Post on 25-Dec-2015
Discussion #27 – MotorsECEN 301 1
Faith and Works
James 2:17-18
17 Even so faith, if it hath not work, is dead, being alone.
18 Yea, a man may say, Thou hast faith, and I have works: shew me thy faith without they works, and I will shew thee my faith by my works.
Solenoids – a simple motor• Use coil to pull (or push) plunger – a “solenoid”
• Current-induced magnetic field is concentrated in core• Field attempts to “center” the core, which can pull or push plunger
• Other configurations such as a rotary solenoid are common• Relays (electrically operated mechanical switch)
Rotary
Pull/push solenoid
Electro-mechanical Relay• Solenoid used to close a switch
• AC or DC can be used to energize coils• Energized coil pulls plunger or lever which closes switch
contacts• Adding a permanent magnet can make a “holding” relay
• Toggle by energizing• Rotary relays• Relay logic• Solid-state relays
Electromechanical Relay
Ladder logic w/relays
Loudspeakers• Moving “voice coil” is attached to a
diagram• Creates high frequency air movement
(vibration) to couple electrical energy to acoustic energy
• Diaphragm size/shape optimized for frequency range
• Woofer, mid-range, tweeter• Cross-over (filter) networks
Transformer – two coupled inductors• An electric transformer converts an AC
voltage from one value to another• Can step up or step down voltage/current• Also used for signal isolation
P
S
P
S
N
N
V
V
S
P
P
S
N
N
I
I
Power distribution• Generated at medium voltage• Stepped up for long-distance distribution
to minimize line losses• Stepped down locally for consumers
• Lower voltages “safer”
• Long-distance > 138 kV • Residential distribution typically 13.8 kV• House wiring 120/220 V
• Computer chips 1.2-1.5 VDC
120/240 V
Types of electric motors• Electric motors
• Convert electrical energy to rotary motion
• (some linear motors)• Changing electric field produces a
changing magnetic field
• DC machines• Motor/generator• Stepper
• AC machines• Synchronous• Induction
DC Motor• Coil (inductor) rotates between magnetic
poles• Commutator acts as switch to reverse field
polarity to keep rotor turning• Motor/generator
Fixed pole DC machine
“Universal” AC Motor (Ch 20)• Conceptually similar to a DC
machine except that the commutator “switch” is replaced by the alternating voltage/current from AC power source
• Motors can have multiple poles to change the rotation rate
• AC motors typically run at fixed rotation rates, i.e. fixed RPM• Will stall (and burn out) if
overloaded
Variable speed drive and Pulse width modulation• Variable speed motors are DC with a
variable supply voltage/ current or an AC motor with a “variable frequency” (non-60 Hz) controller• DC systems often use “pulse width
modulation”
Stepper Motors• Stepper motors use EXTERNAL switching
to rotate• Use an electronic controller• Motor moves or “cogs” one step for each input
change• Multiple coils are “phased” to create rotation
• Can be used as a break with fixed DC current
• Small, low-torque• Commonly used in mechtronic systems
Actuator Summary• All sorts of motors• Electric controllers provide
additional capability for motors and feedback
• “Mechtronics”• Combination of electronics and
mechanical/electrical systems• Interdisciplinary
• EE, ME, AE• Future is now..
Motor Performance (last of Ch. 20)• (Conventional motors)• How do you select a motor?
• Determine requirements• Needed torque and rotation rate
• Don’t forget startup!• Optimize motor size/power input/stall torque
• Chapter 20 has a nice summary of the design procedure• Get help from an expert
3-phase power• High power is more efficiently transferred using a
scheme known as “3-phase” power• One “leg” is conventional “single-phase” 60 Hz power