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ELECTRICAL DRIVES
Introduction
Industrial loads require operation at any one of a wide range of speeds. Such loads are
generally termed as variable speed drives. The loads may be constant torque loads,
requiring constant torque at all speeds. The torque may also be a function of speed, e.g. in
a fan type or pump type load the torque is proportional to the square of the speed. These
loads are driven by hydraulic, pneumatic or electric motors. Drives employing electric
motor are known as electrical drives.
Choice of Electrical Drives
Choice of an electrical drive depends on a number of factors. Some of the important
factors are :
(i)
Steady state operation requirement : nature of speed-torque characteristics,
speed regulation, speed range, efficiency, duty cycle, quadrants of operation,
speed fluctuations if any, ratings.
(ii)
Transient operation requirement: values of acceleration and deceleration,
starting, braking and reversing performance.
(iii) Requirement related to the source : type of source, and its capacity, magnitude
of voltage, voltage fluctuations, power factor, harmonics and their effect on
other load, ability to accept regenerated power.
(iv) Capital and running cost, maintenance needs, life
(v)
Space and weight restrictions if any.
(vi) Environment and location.
(vii) Reliability.
Advantages of Electrical Drives
Electrical drives are widely used because of the following advantages :
1. They have flexible control characteristics. The steady state and dynamic
characteristics of electrical drives can be shaped to satisfy load requirement.
Speed can be controlled and, if required, can be controlled in wide limits. Electric
braking can be employed. Control gear required speed control, starting and
braking is usually simple and easy to operate.
2. They are available in a wide range of torque, speed and power.
3. Electric motors have high efficiency, low no load losses and considerable short
time overloading capability. Can be made in variety of designs to make them
compatible with load. Compared to other prime movers, they have longer life,
lower noise, lower maintenance requirement and cleaner operation.
4. They are adaptable to almost any operating conditions such as explosive and
radioactive environment, submerged in liquids, vertical mounting, and so on.
5. Do not pollute the environment.
6. Can operate in all the four quadrants of speed-torque plane. Electric braking gives
smooth deceleration and increases life of the equipment compared to other forms
of braking. When regenerative braking is possible, considerable saving of energy
is achieved. These features are not available in the other prime movers.
7. Unlike other prime movers, there is no need to refuel or warm-up the motor. They
can be started instantly and can immediately be fully loaded.
8. They are powered by electrical energy which has a number of advantages over
other forms of energy.
One of important features of an industrial drive is its four quadrant operation in the
speed-torque plane. An electric motor can in all the four quadrant of the v-i plane,
corresponding to the mechanical quantities, speed and torque. The normal motoring
operation is in the first quadrant. Sometimes the drive motor is required to maintain a
constant speed, especially when there is a tendency for the load to get accelerated
beyond the safe speed. Electric motors have a natural tendency to become generators
automatically if the speed of the motor goes beyond the no-load value. The energy
responsible for acceleration is successfully returned to the mains, thus maintaining
constant speed. This operation corresponds to the second quadrant. An electric motor
can operate in the reverse direction of rotation with suitable changes in its supply
connections. this operation corresponds to the third quadrant. Operation is also
possible in the fourth quadrant, where the motor acts as a brake. This can be done by
suitably modifying the characteristic, e.g. by inserting a resistance in the rotor circuit
of an induction motor. A suitable torque developed by the motor drives the load at the
desired speed, providing the braking action. An electric motor thus adapts itself for
four quadrant operation. Rapid acceleration of the drive and frequent reversing, if
required, can be easily accomplished when an electric motor is employed.