SPECIFIER GUIDE INTRODUCTION - Baldor.com

1 INTRODUCTION : baldor Electric Company has prepared this Specifiers GUIDE to help you cover all the bases whenyou are specifying electric motors. It will cover in a generic way most of the subjects which youshould consider when writing a specification for an electric motor. Additional specification material isavailable from the National Electric Manufacturers Association. Standard MG-1, and the NationalElectric motor-application engineer matches operating characteristics and load requirements of anapplication with motor characteristics to provide dependable, economic service at minimum high-volume applications, extensive testing is practical to select the motor best suited for anapplication by taking advantage of special electrical or mechanical designs.

2 However, in many appli-cations, because of volume, cost, or expediency, it is desirable to use motors that are available fromthe manufacturers' stock. These motors are generally called standard, stock, or catalog will discuss common industrial applications with guides for the proper use of electric motors the convenience of the motor buyer or SPECIFIER , the National Electrical ManufacturersAssociation (NEMA), a nonprofit trade organization, establishes standards which set forth criticalmounting dimensions, enclosures, and minimum performance. The standards are too long to dis-cuss thoroughly here, but we will discuss them in some detail, because manufacturers build tothese mechanical and electrical GUIDE1 TABLE OF CONTENTS1.

3 POWER SOURCES .. 32. RATINGS .. 33. AMBIENT .. 34. TORQUE .. 45. SERVICE FACTOR .. 46. DUTY CYCLE .. 57. MOUNTINGS .. 58. ENCLOSURES .. 59. INSULATION .. 510. FRAMES AND DIMENSIONS .. 611. EFFICIENCY .. 612. V-BELT APPLICATION .. 613. MULTISPEED MOTORS .. 6 Appendix AFigures ..A-1 SPECIFIER GUIDE2 SPECIFIER GUIDE31. POWER SOURCES:The common power source in the United States is alternating current, 60 power supply may be either single phase or three phase. Generally speaking, single phaseis used on electric motors that are less than 1 horsepower. Motors 1 horsepower and largershould be supplied with three phase power if it is available power supply voltages for single phase are 120 volt and 240 volt.

4 120 volts isgenerally used on motors no larger than 1/3 horsepower. Three phase voltages generallyavailable are 208 volt, 240 volt, 480 volt and 600 volt. Since there is a voltage drop between thepower source and the electric motor, the single phase motors are rated either 115 volt or 230volt. Three phase motors are rated 200 volt, 230 volt, 460 volt and 575 it is rather easy to build a motor suitable for use on two voltages as long as they are in theratio of 2-to-1, it is common to specify single phase motors 115/230 volt. It is also common tospecify three phase motors 230/460 volt. Occasionally three phase motors are designated 200-230/460 volt. This means that the motor can be operated on 200 volts when it is connected for230 volts.

5 When the 230 volt motors operated on 200 volts, there is always a sacrifice in torquecapability and usually a sacrifice in overload RATINGS:Motors are rated in horsepower and RPM. The application always determines the horsepowerload requirement. The RPM is determined by the load and the drive system which is used toconnect the motor to the load. In addition to the horsepower and RPM, consideration shouldalso be given to duty cycle, torques, ambient temperature and service ratings of motors have been standardized by NEMA. See Appendix A. Figure 1,which is an excerpt from the NEMA Standards, lists the horsepower ratings most commonly used motors are squirrel cage induction motors. The synchronous speed ofa motor is determined by the power supply and the number of poles built into the structure ofthe winding.

6 With a 60 cycle power supply the synchronous speeds available are 3600 RPM,1800 RPM, 1200 RPM and 900 RPM. Induction motors develop their torque by operating at aspeed which is slightly less than synchronous speed. Typically they will operate at 3500 RPM,1750 RPM, 1160 RPM or 875 AMBIENT:The ambient conditions are important in selecting the type of motor. Ambient temperature is thetemperature of the air surrounding the motor. Special lubricants and insulation may be necessaryfor either very high or very low ambient temperature. High moisture, humidity, and/or corrosiveenvironments also must be considered when specifying a motor with specific GUIDE4 Standard motors are designed to operate in an ambient temperature of up to 40 degrees C(104 degrees F) and are supplied with standard high temperature grease.

7 At altitudes greaterthan 3300 feet, the lower density of the air reduces the motors cooling ability so altitude as wellas the ambient temperature must be taken into consideration. Refer to the paragraph dealingwith service factors for more information on higher altitude TORQUE:The turning force which a motor develops is known as torque. The amount of torque necessaryto start a load (starting torque) is usually different from the torque required to keep the loadmoving (full load torque). Loads which have a high breakaway friction or that require extratorque for acceleration, should have a motor specified to have high starting specifies design letters to indicate the torque, slip and starting characteristics of threephase induction motors.

8 They are as follows:Design A motors are similar to Design B motors except that starting currents are not limited forDesign A motors by B is the general purpose design used for industrial motors. This design has low startingcurrent and normal torques and slip (approximately 3%) which can be used for many types ofindustrial C motors have high starting torque, low starting current, and also have low slip. Thisdesign is good for hard to start D have very high starting torque, high slip, and low starting current. Design D motors areavailable in 5 to 8% slip and 8 to 13% Appendix A. Figure 2for application SERVICE FACTOR:The service factor shown on the motor nameplate indicates the amount of continuous overloadthe motor can be subjected to, under nameplate conditions, without damaging the motor.

9 Whenthe voltage and frequency are at the same values as shown on the motor nameplate, the motormay be overloaded up to the horsepower indicated by multiplying the rated horsepower by theservice factor. When operated at service factor load, the motor may have an efficiency, powerfactor, and speed slightly different from those shown on the factor can also be used to determine if a motor can be operated continuously ataltitudes higher than 3300 feet satisfactorily. At altitudes greater than 3300 feet, the lowerdensity of air reduces the motor's cooling ability thereby causing the temperature of the motor tobe higher. This higher temperature is compensated for by reducing the effective service factor on motors nameplated with a service factor or greater.

10 If the motor is operatedoutdoors at higher altitudes. it's sometimes possible to use full horsepower and full servicefactor since ambient temperatures are usually lower at those GUIDE56. DUTY CYCLE:A motor should be rated continuous duty if it operates at full load for 60 minutes or more in any24 hour period. If the motor operates less than 60 minutes, it may be given an intermittent dutyrating or a short time rating. In either case, the time designated is that time which will elapsebefore the motor reaches full operating MOUNTINGS:The most popular motor-mounting style for industry is rigid base. This basic mounting is readilyavailable in all frame sizes and enclosures. It is adaptable to direct-connected loads and to beltor chain driven loads, The most common industrial motor will be arranged with the output shaft,base and conduit box located as in See Appendix A.