Digital Mode; Current Limit - PowerTec 2000C Instruction Manual

5.2 DIGITAL MODE

When +24VDC is applied to TB1 terminal 10 with
respect to TB1 terminal 9, the Model 2000C or 3000C is
switched into the DIGITAL MODE. This means that the
speed of the drive will no longer be dictated by the input
reference at TB2 terminal 10, nor will it respond to the
ACCEL and DECEL pots, or to the JOG function. MAX
SPEED will have no effect on speed and the position of the
JP2 jumper will have no meaning in DIGITAL mode.
The control will now respond to a properly applied
signal at TB1 terminal 11 with respect to TB1 terminal 9
(see Section 4.5). For four pole motors, this will be a
square wave frequency of 2 times the motor RPM desired
in hertz (1750 RPM = 3500 Hertz). Eight pole motors
require a frequency of 4 times the motor RPM (1750 RPM
= 7000 Hertz). The reference to frequency is only for the
sake of convenience in describing the input waveform.
The motor control responds pulse for pulse to this input.
The standard four pole motor, after processing by
the pulse circuits, produces 120 feedback pulses for each
revolution of the motor. Each reference pulse applied at
TB1 terminal 11 causes the motor to turn 3 degrees. This
will apply whether the motor is at or near zero speed, or if
it is turning at or near base speed. No acceleration or
deceleration rate is associated with these pulses. They are
applied and responded to immediately.
Each pulse of reference increases the amount of
torque available to the motor (accelerates the motor) and
each motor feedback pulse decreases the amount of torque
available to the motor (decelerates the motor). For any
given load and speed, the amount of torque required to run
the load causes a number of pulses to be accumulated
which is proportional to the torque required to run the load.
Once the amount of torque required to run the load
is developed, every pulse of reference must be answered by
a pulse of feedback from the motor or the control accumu-
lates enough pulses to go into current limit. The only times
that pulses will be lost is if the 2000C or 3000C goes into
current limit, or if a rapid decrease in the pulse rate cannot
be responded to because of high inertia. The Models
2000C and 3000C are not regenerative and cannot brake
the motor to match a rapidly decreasing pulse train. If the
drive does not go into current limit and the load is such that
the motor can slow down as fast as the pulse rate, then each
pulse turns the motor 3 degrees. With eight pole motors,
each pulse results in 1-1/2 degrees of rotation.
A GAIN potentiometer determines how many pulses
will be accumulated to reach current limit. At the mini-
mum setting of the GAIN pot (full counter-clockwise),
about 90 pulses will be needed to reach current limit. At the
maximum setting (clockwise), only about 10 pulses will
accumulate before current limit is reached. Since each
pulse represents 3 degrees of a motor revolution, the motor
cannot be off more than 270 degrees from where it should
be before current limit is reached. If 1750 RPM is being
26
Page
commanded, the motor must run at exactly 1750 RPM, and
can't lose even a single revolution, even over a period of
time, regardless of load short of current limit. This means
the speed regulation is zero percent (no loss of speed from
no load to full load).
The frequency may be ramped from zero to the
desired speed, or ramped from one speed to another, to
avoid current limit. If losing pulses during acceleration or
deceleration is not a concern, the input frequency may be
applied all at once, or even left running into the input while
the control is off. The motor must run at the commanded
speed, or be in current limit.
In DIGITAL mode, a RAMP STOP command will
decelerate the motor to zero speed and shut off ONLY IF
the RAMP STOP jumper JP1 is installed and the frequency
at TB1 terminals 11 and 9 (see Section 4.5) is reduced to
zero when the ramp stop is initiated. If the frequency is not
reduced to zero, the motor will continue to run at the speed
dictated by the frequency until some other action is taken
(such as EMERGENCY STOP). DO NOT USE RAMP
STOP IN DIGITAL MODE UNLESS EMERGENCY
STOP IS CONNECTED TO A PUSHBUTTON OR
EMERGENCY STOP SYSTEM!
The JOG function may be simulated in DIGITAL
mode by applying a string of some number of pulses to the
frequency input, then cutting off the string of pulses. It is
also possible to interrupt the TB1 terminal 10 connection
during the JOG function to allow use of the analog JOG
mode.
THERE IS NO MAXIMUM SPEED PROTEC-
TION WHILE OPERATING IN THE DIGITAL MODE.
IT IS THE RESPONSIBILITY OF THE USER TO
MAKE SURE THAT THE SPEED COMMANDED DOES
NOT EXCEED THE MOTOR'S MAXIMUM SAFE
SPEED WHILE OPERATING IN THE DIGITAL
MODE!!

5.3 CURRENT LIMIT

CURRENT LIMIT is the maximum amount of
output current which the drive will be allowed to give
when it is powering the load, i.e., power is being drawn
by the motor to perform work. The current limit is a
peak-detecting circuit which will limit the current drawn
on every PWM pulse. The amount of current allowed is
determined by the motor rating, allowing no more than
150% of nameplate current.
The current limit and the TRIP levels are calibrated
for each individual control by a Horsepower Calibration
Resistor on Terminals 4 and 5 of TB3 on the Current
Sensor Board (part #4001-144009-XXX). Horsepower
Calibration Resistor values are listed in Section 2.1 on
page 2-1 of this manual. Those values are calculated values
Model 2000C / 3000C Manual