Electric Motors
Part 1

• How much power is needed
• How much electrical power is available
• Do you have enough capacity in service entrance panel (breaker box)

• 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

• Use equipment manufacturer’s recommendation

• Rule of Thumb: 1/3 hp

• 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

• 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

• 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

• 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 = 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

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

• 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

• Direction of Rotation
• Cost
• Maintenance
– motors with brushes cause radio interference
– repulsion-start interferes at starting
– motors with brushes require more maintenance

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

• 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

• 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

• 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

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

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

• 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

• Rigid (fixed to frame)
• Rigid (adjustable screws)
• Sliding Rails

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

• 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

• Each power line needs protection (3)

• 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

• 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

• Shaft Size (Bore)
• Some pulleys come with several bushings to fit several sizes of shafts

• 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

• Standard V-Pulley
• V-Step Pulley
• Adjustable V-Pulley

• 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

• 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

• 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

• 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

• keep pulleys aligned
• adjust belt tension regularly & properly
• keep belts clean
• use proper belts
• never stretch belts or sheaves