AC Motor Selection Procedures – Part Four

In the previous topic,” AC Motor Selection Procedures – Part Three “ I explained some procedures for AC Motor Selection for an application which depends on the characteristics needed in that application which include: 
  1. The power supply, 
  2. System requirements, 
  3. Motor class, 
  4. Motor insulation type, 
  5. Motor Duty Cycle, 
  6. Bearing type, 
  7. Method of mounting the motor, 
  8. The cost and size of the motor, 
  9. Method of speed control, 
  10. Environmental conditions.

Today, I will continue explaining other characteristics of an application needed for AC motor selection and DC Motor selection procedures. 

You can review the following previous topics about motors for more information and good following:

10- Environmental conditions 

10.1 Motor enclosures

The type of enclosure required is dependent upon the surrounding atmosphere in which the motor is installed and the amount of mechanical protection and corrosion resistance required. The two general classes of motor enclosure are: 

1- Open enclosure

An open machine is one having ventilating openings which permit passage of external air over and around the winding of the motor.

2- totally-enclosed 

A totally-enclosed machine is constructed to prevent the free exchange of air between the inside and outside of the motor, but not sufficiently enclosed to be termed air-tight.

I explained the types of motor enclosures in topic “Electrical Motors Basic Components 
“and you can review them there. 

10.2 Altitude

The rating of standard motors assumes operation at sea level in a 40°C ambient. For purposes of standardization it is considered that there is no difference in motor operating temperature between sea level and 3300 feet altitude.

The cooling effect of ventilating air is a function of its density. The atmospheric pressure and density at higher altitudes is reduced and the air cannot remove as much motor heat, causing the motor to run hotter. As a general guide, motor temperature rise increases 1% for every 330 feet above 3300 feet. To keep motor heating within safe limits at altitudes above 3300 feet, there are the following alternatives:

A. Supply a motor designed for standard sea level operation which can either be: 

(1) Operated at less load (a motor with service factor rating of 1.15 or higher can be operated at altitudes up to 9000 feet with a 1.0 service factor), or

(2) Operated in a lower ambient temperature per the following table: 

  • It should be remembered that, although the outdoor ambient temperature at higher altitudes is low, motors probably will be installed indoors in higher ambient temperatures. 
  • Motors applied per A(1) or A(2) above, will have no special altitude or temperature data on the nameplate. 

10.3 Ambient Temperatures 

Standard motors are designed so that the temperature rise produced within the motor, added to the standard 40°C ambient temperature, will not exceed the winding-insulation temperature limit.

The motor may work in two up-normal ambient temperature as follows:

1- High Ambient (If the ambient temperature exceeds 40°C) 

A. in this case, The temperature rise produced in the motor must be offset by:
  • Reducing the load and consequent motor losses. A motor rated for a 40°C ambient temperature and operating in a 50°C ambient, will, if rated 1.15 service factor, carry rated Hp with no overload (1.0 SF) and, if rated 1.0 service factor, carry 90% of rated Hp.
  • Applying a special motor design. 

B. The temperature limit may be raised by the substitution of a higher temperature insulation system, special grease and bearings.

  • Motors applied per A(1) above will not have special ambient temperature or service factor data on the nameplate. 
  • The choice between A(2) and B rests with the motor designer who also may have to use a frame size larger than is standard for the rating. Explosion-proof motors may require frame sizes different from the corresponding totally-enclosed motors. 


  • the maximum allowable ambient temperature for explosion-proof machines is 60°c. 

2- Low Ambient (If the ambient temperature is less than minus 40°C) 

In this case, a Special low-temperature grease and special steel shafts may be required.

DC Motors Selection Procedures

Choosing a dc motor type and associated equipment for a given application requires consideration of several factors which are:

1- Speed range 

The minimum and maximum speeds for an application determine the motor base speed.

2- Allowable speed variation 

Applications requiring constant speed at all torque values should use a shunt-wound motor. If speed changes with load and speed variation must be minimized to less than 2%, a regulator employing tachometer feedback must be used.

3- Torque requirements 

  • The torque requirements at various operating speeds should be determined. 
  • Many applications are essentially constant torque, such as conveyors. 
  • Others, such as centrifugal blowers, require torque to vary as the square of the speed. In contrast, machine tools and center winders are constant horsepower, with torque decreasing as speed increases. Thus, the speed-torque relationship determines the most economical motor. 

4- Reversing 

  • This operation affects the power supply and control. 
  • When the motor cannot be stopped for switching series fields before reverse operation, compound and stabilizing windings should not be used if full load torque is needed in both directions. 
  • Bi-directional operation may also affect brush adjustments. 

5- Duty rating 

DC motors carry one of three ratings:
  • Continuous duty is applied to motors that will continuously dissipate all the heat generated by internal motor losses without exceeding rated temperature rise. 
  • Definite time, intermittent duty motors will carry rated load for specified time without exceeding rated temperature rise. These motors must be allowed to cool to ambient before load is repeated. 
  • Indefinite time, intermittent duty is usually associated with some RMS load of a duty-cycle operation. 

6- Peak torque 

  • The peak torque that a dc motor delivers is limited by that load at which damaging commutation begins. Brush and commutator damage depends on sparking severity and duration. Therefore, peak torque depends on the duration and frequency of occurrence of the overload. 
  • Peak torque is often limited by the maximum current that the power supply can deliver. 
  • Motors can commutate greater loads at low speed without damage. 
  • NEMA standards specify that dc machines must deliver at least 150% rated current for one minute at any speed within rated range, but most motors exceed this requirement. 

7- Heating 

  • The temperature of a dc motor is a function of ventilation and losses in the machine. Some losses in core, shunt-field, and brush friction are independent of load, and vary with speed and excitation. 
  • Several methods can predict operating temperature. The best method is to use thermal capability curves available from the manufacturer. 

This Topic is the end of our course "Introduction to Motors Basics" . so, please keep following our new course for motors " Motors 
Advanced Course"

Note: these topics about Motors in this course EE-1: Beginner's electrical design course is an introduction only for beginners to know general basic information about Motors and Pumps as a type of Power loads. But in other levels of our electrical design courses, we will show and explain in detail the Motor and Pumps Loads calculations.

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