Discussion #27 – MotorsECEN 3011 Faith and Works James 2: 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.

Discussion #27 – MotorsECEN 3012 Lecture 27 – Motors & Transformers

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

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 VDC 120/240 V

Typical house wiring diagram “hot” (two) “neutral” “ground” Grounding & Safety

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

AC to DC Conversion Diode – a Semi-conductor switch Allows current to flow one way but not the other V i Ideal Diode V-i diagram Full-wave Bridge Half-wave Bridge Capacitor across load acts as a low-pass filter to reduce ripple applied to load