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Selecting Electric Motors

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  1. Selecting Electric Motors Electric Motors Part 1

  2. What Size Motor to Select • How much power is needed • How much electrical power is available • Do you have enough capacity in service entrance panel (breaker box)

  3. Replaceing a Motor • Replacing a Small Portable Gas Engine: • rule: 2/3 to 3/4 as much power as engine • Replacing an Industrial Engine: • “Maximum Brake” or “Rated Brake” hp, use same rule • “Continuous Brake” or “Kilowatts”, use same hp as engine • Replacing Tractor PTO: • same hp as tractor

  4. Motor on New Equipment • Use equipment manufacturer’s recommendation

  5. Installing a Motor on Hand-Powered Equipment • Rule of Thumb: 1/3 hp

  6. Power Supply • Single Phase, 115 or 230 volts • limited to 7 1/2 hp • most farms and homes • many motors will run on 115 or 230 volts

  7. Power Supply • 3-Phase, 208, 230 or more volts • 4 wires in power line • up to 1,000 hp • little or no light flickering • cost less • last longer • pay extra to install 3-phase power lines

  8. Service Entrance Capacity • SEP must have about 3 times more amperage capacity than the amp rating on the nameplate of the motor • for extra amps for starting the motor • if motor is 20 amps, SEP must be at least 60 amps • May need a separate SEP

  9. What Motor Speed to Select • Determine speed of equipment • Speed is in RPM’s • Most common: 1750 • If different speed is needed, use pulley, gear, or chains to convert

  10. Motor Duty • Motor Duty = amount of time the motor is operating under full load, and how much time it is stopped • Continuous Duty: constant full load for over 60 minutes at a time • Intermittent Duty: fully loaded for 5, 15, 30, or 60 minutes

  11. Starting Loads • Easy Starting Loads: • Shaded Pole Induction • Split Phase • Permanent-Split, Capacitor-Induction • Soft-Start

  12. Starting Loads • Difficult Starting Loads • Capacitor-Start, Induction-Run • Repulsion-Start, Induction-Run • Capacitor-Start, Capacitor-Run • Three-Phase, General-Purpose • Perkey Concept: use tractor PTO to start • Repulsion-Start, Capacitor-Run

  13. Other Factors to Consider • Direction of Rotation • Cost • Maintenance • motors with brushes cause radio interference • repulsion-start interferes at starting • motors with brushes require more maintenance

  14. Bearing Types • Sleeve Bearings: brass, bronze or tin lined cylinder • Ball Bearings: round steel balls surround the shaft in a special cage

  15. Lubrication • Sleeve Bearings: SAE 20 non-detergent or electric motor oil • avoid over oiling • wipe off excess oil • Oil Wick: wick into small oil well under the sleeve bearing • refill well at least twice / year

  16. Lubrication: Sleeve Bearings • Yarn Packed: add few drops of oil every few months to yarn • Ring Oiled: ring spins freely in oil reservoir • keep oil level up to fill plug

  17. Lubrication: Ball Bearings • Prelubricated and Sealed: no maintenance required • Hand Packed: disassemble bearing and hand pack with grease every 2-5 years • Special Fittings: filler and drain plug • remove bottom plug before greasing

  18. Mounting Position • Sleeve Bearings: parallel to floor • may need to rotate end shield to prevent oil from running out of reservoir • Ball Bearing: any position

  19. Enclosures • Motors produce heat • Cooling: fan on shaft, openings in end • Must protect from dust, water etc

  20. Enclosures • Dripproof: (open-type) • must provide clean air & keep water away • Totally-Enclosed: no openings for circulation of outside air through motor • may use external fan • higher operating temperature • Explosion Proof: hazardous locations

  21. Mounting Base • Rigid (fixed to frame) • Rigid (adjustable screws) • Sliding Rails

  22. Overload Protection • Excessive Current will flow to the motor if: Load is too heavy Voltage is too low

  23. Types of Overload Protection • Built-In Overload Protection in Motor • Manual-Reset Type • Automatic-Reset • Manual Starting Switch with Overload Protection (breaker in switch) • Magnetic Starting Switch with Overload Protection (power tools) • Time-Delay Fuse in Motor Disconnect Switch • Current-Limiting Starters

  24. Three-Phase Protection • Each power line needs protection (3)

  25. Motor Drives • Direct: connect motor to equipment Flexible-Hose Coupling Flange Coupling: flange attaches to motor, another to equipment, flanges attach to flexible disk Cushion-Flange Coupling: tire shaped cushion between flanges Flexible Shaft: direction of rotation is important

  26. Speed-Conversion Drives • Gear Drive • Chain-and-Sprocket Drive • Pulley-and-Belt Drive: pulleys connected by continuous belt loop V-Belt Webbed Multi-V-Belt Flat-Belt V-Flat

  27. What Size of Drive to Select • Shaft Size (Bore) • Some pulleys come with several bushings to fit several sizes of shafts

  28. Sizing Drives • When operating speeds are changed, horsepower changes in same proportion • if equipment speed doubles, horsepower requirement doubles • Fans, Blowers, Centrifugal Pumps: • speed increases, horesepower requirement increases by cube of increment of increase 3 hp motor, double speed: (3hp x 2 x 2 x 2) = 24 hp

  29. Pulley Types • Standard V-Pulley • V-Step Pulley • Adjustable V-Pulley

  30. Sizing Pulleys • Pulley Selection Chart (p.49) • Size of pulley on motor • under 1/2 hp, keep pulley under 2” diameter • over 1/2 hp, pulley 3” or larger • Move across chart to desired equipment speed • Move up to find equipment pulley size

  31. Sizing Pulleys • RPM of motor pulley X Dia. of motor pulley = RPM of equip. pulley X Dia. equip. pulley Example: Motor = 1725 rpm’s with 3” pulley Desired rpm of equipment = 2100 What size pulley is needed on equipment? 1725 X 3 = 2100 X pulley 5175 = 2100 x pulley 5175 / 2100 = 2.46 or 2 1/2: pulley

  32. Sizing Pulleys (Jack Pulley) p.50 • 1725 rpm X 2” pulley = rpm X 12” pulley • 287.5 rpm’s on Jack pulley • 287.5 rpm X 2” pulley = 70 rpm X Pulley • 8” pulley on equipment

  33. Belt Types • FP = Fractional Power (3L): 2 1/2” pulley or smaller, less slippage • A-Section: (4L): heavier, larger pulleys to prevent slippage (3” or larger) • B, C, D, E: larger belts, larger pulleys • Belt should have same width of groove as pulley • Top of belt should sit flush with top of pulley

  34. Factors Affecting Belt Life • keep pulleys aligned • adjust belt tension regularly & properly • keep belts clean • use proper belts • never stretch belts or sheaves