The factor is shown below for the selection of an electrical drive
(a) Electrical characteristics
1. Starting Characteristics-
The starting characteristics of the drive are to find the relation between current-speed and current torque. The different motor has different starting characteristics.In the case of a DC motor, the stated characteristics are shown below-
We have seen that torque is proportional to the product of flux and armature current. In this case, as field windings also carry the armature current, therefore, flux is proportional to the armature current up to the point of magnetic saturation. Hence, before saturation, torque is proportional to the product of flux and armature current and after saturation torque is proportional to the square of armature current.
Before saturation
After saturation
2. Running Characteristics-
The running characteristics of the drive are to find the relation between speed-torque. The different motor has different running characteristics. Running characteristics are also known as mechanical characteristics.Running characteristics for the DC motor is shown below
The running characteristics of the induction motor are shown below
3. Speed control-
Different drive operation tasks employed different speeds, so speed control is one of the important parameters for the selection of an electrical drive.
For example-
Squirrel Cage Induction Motor. This motor is quite simple but rugged and possesses high over-load capacity. It has a nearly constant speed and poor starting torque. Hence, it is used for low and medium-power drives where speed control is not required as for water pumps, tube wells, lathes, drills, grinders, polishers, wood planers, fans, blowers, laundry washing machines and compressors etc.
D.C. Series Motor. Since it has high starting torque and variable speed, it is used for heavy-duty applications such as electric locomotives, steel rolling mills, hoists, lifts, and cranes.
4. Braking-
There are three types of electric braking applicable to electric motors is given below
1. Plugging or reverse-current braking.
2. Rheostatic or dynamic braking.
3. Regenerative braking.
Sometimes it is desirable to stop a motor quickly. This may be necessary in the case of an emergency or to save time if the motor is being used frequently repeated operations. In electric braking, the kinetic energy of the moving parts is converted into electrical energy which is dissipated in resistance as heat or alternatively, it is returned to the supply source).
For example, a machine required a quickly stopped at the end of its stroke in order to achieve a high rate of production. In other cases, rapid stops are essential for preventing any danger to the operator or damage to the product being manufactured. Similarly, in the case of lifts and hoists, effective braking must be provided for their proper functioning.
(b) Mechanical considerations
1. Type of enclosure
2. Type of bearings
3. Method of power transmission
4. Type of cooling
5. Noise level
(c) Size and rating of motors
1. The requirement for a continuous, intermittent or variable load cycle
2. Overload capacity
(d) Cost
1. Capital cost
2. Running cost