Rockwell Automation WebPak 3000 DC Drive Application Workbook Version 1.1 - Manual (Page 94)

Glossary-2 WebPak 3000 Drive Application Workbook, Version 1.1
• speed control - Velocity feedback is used when an absolute value of motor shaft
speed is required. Shaft speed is measured by a tachometer whose signal is
compared to the speed reference signal.
• tension control - Tension control systems are used where direct tension
measurement is required. The feedback device is a force transducer mounted on
each end of a tension-sensing roll.
• voltage control - A voltage signal is obtained directly from the output terminals of
the drive and compared to the reference signal from the speed-setting
potentiometer.
counts - The internal units used by the drive. Most parameters that are entered in
engineering units are usually converted to a value in counts for use by the drive. By
converting to counts, the drive is better able to maintain consistency and accuracy
when making calculations.
When parameters expressed in units of RPM are entered into the drive, the value is
converted to counts and normalized so that 4095 counts corresponds to the gear-in
speed in engineering units. For example: If the gear-in speed is 1000 RPM, then a
reference of 250 RPM is converted to 1023 counts, or 25% of 4095 counts. The value
4095 is used because this corresponds to the maximum value produced by the drive’s
12-bit A/D and D/A converters.
cross over frequency - Also known as the cut-off frequency and indicates bandwidth.
In the case of the speed loop or outer loop, only input frequencies between zero and
the corresponding cross over frequency will be passed through the loop. Frequencies
outside this range will be attenuated.
current memory - A function that changes the drive on the outgoing roll from its
primary tension regulator to a current regulator whose current reference is the
memorized value of its armature current just prior to being switched out of tension
operation.
engineering units - Units of measurement (such as pounds, pounds per square inch,
degrees Celsius) relative to the process. An input signal is often a percentage of the
full-scale range (such as 5 volts or 4-20 mA) of direct measurement that must then be
converted, through scaling, into engineering units.
See scaling.
field economy - A circuit design feature of a DC motor shunt field supply that reduces
the supply voltage output after a predetermined period of time after the motor is
stopped. On many field supplies, this means a 50% reduction in output voltage 2 to 3
minutes after machine shutdown (idle). A field economy circuit serves to reduce
standby power consumption and prolong the insulation life of the motor field windings.
field shaping - A function that enables center-driven winders to meet wide speed
range requirements by operating in the field weakening range of the motor.
field weakening - The action of reducing the current applied to a DC motor shunt
field. This action weakens the strength of the magnetic field and thereby increases the
motor speed while decreasing the available motor torque.
gain - The ratio of the magnitude of the output signal with respect to that of the input
signal.