AC Motor Selection Procedures – Part Three

In the previous topic,” Motor selection procedures – Part Two “ 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.

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

6- Bearing Types 

Bearings, mounted on the shaft, support the rotor and allow it to turn. Not all bearings are suitable for every application; a universal, all-purpose bearing does not exist. The choice of bearing arrangement is based on the following qualities:
  • Load carrying capacity in the axial and radial direction. 
  • Overspeed and duration. 
  • Rotating speed. 
  • Bearing life. 

The size of the bearing to be used is initially selected on the basis of its load carrying capacity, in relation to the load to be carried, and the requirements regarding its life and reliability.

Other factors must also be taken into consideration, such as operating temperature, dirty and dusty environmental conditions, and vibration and shocks affecting bearings in running and resting conditions.

There are many types of bearings on the market, each with different characteristics and different uses, these types were listed before in topic “Electrical Motors Basic Components
and you can review them there. 

7- Method of mounting the motor

7.1 Motor Mounting Configurations

Motors may be furnished for any of twelve mounting configurations classified under 3 main mounting positions:

  • Floor Mounting Positions,
  • Ceiling Mounting Positions,
  • Wall Mounting Positions.

A- Floor Mounting Positions 

The typical floor mounting positions are illustrated in fig(1) , and are referred to as F-1 and F-2 mountings.


The conduit box can be located on either side of the frame to match the mounting arrangement and position. The standard location of the conduit box is on the left-hand side of the motor when viewed from the shaft end. This is referred to as the F-1 mounting. The conduit opening can be placed on any of the four sides of the box by rotating the box in 90° steps.

B- Ceiling Mounting Positions

With modification, a foot-mounted motor can be mounted on a ceiling. Typical ceiling mounts are shown in fig (2) . Ceiling mounted positions have the prefix C.


C- Wall Mounting Positions,

With modification, a foot-mounted motor can be mounted on a wall. Wall mounting positions have the prefix W. Wall mounting positions can be divided to two types according to the shaft direction as follows:

C.1 Shaft Horizontal
Typical wall mounts, Shaft Horizontal are shown in the fig (3).


C.2 Shaft Vertical
Typical wall mounts, Shaft Vertical are shown in the fig (4).


Note: Motors which are to be mounted vertically must be specified shaft up or down to obtain a suitable bearing and lubrication system.

7.2 Motor Mounting Faces


It is sometimes necessary to connect the motor directly to the equipment it drives as in fig (5) where a motor is connected directly to a gear box. The motor be connected to the equipment by one of the following two methods:

(1) C-face
The face, or the end, of a C-face motor has threaded bolt holes. Bolts to mount the motor pass through mating holes in the equipment and into the face of the motor.

(2) D-flange
The bolts go through the holes in the flange of a D-flange motor and into threaded mating holes of the equipment.

8- The cost and size of the motor

This point will be explained later in our “Motor advanced course” that will be issued shortly.

9- Method of Motor speed control

There are four major motor control topics or categories to consider. Each of these has several subcategories and sometimes the subcategories overlap to some extent. These four categories are:

  • Motor Starting,
  • Motor Protection,
  • Motor Stopping,
  • Motor Operational Control.

The motor speed control is classified under the forth category “Motor Operational Control” which – and other categories- will be explained in detail in our “Motor advanced course” that will be issued shortly.

However, Motor speed controls can be divided into two basic categories:
  • Passive device speed controls, 
  • Solid state controls. 

A- Passive device speed controls:

Passive device controls consist of fixed or variable resistors, or variable transformers that are used to adjust the magnetic field strength, voltage levels or other motor characteristics (depending on the motor type), in order to control motor speed.

Passive device speed controls

Passive devices such as resistors increase the motor circuit resistance, causing increased power dissipation in the form of heat. This additional heat produces no useful work and decreases the overall efficiency of the system.

B- Solid state speed controls:

Solid state controls utilize more complex circuits consisting of active devices like diodes, thyristors, transistors, integrated circuits and in some cases, microprocessors to control motor voltage, power supply frequency, or to provide electronic commutation and thereby control motor speed.

Solid state speed controls
With the development of semiconductors, it became possible to vary motor speed through voltage switching rather than by adding resistance to the drive circuit.

Instead of varying the level of resistance, switching amplifiers vary the time during which full line voltage is applied to the armature. The net effect is an average voltage which is roughly equivalent to a voltage level obtained by the variable resistance- type control.

Examples for Solid state controls are:

  • Half-Wave SCR Controls.
  • Full-Wave SCR Controls.
  • Pulse Width Modulation Control.

In the next Topic, I will continue explaining the AC Motors Selection Procedures and DC 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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