24-07-2012, 03:45 PM
D.C.MOTOR USING FULLY CONTROLLED CONVERTER
SPEED CONTROL OF SEPERATELY EXCITED.docx (Size: 1.17 MB / Downloads: 58)
Introduction
Seperatelyexcietedd.c.motors are those in which supply is provided separately
They are in all aspects similar to that of shunt motors but the only thing that makes the
difference is that its field winding is separated from armature winding.So both the
windings are to be separately fed from sources.Usually the armature resistance is very
low and shunt resistance is comparatively high.The inductance effects are neglected in
all the dc motors .
FLUX CONTROLMETHOD :
It can be observed from the above expression that speed is inversely proportional to flux
per pole..By decreasing the flux of d.c.motor we can increase the speed By varying the
shunt field rheostat.tfe field current can be varied.the current carried by shunt field is
usually very small.So the copper losses due to the resistance effect is usually very small
Any further weekening of flux below its limit causes the system to be adversely effected
Usually a common machine has speed control in 2:1 ratio.Machines with interpole has
thespeed control ratio of 6:1.
ARMATURE OR RHEOSTATIC CONTROL :
This method is used when speeds below rated are needed.In this method a resistance
is placed in series with the armature .As we increase its value the armature voltage (E)
decreases .thus following the relation of speed being directly proportional to E the
speed decreases .Here in this case the supply voltage is constant.for a load constant
torque,speed is approximately proportionaltop.d.across the armature terminals.
From the Speed/armature characteristicsit is seen that greater the resistance
in the armature greateris the fall in speed
Let some controller resistance of value R be added to armature circuit resistance so that
its value becomes
Rr=Ra+R
N2/N1=Eb2/Eb1
As this method is very wasteful ,expensive ,unsuitable for rapidly changing loads
because for a given Rr speed will change with load.Amore suitable way of control can
be obtained by using diverter in across armature in addition to armature control
resistance .now the changes in armature current will not be so effective in changing
p.d.across armature.
SPEED CONTROL BY FULLY CONTROLLED THYRISTORS
SINGLE-PHASE, FULL-WAVE,CONTROLLED RECTIFIER
The electricdrivediscussed is a single-phase, full-wave controlled rectifier. This
adjustable speed drive is similar to the single-phase,half-wave controlled rectifier. As
an example, this rectifier is presented to control a separately excited DC motor. The
single-phase, full waveControlled rectifier consists of four thyristors. The increase in
thyristors provides for better control compared to the half wave controlled rectifier. The
obvious disadvantage the full-wave rectifieris the increase in price because of the
increase in the number ofthyristor.