Download Selecting Electric Motors

Selecting Electric Motors
Electric Motors
Part 1
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)
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
Motor on New Equipment
• Use equipment manufacturer’s
recommendation
Installing a Motor on HandPowered Equipment
• Rule of Thumb: 1/3 hp
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
Power Supply
• 3-Phase, 208, 230 or more volts
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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
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
What Motor Speed to Select
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Determine speed of equipment
Speed is in RPM’s
Most common: 1750
If different speed is needed, use pulley, gear,
or chains to convert
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
Starting Loads
• Easy Starting Loads:
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Shaded Pole Induction
Split Phase
Permanent-Split, Capacitor-Induction
Soft-Start
Starting Loads
• Difficult Starting Loads
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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
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
Bearing Types
• Sleeve Bearings: brass, bronze or tin lined
cylinder
• Ball Bearings: round steel balls surround
the shaft in a special cage
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
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
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
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
Enclosures
• Motors produce heat
• Cooling: fan on shaft, openings in end
• Must protect from dust, water etc
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
Mounting Base
• Rigid (fixed to frame)
• Rigid (adjustable screws)
• Sliding Rails
Overload Protection
• Excessive Current will flow to the motor if:
Load is too heavy
Voltage is too low
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
Three-Phase Protection
• Each power line needs protection (3)
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
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
What Size of Drive to Select
• Shaft Size (Bore)
• Some pulleys come with several bushings to
fit several sizes of shafts
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
Pulley Types
• Standard V-Pulley
• V-Step Pulley
• Adjustable V-Pulley
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
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
Sizing Pulleys (Jack Pulley) p.50
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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
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
Factors Affecting Belt Life
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keep pulleys aligned
adjust belt tension regularly & properly
keep belts clean
use proper belts
never stretch belts or sheaves