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Field weakening in a DC drive...
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Speed of a DC motor is governed by
two variables viz. Armature voltage and field voltage. Armature voltage and
speed are directly proportional to each other. Field voltage and speed are
inversely proportional to each other. i.e. if the field voltage is reduced
keeping armature voltage constant, the speed of the motor goes up. On the
other hand, if the field voltage is kept constant and armature voltage is increased,
the speed goes up. For getting speeds up to base
speed of the motor ( designed speed which is achieved when armature voltage
and field voltage are at their full rated values ), the armature voltage is
varied from zero up to full rated voltage. For getting speed above the base
speed, the armature voltage is kept constant and field is gradually reduced.
This is represented in the sketch below. |
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Electronically, using dc drives
this is achieved as follows. Armature of the
dc motor is connected to a normal dc drive which varies the speed of the motor
from zero up to its rated base speed. This is achieved, for example, with a
reference voltage of zero to 5 volts. At 5 volts reference level, the motor
reaches its maximum level of armature voltage allowed. As the reference is
further increased from 5 to 10 volts, another thyristor unit which feeds the
field current, takes over the control and starts reducing the field slowly.
This changeover of controls is smooth and automatic. Referring to the block
diagram below makes it clear. Such controls are useful where the load on the
motor ( kW ) does not increase proportionately at speeds abvoe certain speed.
In this zone of speed, the higher speed of motor can be achieved through
field weakening. In the zone where speed is increased by weakening of the field,
the Horse Power of the motor remains constant and is therefore refered to as
“Constant HP” operation. Below the point where field weakening just begins,
the operation is called “Constant Torque”. These two names are slightly
misleading. In the Constant HP zone, the horsepower actually developed is
constant. However, in the Constant Torque zone, it is not the torque
developed which is constant, but it is the Maximum Toruqe Available
is constant.
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Speed control not satisfactory
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