|
Speed drops when load increases....
|
|
|
|
|
|
Some times, even when the load
is well within the limit of the rating of the drive and that of the motor, is
is observed that speed drops when the load is increased. There can be following
reasons for this. 1. The drive is working under Voltage
Feedback Control and that IR compensation ( i.e. the compensating action to
compensate for the Resistive Drop of voltage in the Armature Resistance ) is
not adjusted properly. IR compensation is not to be applied when using
Tachogenerator Feedback arrangement. 2. If only four thyristors are firing out of
six, an increase in load can push the drive in current limit it it is set
very critically without any margin left. When four thyristors fire, average
current in the motor is less but the peaks are higher. These higher peaks
cause the drive to go in Current Limit. A quick check on Oscilloscope will
reveal this immediately. More on IR compensation... IR stands for I into R. i.e.
Armature Current multiplied by Resistance of the Armature winding. In a dc
motor the speed N is given by the formula
N= (V- IR )/Flux. V is the
applied voltage, Flux is the magnetic filed in the air gap between stator and
the rotor. Considering the magnitude of IR compared to that of Voltage
applied, one can ignore it. IR is not more than 5 % of full armature voltage.
Speed of the motor therefore, can be simply assumed to be proportional to the
applied voltage. To get somewhat closer to the real fact, one can generate a
signal proportional to I x R by sensing actual armature current and use to
compensate the error in N by subtracting IR from V. Thus, a feedback signal
which is proportional to V-IR rather than only V is more accurate and able to
control the speed more closely. Excessive compensation of IR can cause
overcompensation and may tend to increase the speed as the load increases.
This is not desired. |
|
|
|
|
Download PDF version
of these pages.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Speed control not satisfactory
|
|
|
|
|
|
|
|
|
|
|
|
|