Product Manual

torque applications, including conveyors, positive displacement
pumps, extruders, mixers or other “machinery” require the same torque
regardless of operating speed, plus extra torque to get started. Here,
high overload capacity is required.
Smaller-horsepower drives are often built to handle either application.
Typically, only a programming change is required to optimize efficiency
(variable volts-to-hertz ratio for variable torque loads, constant volts-to-
hertz ratio for constant torque loads). Larger horsepower drives are usu-
ally built specifically for either variable or constant torque applications.
Speed: As mentioned, AC drives provide an extremely wide speed range.
In addition, they can provide multiple means to control this speed. Many
drives, for example, include a wide selection of preset speeds, which can
make set-up easier. Similarly, a range of acceleration and deceleration
speed “ramps” are provided. Slip compensation, which maintains
constant speed with a changing load, is another feature that can be
helpful. In addition, many drives have programmable “skip frequencies.”
Particularly with fans or pumps, there may be specific speeds at which vibra-
tion takes place. By programming the drive to avoid these corresponding
frequencies, the vibration can be minimized. Another control function,
common with fans, is the ability for the drive to start into a load already
in motion – often called a rolling start or spinning start. If required, be
sure your drive allows this or you will face overcurrent tripping.
Current: The current a motor requires to provide needed torque (see
previous discussion of torque) is the basis for sizing a drive. Horsepower
ratings, while listed by drives manufacturers as a guide to the maxi-
mum motor size under most applications, are less precise. Especially
for demanding constant torque applications, the appropriate drive
may, in fact, be “oversized” relative to the motor. As a rule, general-
purpose constant torque drives have an overload current capacity of
approximately 150% for one minute, based on nominal output. If an
application exceeds these limits, a larger drive should be specified.
Power Supply: Drives tolerate line-voltage fluctuations of 10-15% before
tripping and are sensitive to power interruptions. Some drives have
“ride-through” capacity of only a second or two before a fault is trig-
gered, shutting down the drive. Drives are sometimes programmed for
multiple automatic restart attempts. For safety, plant personnel must be
aware of this. Manual restart may be preferred.
Most drives require three-phase input. Smaller drives may be available for
single-phase input. In either case, the motor itself must be three-phase.
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