Selecting Electric Motors

Electric MotorsPart 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 Hand-Powered 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– 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

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

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

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 = 2100What size pulley is needed on equipment?1725 X 3 = 2100 X pulley5175 = 2100 x pulley5175 / 2100 = 2.46 or 2 1/2: pulley

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

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

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