Introduction

Introduction to Electric Drives

For almost three quarters of the twentieth century, most of the ac drives for industrial and domestic uses have been designed to operate at constant speed. Constant speed drives are highly inefficient for variable speed operations. Now, with the availability of economical variable frequency electric supplies, resulting from advances in power electronic switching devices, and the low cost microprocessor based control, electric drives are increasingly being used for operating industrial loads at any one of a wide range of speeds. In many modern industries adjustable speed drives are required for precise and continuous control of speed, torque, or position, with long term stability, good transient performance and high efficiency. The industrial loads may be of constant torque type, constant power type, or may have torque as a function of speed. The electric drive makes use of electric motors as prime mover due to the following advantages. They can be brought very close to the working machine (load), can be operated at any desired speed through power electronic control, and can be started and reversed in very short periods of time. The electric motors are also available in the market in a wide range of power ratings; from few watts to few thousand of kilowatts capacity. Another advantage of electric drives is that speed- torque characteristic of electric motors can be easily modified to suit the load characteristic.

A modern electric drive has four components:

(i) Electric motor

(ii) Power electronic converter

(iii) Controller

(iv) Actual apparatus or equipment (Load)

These components are described in the following sections.