1. Selecting 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 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