Field Current Control - Allen-Bradley 1395 Troubleshooting Manual

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Field Current Control

Field Current Control Malfunctions
Malfunction
The exterior surface of the
motor reaches a high
temperature when the motor is
stopped.
The field current and resulting
CEMF vary above base speed
with a constant speed
reference.
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Field current control is performed in both the Velocity Processor and the
Current Processor. The Velocity Processor controls the field current
reference based on velocity, armature voltage, mode of operation, etc. The
Current Processor performs the field current regulation based on the
reference it receives from the Velocity Processor and the field current
feedback measured by the Current Processor.
Most control related malfunctions are experienced when initially calibrating
the field control for operation below Base Speed. The field must be
calibrated for operation in both constant Torque mode and constant
Horsepower mode. Constant Torque mode occurs below base speed when
the motor has full field current applied and maximum rated torque can be
achieved. Constant Horsepower mode occurs above base speed. The field
control reduces the field current reference to maintain a constant armature
voltage as velocity increases. The torque output decreases with an increase
in velocity to maintain a constant horsepower output. This functionality is
commonly referred to as "Field Weakening".
The following is a list of malfunctions, symptoms, and possible solutions
for the field control.
Symptom
The motor case temperature is
excessive while the drive is in a stand-by
mode. The drive is powered on, but not
enabled.
With a constant speed reference above
base speed, the velocity and field
current vary.
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Malfunctions Not Indicated by a Fault
Solution
• The Field Economy function in the drive is not enabled. The
Field Economy function reduces the field reference when the
motor is stopped to reduce the power in the motor. Bit 0 of
parameter 627 "Flux Mode Select" should be set to 1 to enable
the Field Economy function. The field flux reference will be
reduced to the level specified in parameter 674 "Fld Economy
Ref" after the motor has been stopped for a period of time
exceeding the value specified in parameter 675 "Fld Economy
Dly". A typical value of field economy is 30-50%.
• Setting the field economy too low may cause a drive fault to
occur upon starting the drive. This would happen when the
field bridge goes into discontinuous conduction when in field
economy mode. If this occurs, it will take too long for the field
current to increase to the commanded level once a START
command is issued and the drive will fault on VP-13 "Motor
Field Loss" or VP-36 "Excess Arm Voltage". The field economy
reference should be set so the field bridge is in continuous
conduction.
• The CEMF regulator is unstable, causing the field current
reference to oscillate. The gains of the regulator are
determined by parameter 672 "KI Flux" and parameter 673 "KP
Flux". Decrease parameter 672 until stable operation is
achieved.
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