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What are the various types of electric drives? Compare their advantages and disadvantages. State and explain the various factors which affect the selection of an...

What are the various types of electric drives? Compare their advantages and
disadvantages. State and explain the various factors which affect the selection of an
electric motor as industrial drive.

Answers

: Various type of electric Drives: A variety of thyristor or power semiconductor
control circuitry has been devised for motor control (also called electric drives),
depending upon the type of supply (ac/dc) and the type and size of the motor.

For dc motor control, controlled dc power from a constant voltage ac supply is
obtained by means of controlled rectifiers employing thysistors and diodes. The
control of d.c. voltage is achieved by firing the thyristors at an adjustable angle with
respect to the applied voltage. This angle is known as the “iring angle’ and the scheme
of control is called the phase control. Control of d.c. motors fed from dc supply is also
achieved by means of d.c. choppers which are d.c.-d.c. converters. This type of
controller periodically opens and closes the switch; thus the average voltage is
controlled by varying the ‘on’ and ‘off’ durations. It gives an efficient and stepless
control of dc motors. However the choppers controller requires forced commutation
of thyristors which involves special thyristorized circuitry.
Closed-loop control of d.c. drives usually involves one or more feddback loops. The
relevant signals are sampled through transducers at discrete instants for computational
purposes; also switching of power semiconductor devices is accomplished by
gate/base control. . The motor speed is sensed by a tachometer, the output of which is fed
through an A/D converter to the microcomputer. However a shaft encoder can also be
used for this purpose, and the digital output of the shaft encoder can be directly fed to
the microcomputer. The armature current of the motor can be sensed by measuring the
voltage across a standard resistor
Till recent times variable speed drive application were dominated by d.c.
motors. Squirrel cage induction motor which costs approximately one – third
of a d.c.-motor of the same rating, is quite rugged, maintenance free, can be
built for higher speeds, torques and power ratings. Wound rotor induction
motors, through costlier than squirrel cage ones, also need less maintenance
and are available in higher power ratings. Thus a.c. drives have succeeded in
replacing dc drives in a number of variable speed applications. For control of
a.c. motors that include both types of induction motors as well as synchronous
motors fed from d.c. supply, thyristor, transistor or MOSFET based inverters
are employed. These semiconductor switching circuits transfer energy from dc
supply to an a.c. load of variable frequency and/or variable voltage. The
switching operation results in considerable harmonics in the input a.c.
waveforms which are filtered out by the a.c. motor. The normal power supply
being of the a.c. type, the complete scheme involves a converter and then an
inverter.
A cycloconverter is a direct a.c. to a.c. converter for obtaining variable
voltage low frequency a.c. supply from a fixed frequency one. Thus it
eliminates the necessity of on intermediate d.c. stage. Inspite of their direct a.c
to a.c. conversion the cycloconverters have the disadvantages that
(i) They produce a subfrequency ouptput,
(ii) their output contains a high harmonic content, and
(iii) the input power factor is low. Cycloconverters are used for low-speed
drives and controlling linear motors in high-speed transportation systems.

gregorymasila answered the question on February 20, 2018 at 16:19

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What are the various types of electric drives? Compare their advantages and disadvantages. State and explain the various factors which affect the selection of an...

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