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AC Motor Basics
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TopicsTopics
�Motor Basics
�Motor Construction�Motor Construction
�Application of Variable Speed Motors
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Motor BasicsMotor Basics
�Magnetism and Induction
�Motor Speed Relationship
�Motor Performance
�Motor Nameplate Information�Motor Nameplate Information
�Motor Construction
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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!”
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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
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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
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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
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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
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Motor ConstructionMotor Construction
�Enclosures
�Motor Construction and Components
�Nameplate Nomenclature�Nameplate Nomenclature
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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
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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
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torque (ft. lb.) x speed
5250Horsepower =
Torque and Horsepower
horsepower x 5250
speed
Torque =
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Load and Speed RangeLoad and Speed Range
� Type of Load◦ Constant Torque
◦ Variable Torque
◦ Constant Horsepower◦ Constant Horsepower
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�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
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�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
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�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