Page 203 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

DC drive input : Some thyristor based DC drives operate on a single phase supply and use four thyristors for
full wave rectification. For larger motors 3 phase power supply is needed because the waveforms are much
smoother. In such cases, six thyristors are needed for full wave rectification.

Rectifier Bridge : The power component of a controlled DC drive is a full wave bridge rectifier which can be
driven by three phase or single phase supply. As mentioned above the number of thyristor may vary depends on
the supply voltage.

A six - thyristor bridge (in case of three phase converter) rectifies the incoming AC supply to DC supply to the
motor armature. The firing angle control of these thyristors varies the voltage to the motor.
Field Supply Unit (FSU) : The power is to be applied to the field winding is much lower than the armature power.

In many cases a two - phase supply is drawn from the three phase input (that supplies power to the armature) and
hence the field exciter is included in the armature supply unit.
The function of the field supply unit is to provide a constant voltage to the field winding to create a constant field
or flux in the motor. In some cases, this unit is supplied with thyristors to reduce the voltage applied to the field
so as to control the speed of the motor above the base speed incase of permanent magnet DC motors, the field
supply unit is not included in the drive.
Speed Regulation unit : It compares the operator instruction (desired speed) with feedback signals and sends
appropriate signals to the firing circuit. In analog drives, this regulator unit consists of both voltage and current
regulators. The voltage regulator accepts the speed error as input and produces the voltage output which is
then applied to the current regulator.
The current regulator then produces required firing current to the firing circuit. If more speed is required,
additional current is drawn from the voltage regulator and hence thyristors conduct for more periods. Generally,
this regulation (both voltage and current) is accomplished with proportional -integral- derivative controllers.
The field current regulator is also provided where speed greater than the base speed is required.

Parts of DC drive : DC drives of various brands with different ratings are available in the market. It is generally
assembled in a metallic enclosure. The front panel has the power terminals, control terminals, keypad for
controlling the drive etc. It has provision for connecting to PC for programming the drive.
The main parts of DC drive are given below. (Fig 4&5)

Fig 4 Fig 5

190

CITS : Power - Electrician & Wireman - Lesson 93-100