FACTORS GOVERNING THE SELECTION OF MOTORS

                               FACTORS GOVERNING

                                             THE

                             SELECTION OF MOTORS :

                The following factors are taken into consideration while selecting a motor for industrial applications. 

1. ELECTRICAL CHARACTERISTICS 

a) Running.characteristics

b) Starting characteristics

c) Speed control 

d) Braking 

a) RUNNING CHARACTERISTICS :

                                    The running characteristics of a motor include the speed-torque or the speed-current characteristics, losses, magnetising current, efficiency and power factor at various loads. The magnetising current and power factor are to be considered in case of A.C motors only.

 b) STARTING CHARACTERISTICS :

 The starting torque developed by a motor should be sufficient to start and, accelerate the motor at its load to the rated speed in a reasonable time. Some motors may have to start against full load torque. e.g-motors driving grinding mills or oil expellers, traction work etc.

 At the time of starting a motor, two torques come into play.

            1. The torque required to overcome the static friction pun 

            2. The torque necessary to accelerate the motor and its load to the desired speed. 

c) SPEED CONTROL :

               In D.C motor the speed can be controlled by following methods

       1. Armature control method

       2. Field control method

 In AC motors, the speed can be controlled by following methods 

1. By changing the supply voltage 

2. By changing the supply frequency 

3. By changing the number of poles of motor 

4. By injecting emf in the rotor circuit 

5. By cascading of motors 

6. By injecting resistance in the rotor circuit

 d) BRAKING :

                If the load is removed from an electric motor and supplied it to be disconnected it will continue io run for sometime due to inertia. To avoid danger to the worker or damage to the products manufactured quick stopping of motor is required. It is done by braking. The braking system should be reliable and quick in action. The braking torque must be controllable. There are two types of braking. 

                            

                         1. Mechanical braking

                         2. Electrical braking

2. MECHANICAL CHARACTERISTICS :

While selecting a motor for a particular drive, the mechanical characteristics are also taken into account. The following mechanical features determine the suitability of the motor. 

a) Types of enclosures 

b) Bearings

c) Noise 

d) Transmission of drive (method of coupling) 

a) TYPES OF ENCLOSURES :

 All the major parts of the motors such as windings, bearings, insulation etc are to be protected from the surrounding contaminated air. In industry surrounding the motors may contain metal, dust, oil, mist, water, dust inflammable fumes etc. Also accidents may occur to persons coming in contact with the moving parts. necessary to provide proper enclosures. Therefore it The different types of enclosures are as follows. 

1. Open type

2. Screen protected type 

3. Drip proof type 

4. Totally enclosed type 

5. Splash proof type 

6. Flame proof type 

7. Pipe ventilated type

b) BEARINGS  :

                   bearing are the parts of machines which house and support the main shaft. Bearings provide free rotation of the moving parts with minimum friction. There are two types of bearings usually employed in motors.

                      

                        1. Ball or roller bearing 

                        2. Sleeve or bush bearing 

c) NOISE :

                  This is another important feature to be considered while making the selection of a motor. Noise should be kept as low as possible in the workshops, hospitals and other domestic purposes. vibrations, magnetic pulsations, and faulty foundations. The noise may be due to bearing, To reduce noise, journal bearing may be used in place of ball bearings. The motor should be mounted on a heavy concrete or cast iron block. The electrical connections should be made through flexible conduits. 

d) TRANSMISSION OF DRIVES:

                 For transmilt power from driving machine to driven machine power different types of drives are used. They are 

                                 1. Direct drive 

                                 2. Belt drive 

                                 3. Rope drive 

                                 4. Chain drive 

                                 5. Gear drive

3. SIZE OF MOTOR IE., STANDARD RATING OF MOTORS :                                Whether the motor is required for continuous load, intermittent load or variable load. 

4. COST :

a) Capital cost b) Running cost, power factor, losses, maintenance and depreciation etc. 

5. NATURE OF LOAD :

          there are three types of  nature of load of the motor are their are. 

                                   a) resistive load 

                                   b) inductive load

                                   c) capacitive load

6. APPEARANCE :

               The motor selected must fulfil all the necessary load requirements and at the same time it should not be very costly. 

7. POWER SUPPLY :

            Whether for single phase AC, 3 phase AC or D.C supply.

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