Transcript A DC motor consists of two electromagnetic fields

Ohm’s Law
E=I*R
I=E/R
R=E/I
I
+
E
R
-
E = Voltage
I = Current
R = Resistance
Bar Magnet
N
S
Lines of Flux
Flux Density =
# of Flux Lines / per unit area
Magnetic Poles
Unlike Poles Attract
S
N
S
N
S
N
N
S
Like Poles Repel
Current Carrying Conductor
Conductor
Lines of Flux
Current Flow
Electromagnetic Coil
Flux Density
Dependant on:
Lines of Flux
• Current
• # of Coils
• Core Material
Electro-magnetic Coils
Direction of Current determines Magnetic Polarity
DC Motor
Rotation Considerations
• Speed
• Torque
Speed
Rotation of the shaft
Rpm - revolutions per minute
Torque
Torque is the product of
Force x Lever Arm Length (Radius)
Clockwise and Counter-Clockwise
efforts are distinguished by
differences in sign, + or -
DC Motor
A DC motor consists of two electromagnetic fields
• 1) Armature
•
- coils of wire on the shaft
• 2) Field (Shunt Field)
•
- coils of wire built into stationary
frame
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)
A2
A1
Brush
Commutator Bars
Main Field
Main Field
Armature
Coils
F2
Brush
F1
Motor Armature, Commutator and
Field Wiring Arrangement
Motor General Equation
ET = KMN + IARA
ET
KM

N
IA
RA
= Armature (Terminal) Voltage
= Motor Constant
= Motor Field Flux Density
= Motor Speed
= Armature Current
= Armature Resistance
DC Motor Speed
Motor Speed Varies by:
N = ET
KM
ET
KM

N
= Armature (Terminal) Voltage
= Motor Constant
= Motor Field Flux Density
= Motor Speed
DC Motor Torque
DC Motor Torque varies by:
T = KTIA
T
KT

IA
= Motor Torque
= Motor Constant
(# poles, armature conductors)
= Motor Field Flux Density
= Armature Current
DC Motor Horsepower
DC Motor Horsepower
Can be Determined By:
HP = T x N
5252
HP
T
N
= Motor Horsepower
= Motor Torque
= Speed
Speed Power Curve
Armature Voltage Control
Field Current Control
Constant Field Current
Constant Armature
Voltage
Power (% of Rated)
100
Constant Power
ta
s
n
o
T
t
u
rq
e
n
Co
100
Speed (% of Base Speed)
Speed Power Curve
Armature Voltage Control
Field Current Control
Constant Field Current
Constant Armature
Voltage
Power (% of Rated)
100
Constant Power
ta
s
n
o
T
t
u
rq
e
n
Co
100
Speed (% of Base Speed)
Types of DC Motors
• Shunt Wound
– Straight Shunt
Most DC Motors are:
• Compound Wound
– Stabilized Shunt
• Permanent Magnet
• Series Wound
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
DC Motor Control
One possibility…
• Connect motor
directly to the I/O pins
Two directions:
• PD2: 1; PD3: 0
• PD2: 0; PD3: 1
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-Bridge
+5V
Simple H-Bridge
What
happens
with
these
inputs?
1
0
Pulse Width Modulation (PWM)
Time On
Time On
Total Cycle Time
Total Cycle Time
Duty Cycle =
Time On
Total Cycle Time
Time On
Total Cycle Time
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