VoltageKM = Motor Constant = Motor Field Flux DensityN = Motor SpeedIA = Armature CurrentRA = Armature Resistance
DC Motor Speed
N = EN = ETT
KKMM
Motor Speed Varies by:
ET = Armature (Terminal) VoltageKM = Motor Constant = Motor Field Flux DensityN = Motor Speed
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
DC Motor Horsepower
DC Motor Horsepower Can be Determined By:
HP = T x N 5252
HP = Motor HorsepowerT = Motor TorqueN = Speed
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
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
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
DC Motor Review• Speed is primarily determined by
Armature Voltage
• Torque is determined by
• Armature Current
DC Motor Review• Speed is primarily determined by
Armature Voltage
• Torque is determined by
• Armature Current
One possibility…
• Connect motor directly to the I/O pins
Two directions:
• PD2: 1; PD3: 0
• PD2: 0; PD3: 1
DC Motor Control
What is wrong with this implementation?
• Our I/O pins can source/sink at most 20 mA of current
• This is not very much when it comes to motors…
How do we fix this?
Simple H -B ridge
+ 5 V
What happens with these inputs? 1 0
Simple H -B ridge
Pulse Width Modulation (PWM)
Time On Time On Time On
Total Cycle Time Total Cycle Time Total Cycle Time
Time OnTotal Cycle Time
Duty Cycle =
When we wish to control the speed of a motor we adjust its voltage. This being the age of digital electronics we have found a very fast and efficient way to vary a motor’s voltage. Using powerful transistors (MOSFETS), we switch the voltage supplied to the motor off and then back on very fast (sometimes millions of times a second). The amount of time the voltage is switched on compared to the amount of time it is switched off is also controlled. This is referred to as Pulse Width Modulation (PWM). The most important factor of the PWM signal is the duty cycle.
L293 H-bridge chip
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