AC Motor Basics

TopicsTopics

�Motor Basics

�Motor Construction�Motor Construction

�Application of Variable Speed Motors

Motor BasicsMotor Basics

�Magnetism and Induction

�Motor Speed Relationship

�Motor Performance

�Motor Nameplate Information�Motor Nameplate Information

�Motor Construction

Magnetism and InductionMagnetism and Induction

◦ A Motor Converts Electrical Energy Into Mechanical Energy

◦ AC Alternating Current Reverses Poles of Electro-magnetic Coil 120 Times Per Sec (60hz)

◦ Coils Wound into the Stator of the Motor Create Rotating Magnetic FieldRotating Magnetic Field

◦ Magnetic Field Passing Through the Conductors of the Rotor Induces Current to Flow

◦ Induced Current Produces Magnetic Field Around Rotor Conductors Which Creates Magnet

◦ Rotor and Shaft Follow the Rotating Magnetic Field of the Stator

◦ “And the chase is on!”

Motor Speed RelationshipMotor Speed Relationship◦ The Synchronous Speed of an AC Motor Is the Speed

of the Rotating Magnetic Field Set up by the Stator Winding

◦ Synchronous Speed (RPM) = (120 X Frequency) Divided by the Number of Poles. i.e. 120 x 60Hz / 4-Poles = a Base Speed of 1800 RPM

◦ As Load Torque Is Required From the Motor, Rotor ◦ As Load Torque Is Required From the Motor, Rotor Speed Will Decrease Relative to the Rotating Field. This Is Called “Slip”

◦ “Slip” Induces Current to Flow in the Rotor Which Interacts With Rotating Field and Produces Torque Which Is Transmitted to the Load

◦ Synchronous Motor With Reluctance or Permanent Magnet Rotor Have No Slip and Provide Exact Speed up to Full Load Rating

300

250

200

BREAKDOWN TORQUE

FULL VOLTAGE

PULL-UP TORQUE

P

E

R

C

E

N

STARTING TORQUE

Speed - Torque Curve

Motor PerformanceMotor Performance

Speed - RPM

200

150

100

50

0

FULL LOAD

TORQUE

1800

1740 rpm

N

T

T

O

R

Q

U

E0

Motor PerformanceMotor Performance

◦ Standard NEMA Designs of Various Torque Characteristics to Meet Different Application Loads

NEMA

Design

Starting

Torque

Starting

Current

Breakdown

Torque

Full Load

Slip

Typical sine wave

Applications

A Normal High High Low Mach. Tools, Fans

B Normal Normal Normal Normal General Industrial

C High Normal Normal NormalLoaded Compressor,

Loaded Conveyor

D Very High Low High Punch Press or Hoists

Motor Nameplate InformationMotor Nameplate Information

- The Service Factor on a Motor Nameplate Indicates How Much Above the Nameplate Rating a Motor Can Be Continuously Loaded.

- Example: 40hp 1.15sf Is Good for 46hp Continuously

- The Motor Nameplate Will also Provide Key Application Information to Include: Full Load Application Information to Include: Full Load Current, Locked Rotor Code Letter to Determine Locked Rotor Current, Ambient Temperature, Insulation Class, and Environmental Rating

- ALWAYS USE NAMEPLATE DATA WHEN SELECTING MOTOR OVERLOAD PROTECTION

Motor ConstructionMotor Construction

�Enclosures

�Motor Construction and Components

�Nameplate Nomenclature�Nameplate Nomenclature

Motor EnclosuresMotor Enclosures

� Open◦ Protected - PROT◦ Drip-proof Force Ventilated - DPFV◦ Weather Protected - Type I & II

� Totally Enclosed◦ Non-ventilated - TENV◦ Non-ventilated - TENV◦ Fan Cooled - TEFC◦ Blower Cooled - TEBC◦ Air Over - TEAO◦ Dual Cooled - TEDC-A/A (Air to Air) or -A/W

(Air to Water)◦ Explosion Proof - TEFC-XP

Typical TEFC Motor ConstructionTypical TEFC Motor Construction

Rotor

Stator

Frame

Rear CoverFront Cover

Air Inlet / Fan Guard

Cooling Fan – Shaft

Driven

Output Shaft – to LoadMotor Terminal Housing

Motor Data Nameplate

torque (ft. lb.) x speed

5250Horsepower =

Torque and Horsepower

horsepower x 5250

speed

Torque =

Load and Speed RangeLoad and Speed Range

� Type of Load◦ Constant Torque

◦ Variable Torque

◦ Constant Horsepower◦ Constant Horsepower

�Constant Torque

◦ Heat Generated Is Same at All Speeds

◦ Cooling System Deteriorates at Reduced Speed Unless Equipped

Load and Speed Range

Reduced Speed Unless Equipped With Blower or the Motor is TENV

�Variable Torque◦ Typical of Centrifugal Pumps and Fans◦ Torque Drops As Square of Speed Reduction◦ Operation at Low Speed Not a

Load and Speed Range

◦ Operation at Low Speed Not a Problem Due to Low Torque Requirement◦ Motor Enclosure Dependant Entirely on Environmental Needs - Not Thermal

�Constant Horsepower Operation◦ Torque Falls Off Above Base Speed◦ Constant Power Required at High RPM◦ Common Applications Include:

Load and Speed Range

◦ Common Applications Include:�Winders�Machine Tools�Transmission Test Stands / Dynamometers