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.

Discussion #27 – MotorsECEN 301 2

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

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

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

iIdeal DiodeV-i diagram

Full-wave Bridge

Half-wave Bridge

Capacitor across load acts as a low-pass filter to reduce ripple applied to load