Selecting Electric Motors

1
Selecting Electric Motors

• Electric Motors
• Part 1

Original Power Point Created by Casey
Osksa Modified by Georgia Agricultural Education
Curriculum Office June 2002
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 manufacturers 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 RPMs
• 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

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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

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Three-Phase Protection

• Each power line needs protection (3)

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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

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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

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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

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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

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Sizing Pulleys

• RPM of motor pulley X Dia. of motor pulley RPM
  of equip. pulley X Dia. equip. pulley
• Example Motor 1725 rpms 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

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Sizing Pulleys (Jack Pulley) p.50

• 1725 rpm X 2 pulley rpm X 12 pulley
• 287.5 rpms on Jack pulley
• 287.5 rpm X 2 pulley 70 rpm X Pulley
• 8 pulley on equipment

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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