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Current Amplifer of a DC drive...
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Current amplifier in a dc drive
does not amplify current ! It derives its name from the simple
fact that the feedback signal given to this amplifier is that of current
of the dc motor. This amplifier is also called more appropriately as MINOR
LOOP. This amplifier forms a minor loop ( inner loop) in the overall closed
loop system of a dc drive, the external or MAJOR loop being that of speed
with speed feedback. This amplifier, in a simple
analog dc drive is built around single operational amplifier in PI
configuration. i.e. it has a resistor and capacitor in the feedback loop
deciding PROPORTIONAL GAIN and INTEGRAL TIME of the amplifier. Typical values
in an analog dc drives are 10 kilo ohms and 1 micro farad. Some designs may
have Current Gain, like that in Speed Amplifier. Output of the Current
Amplifier acts as the input to the firing circuit. The output of this
amplifier is restricted within two minimum and maximum limits set by two
preset potentiometers called minimum and maximum voltage or
"conversion" and "inversion" limits. This dc level
decides the instance in sinewave at which the thyristors are fired. If we
disconnect the output of this amplifier and connect a potentiometer to feed
dc level to the firing circuit, we can test any drive in "open"
loop manner. This is normally done to diagnose faults in the firing section
of the drive. Since there are no safety limits left when the drive is tested
is this fashion, it is advisable to take initial tests with Lamp load instead
of dc motor. |
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Speed control not satisfactory
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