Instruction manual

22
velocity control and another one for current limit. A
simple closed loop system is shown in Figure 3-1.
POWER AMPLIFIER
The power amplifier is a high current capacity SCR
power section capable of supplying four times rated
voltage (field forcing) to the clutch and brake coils. Field
forcing reduces the effective coil time constant. One
time constant is defined as the time required for the
current in a coil to reach 63% of its final value with a step
change in voltage. With a step change in voltage, the
current rises exponentially. Over-voltaging, or field
forcing, is a method for reducing the effective time
constant. A current feedback loop is used with the CES
controller to limit the current to its rated value by cutting
back on the voltage as 100% current is approached.
For proper operation of the press drive, a separation
must be maintained between the brake and clutch. If the
proper separation is not maintained, an overlap condition
can result which pre-loads the motor. Separation
between clutch and brake action is necessary to ensure
against simultaneous braking and clutching.
When the speed reference is increased, a positive
voltage is applied at the positive input of the clutch error
amplifier to turn the clutch power amplifier on. This
same voltage is applied to the negative input of the
brake error amplifier to turn the brake power amplifier
off. The clutch remains on until the voltage output from
the tachometer feedback equals the speed reference
voltage. At that time, the error amplifier instantly turns to
a zero volt output to the power amplifier thereby
discontinuing power to the clutch.
If the speed reference voltage is reduced, the opposite
effect takes place. The tachometer feedback voltage
then exceeds the reference and the clutch error amplifier
drives the clutch power amplifier off. This puts a positive
input on the brake error amplifier, driving the brake
power amplifier on. By braking to slower speeds, the
tachometer feedback voltage is reduced in direct
proportion to the reduction of the press speed until the
speed reference voltage equals the tachometer
feedback voltage. The brake then turns off. Thus,
changes in speed setting can be reacted to quickly with
either the clutch or brake being energized as required.
The current limit loop function is similar to the speed
control loop, except that current is sensed by an LEM in
series with the clutch and brake coils. This network acts
as a current limit to hold control current output to a safe
level while providing maximum voltage forcing for
response.
CES LOGIC BOARD
The primary function of the logic board is to issue
properly timed output gate signals to the brake and
clutch gate driver circuit boards that are based upon the
following four inputs: reference signal input from the
digital main regulator (DMR); coil current feedback from
the LEM sensors; amplitude and phasing of the ac lines;
and speed feedback from the DMR. The abbreviations,
"DFE" and "CDB" which were used previously for Digital
Front End and Computer Digital Board respectively, may
be interchanged with "DMR" in this manual and
elsewhere.
In addition, other logic conditions determine whether
clutch phase up is permitted, such as run signal and
DMR inhibit. Coil voltage is sensed and used in
separate circuits to generate free wheel gate commands.
The reference and feedback voltages are processed
separately for the clutch and brake, including separate
microprocessor full converter phase shifters. This
provides a clear separation of signals and functions.