A Simple Electric Motor:

Variable speed drives in the industry employ a simple electric motor as their drive motors mainly because they enjoy several specific advantages, such as overload capacity, smooth speed control over a wide range, capability of operating in all the four quadrants of the speed-torque plane, etc.

Till the advent of thyristors and thyristor power converters the dc motor had been very popular in the area of adjustable speed drives, even though it suffered from the disadvantages imposed by the presence of a mechanical commutator. Thyristor power converters capable of providing variable voltage, variable frequency supplies have now made ac motors increasingly popular.

Industrial loads have different types of speed-torque characteristics. Once the speed-torque of the load to be driven is determined a proper motor has to be selected for driving the load.

Some of the factors that can influence the choice of a simple electric motor selection to drive the load are:

1. the available ratings, capital and running costs involved
2. the limits of speed range, hardness of speed control and speed regulation
3. the efficiency during variable speed operation
4. controllability
5. braking requirements
6. reliability of operation
7. starting requirements
8. power/weight ratio
9. power factor
10. capability of operating on a load factor or duty cycle
11. availability of supply
12. effects of supply variations
13. loading of the supply, and
14. environmental effects.

Therefore, a knowledge of the behaviour of a simple electric motor and with regard to the factors listed above is required. It may also be possible to modify the system to improve its performance and make it more economical and efficient.