Motor Basics. Application of Variable Speed Motors

Transcription

1 AC Motor Basics

2 Topics Motor Basics Motor Construction Application of Variable Speed Motors

3 Motor Basics Magnetism and Induction Motor Speed Relationship Motor Performance Motor Nameplate Information Motor Construction

4 Magnetism and Induction A Motor Converts Electrical Energy Into Mechanical Energy AC Alternating Current Reverses Poles of Electromagnetic Coil 120 Times Per Sec (60hz) Coils Wound into the Stator of the Motor Create Rotating 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!

5 Motor 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 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

6 Motor Performance Speed - Torque Curve P E R C E N T T O R Q U E STARTING TORQUE PULL-UP TORQUE Speed - RPM BREAKDOWN TORQUE FULL VOLTAGE rpm FULL LOAD TORQUE

7 Motor 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 Normal Loaded Compressor, Loaded Conveyor D Very High Low High Punch Press or Hoists

8 Motor 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 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

9 Motor Construction Enclosures Motor Construction and Components Nameplate Nomenclature

10 Motor Enclosures Open Protected - PROT Drip-proof Force Ventilated - DPFV Weather Protected - Type I & II Totally Enclosed 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

11 Typical TEFC Motor Construction Rear Cover Stator Front Cover Air Inlet / Fan Guard Frame Rotor Cooling Fan Shaft Driven Motor Data Nameplate Motor Terminal Housing Output Shaft to Load

12 Torque and Horsepower Horsepower = torque (ft. lb.) x speed 5250 Torque = horsepower x 5250 speed

13 Load and Speed Range Type of Load Constant Torque Variable Torque Constant Horsepower

14 Load and Speed Range Constant Torque Heat Generated Is Same at All Speeds Cooling System Deteriorates at Reduced Speed Unless Equipped With Blower or the Motor is TENV

15 Load and Speed Range Variable Torque Typical of Centrifugal Pumps and Fans Torque Drops As Square of Speed Reduction Operation at Low Speed Not a Problem Due to Low Torque Requirement Motor Enclosure Dependant Entirely on Environmental Needs - Not Thermal

16 Load and Speed Range Constant Horsepower Operation Torque Falls Off Above Base Speed Constant Power Required at High RPM Common Applications Include: Winders Machine Tools Transmission Test Stands / Dynamometers