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Class 8839 58M Enclosed AC Drives
Application Information
172
09/2003
© 2000–2003 Schneider Electric All Rights Reserved
Accelerating Torque
AC induction motors built to NEMA standards are
designed to provide starting torque which must
meet certain minimum ratings. This is normally
expressed as a percentage of full load torque.
These torque ratings are valid only for full voltage
starting where inrush current can be
approximately 600% of motor full load current.
The Class 8839 58M Enclosed AC drive will limit
starting current to a value of usually not more than
150% (CT rated) to 110% (VT rated) of drive full
load current, which provides approximately 150%
starting torque for CT loads and 110% for VT
loads.
AC Drives provide better torque per ampere than
any other reduced inrush method, but the starting
torque available may be less than the starting
torque available with an across-the-line starter.
Applications with known high starting torque
requirements should be carefully evaluated. It
may be necessary to oversize the AC Drive, or the
motor to provide the necessary accelerating
torque.
Dynamic Braking
Dynamic braking directs the regenerative energy
from an AC induction motor dissipated in the form
of heat through a resistor. This condition presents
an electrical load, or retarding torque, to the
motor, which is acting as a generator. The
thermal capacity required for this resistor is
determined by the stopping duty cycle for the load
and the energy dissipated for each deceleration.
Dynamic braking requires the motor to remain
energized to maintain the rotating magnetic field.
Dynamic braking cannot operate during periods
where power is lost and cannot maintain holding
torque when the AC Drive is stopped. A
mechanical brake must be used when the
application requires a holding torque at zero speed.
A dynamic braking resistor configuration is
available as optional equipment. The dynamic
braking resistor is sized to be capable of
absorbing six times the stored energy of a motor
at maximum speed, which means it could make 6
consecutive stops from rated speed without
overheating. Applications with high inertia are
typical candidates for dynamic braking.
Follower Signals
The Class 8839 58M Enclosed AC drives are
designed to accept a 4–20 mAdc or optional
0–10 Vdc analog input, or an optional 3–15 psig
analog input. Other follower signals may be
accommodated which will require additional
hardware or signal conditioners as optional
equipment.
PI Regulator
The Class 8839 58M Enclosed AC drive has a
build in PI regulator to provide set-point control
from the key pad or remote analog signal.
Selection parameters are set via the key pad to
automatically control a level, pressure or flow
process. This PI function does not require any
additional hardware, such as options boards or
separately mounted equipment.
Bypass Operation
Although the Class 8839 58M Enclosed AC drive
is designed for maximum reliability, it is possible
that a controller could be out of service when
required to operate. Critical operations which can
tolerate little or no down time should be
considered as candidates for bypass (full speed)
operation.
This involves an isolation contactor to disconnect
the motor from the AC Drive and a full voltage
starter to bypass the controller and operate the
motor across-the-line or by an alternate starter
such as a reduced voltage autotransformer starter
or a solid state reduced voltage starter.
Motor Selection
The Class 8839 58M Enclosed AC drives are
designed to operate with any three phase AC
squirrel cage induction motor or synchronous
reluctance motor having voltage and current
ratings compatible with the drive.
It is recommended that all motors used with AC
Drives be equipped with thermostats in the stator
windings. This affords the ultimate motor
overload protection much better protection than
overload devices sensitive to motor current,
because motor temperature may rise due to loss
of cooling resulting from low speed operation and
not necessarily because of an overcurrent
condition.
The motor should meet NEMA MG-1, Part 31
standards. This motor spec calls for 1600 volt
rated magnet wire, while the NEMA MG-1, Part 30
standard calls for 1000 volt rated magnet wire.
The higher voltage rated magnet wire will protect
against possible premature motor failures due to
voltage stress from fast dv/dt rise times commonly
seen with IGBT based AC Drives.