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velocity of the motor to the velocity commanded by the drive and adjusting the current
command as needed to maintain the commanded velocity.
Velocity Loop
In the velocity loop circuit the error signal is processed by a P.I.D. (Proportional, Integral
and Derivative). The output of the P.I.D. filter is the current reference signal also
available for test on terminal 2 of the front connector. The voltage on this point is ± 10
VDC. At ± 10V the drive generates the maximum current in the designated direction.
All the adjustments shown on the block diagram in Figure 1-3. Zero offset, proportional
gain, response, accel/decel ramp gradient and full scale speed are located on the
personality board and the potentiometers are accessible from the front panel.
Current Loop and Limiting
The current error signal is generated comparing the output of the current limiting stage
with the actual current in the motor. The current error signal is computed to generate
the PWM signals driving the IGBT final stage. In the block diagram in Figure 1-3, the
IGBT devices are shown as switches.
Current Limiting
In the current loop circuit there is a current limiting circuit referred to as Ixt, which
continuously monitors the current commanded and delivered into the motor.
The Ixt limiting circuit is not operational in the current command
mode. See Chapter 6 for details on implementing current command
mode.
This limiting circuit estimates the heating of the motor by continuously monitoring the
amount of current in the motor and the length of time this current has been flowing. The
limiting value is determined by the setting of dip switches on the personality board. If the
current requested exceeds the value set by the dip switches, the Ixt control circuit will
determine how long the commanded peak current will be allowed before limiting the
delivered current to the dip switch value. This current limiting is not a fault condition but
rather an Ixt current fold back limiting and is so indicated by the High Irms LED and
High Irms output. When the drive is in the Ixt limit status, the RED led (HIGH Irms) lights
and terminal 12 becomes open circuit. Once current fold back is engaged the drive will
continue in the limited current condition until the current commanded is reduced below
the dip switch level for length of time sufficient to reset the Ixt limiting circuitry. The
amount of time allowed above the continuous level before Ixt limiting varies is
dependent on the percent of RMS current the drive has been running. Peak current
availability is also dependent on the level of current demand below and the amount of
time below the dip switch level. In addition to the current foldback limiting, the LX
amplifiers also have short circuit protection. This prevents destruction of the amplifiers
due to short circuits either from a short that is applied while in operation or from a short
circuit in effect at power ON.
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Section 1 – Page 8
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