MAGM 225

Mr. Conrado

Electric Motor Basics

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?

150 rpm

12”

225 rpm

?

Example 3Find 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?

12” 8”

225 rpm?

Example 4Find 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?

12” 8”

225 rpm

?

150 rpm

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?

30”5”

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.