Motor selection procedures – Part Two

In the previous topic,"Motor selection procedures – Part  oneI 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.

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

NEMA has designated several specific types of motors (see Fig.1), each type having unique speed/torque relationships (see Fig.2) and these designs are described below along with some typical applications for each.

Fig (1): NEMA Motor Class

Fig (2): NEMA Motor Speed/torque Curves

Table (1): Locked-Rotor Torque 

The pull-up torque for NEMA design A and B motors listed in the above Locked-Rotor Torque Table with rated voltage and frequency applied will not be less than the following:

Table (2): Pull-up Torque

note: The pull-up torque of Design C motors, with rated frequency and voltage applied will not be less than 70 percent of the locked-rotor torque in the above Locked-Rotor Torque Table.

Table (3): Breakdown Torque

3.2 Poly-phase Motors Larger Than 500 HP

Ratings larger than those listed in Tables 1, 2 and 3 are not covered by NEMA design letters, but have minimum torques established by NEMA MG1 as follows:

Locked-rotor current of these designs will normally not exceed 650% of full-load current, and will normally be within NEMA locked-rotor KVA limits for a code G or H motor.

3.3 Single-Phase Motors NEMA Design L

NEMA design L motor is a single-phase integral horsepower motor designed to withstand full-voltage starting. Starting performance of these motors is dependent upon a “start” winding controlled by a centrifugal mechanism and switch.

Upon energization of the motor, both the “start” winding and the “run” winding of the motor are connected to the line. As the motor comes up to speed, the centrifugal mechanism will actuate and open the switch, removing the start provisions from the line. The rpm at which this occurs is known as “switch speed”. The motor will then operate at full-load with only the run windings connected.

Maximum locked-rotor currents for Design L, 60 Hz motors are shown in the following table:

3.4 Service Factor

Service factor is defined as the permissible amount of overload a motor will handle within defined temperature limits.

When voltage and frequency are maintained at nameplate rated values, the motor may be overloaded up to the horsepower obtained by multiplying the rated horsepower by the service factor shown on the nameplate.
However, locked-rotor torque, locked-rotor current and breakdown torque are unchanged. 

NEMA has defined service factor values for standard poly-phase drip-proof, 60 Hz motors as shown in the following table:

4- Motor Insulation Type

NEMA has classified insulation systems by their ability to provide suitable thermal endurance. The total temperature is the sum of ambient temperature plus the motor’s temperature rise.

The following image illustrate the temperature rise limits established for various insulation classes per NEMA MG1, Part 12.

Note: An additional 10 degrees C measured temperature rise is permitted when temperatures are measured by detectors embedded in the winding.

4.1 Motor Temperature

A major consideration in both motor design and application is heat. Excessive heat will cause the following effects on motors:
  1. Accelerate motor insulation deterioration and cause premature insulation failure. 
  2. Cause a breakdown of bearing grease, thus damaging the bearing system of a motor.

and, The total temperature a motor must withstand is the result of two factors:
  1. External or ambient temperature.
  2. Internal or motor temperature rise. 
Most motors are designed to operate in a maximum ambient temperature of 40°C. If the ambient temperature exceeds 40°C, the motor may need to be modified to compensate for the increase in total temperature.
The temperature rise is the result of heat generated by motor losses during operation as follows:
  • At no-load, friction in the bearings, core losses (eddy current and hysteresis losses), and stator I2R losses contribute to temperature rise.
  • At full-load, additional losses which cause heating are rotor I2R losses and stray load losses. (NOTE: I = current in amps and R = resistance of the stator or rotor in ohms).

Since current increases with an increase in motor load and under locked-rotor, temperature rise will be significantly higher under these conditions. Therefore, applications requiring frequent starting and/or frequent overloads may require special motors to compensate for the increase in total temperature.

4.2 Motor Cooling

Since the total temperature of a motor is greater than the surrounding environment, heat generated during motor operation will be transferred to the ambient air. The rate of heat transfer affects the maximum load and/or the duty cycle of a specific motor design. Factors affecting this rate of transfer are:

a. Motor enclosure

Different enclosures result in different airflow patterns which alter the amount of ambient air in contact with the motor.

b. Frame surface area

Increasing the area of a motor enclosure in contact with the ambient air will increase the rate of heat transfer. General Electric motor enclosures often are cast with many ribs to increase their surface area for cooler operation.

c. Airflow over motor

The velocity of air moving over the enclosure affects the rate of heat transfer. Fans are provided on most totally-enclosed and some open motors to increase the velocity of air over the external parts.

d. Ambient air density

A reduction in the ambient air density will result in a reduction of the rate of heat transfer from the motor. Therefore, total operating temperature increases with altitude. Standard motors are suitable for operation up to 3300 feet; motors with service factor may be used at altitudes up to 9900 feet at 1.0 service factor.

5- Motor Duty Cycle Applications

The duty cycle of a motor describes the energization / de-energization, and load variations, with respect to time for any given application.
Duty cycle applications may be divided into three basic classifications:

1. Continuous duty

It is a requirement of service that demands operation at an essentially constant load for an indefinitely long time.
This is the most common duty classification and accounts for approximately 90% of motor applications. To size a motor for a specific application with this duty cycle classification, select proper horsepower based upon continuous load.

2. Intermittent duty

It is a requirement of service that demands operation for alternate intervals of load and no-load; or load and rest; or load, no-load and rest; each interval of which is definitely specified.

 Intermittent duty

Select a motor for these applications to match the horsepower requirements under the loaded condition. In some instances, such as a hoist or elevator application, savings in the purchase price of a motor may be possible by designing for the intermittent duty cycle. 30 or 60 minute motors are normally specified. Frequent starts, however, can increase motor heating.

3. Varying duty

It is a requirement of service that demands operation at loads and for intervals of time, which may be subject to wide variation. For this classification of duty cycle, a horsepower versus time curve will permit determination of the peak horsepower required and a calculation of the root-mean-square (RMS) horsepower will indicate the proper motor rating from a heating standpoint.

The following example demonstrates the method for selecting a motor for a varying duty cycle based upon peak horsepower and RMS horsepower requirements assuming constant frequency.

Example for a Varying duty Motor

In the next Topic, I will continue explaining the AC Motors Selection Procedures. So, please keep following.

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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