Why use electric motors?• Adaptable – can be used anywhere• Automatic – can be controlled with automatic devices.• Compact – large power in small unit.• Dependable – if chosen properly they give little trouble.• Economical – can replace many man hours.• Efficient – can be up to 95% efficient.• Low-maintenance – require little or no maintenance.• Quiet• Safe• Simple to operate
Selecting the proper motor• Power supply – What power is available 120v, 240, single phase, three phase? Most motors larger than ½ HP are run on 240v for greater efficiency and three phase for even more efficiency.• Power Requirements – How much HP is required? – Is machine easy to start? • If not a high torque motor may be needed. – What speed is needed? – Generally when replacing motors the replacement should match as closely as possible to the old motor.
Selecting the proper motor• Base, Bearings, & Frame – Generally the motor base or the mounting frame are slotted to accommodate bolts for mounting. – Often rubber mounts are used to reduce vibration.• Ball Bearing vs. Bushing – Ball/Needle bearings are a set of hardened balls or needles set in a metal frame. They are usually permanently lubricated, have a higher initial cost, but last longer. They can be mounted in any position, create less friction, withstand greater shaft pressure, and rarely need replacing. – Bushings is a sleeve in a frame that keeps the shaft centered. Bushing are cheaper but require lubrication, wear out faster, and withstand less shaft pressure.
Selecting the proper motor• Starting and running Characteristics – Generally electric motors are connected directly to the load with no clutch or drive train. Therefore must have sufficient torque to start.
Interpreting nameplate information• Model and serial number – Enable the manufacture to identify the motor and useful for warranty and replacement• Type, Horsepower, RPM, and duty ratios all indicate the capabilities of the motor. – Horsepower • 1HP = 33,000lbs lifted one foot in minute. • Calculated HP = lbs. lifted x Distance in feet x time in seconds ÷ 550 • W(LBS) X D(FT) X T(SEC) ÷ 550 • An efficient motor will generate one HP while using about 800 watts. – Duty Rating refers to the percent time a motor may run without overheating. – Temperature rise indicates how hot a motor can get without internal damage.
Motor Drives• Motor Drives – Direct drive couplers • Flexible-hose, Flange, Cushion Flange, and Flexible shaft – Belts • Permit the motor and machine to run at different speeds. • Flexible and quiet • Classified by shape and size
Motor Drives • Pulley replacement – To change desired output speed – Lost or destroyed pulleys • Formula – The formula can be rearranged to find the needed component.Formula:
Example 1 Find Driven Pulley Speed• Example – What is the speed of an 8” driven pulley if the diameter of the drive pulley is 12” and a speed of 150 RPM? ? 150 rpm 12” 8”
Example 2 Find Drive Pulley Diameter• Calculating drive pulley diameter is based on knowing the speed of the driven pulley and the diameter of the drive pulley. The formula:• Example – What is the diameter of a driven pulley rotating at 225 rpm if the diameter of the drive is 12” and its speed is 150 rpm? 225 rpm 150 rpm 12” ?
Example 3 Find Drive Pulley Speed• Calculating drive pulley speed is based on knowing the diameters of the drive and driven pulleys and the driven pulley speed. The formula:• Example – What is the speed of a 12” drive pulley if the diameter of the driven pulley is 8” and its speed is 225 RPM? 225 rpm ? 12” 8”
Example 4 Find Drive Pulley Diameter• Calculating drive pulley diameter is based on knowing the speed of the drive pulley and the speed and diameter of the driven pulley. – The formula:• Example – What is the speed of a 12” drive pulley if the diameter of the driven pulley is 8” and its speed is 225 RPM? 225 rpm 150 rpm 12” ? 8”
Belt Length• Belt length can be determined by using the pulley diameters and the proper formula. •Example: •What is the belt length for two pulleys 5” and 10” in diameter and 30” apart at center? 5” 30” 10”
Step Pulley Determination• Given: A motor speed of 1725rpm and a step pulley of 2”,3”,4”, & 5” inverted pulleys.• Find the speed at a given belt location.• Calculation: 1. Divide the diameter of the motor pulley by the size of the corresponding driven pulley. In example below 3” ÷ 4” = 0.75. 2. Then multiply the motor speed by the result of the above calculation to get the spindle speed. 1725 x .75 + 1293 rpm.
Motor Switches• Types of switches – Toggle, rotary, Push-button, trigger, or magnetic. – Magnetic switches are recommended for motors 1hp or more.• Reversing switches – Allows the machine to run in either direction.