68 Rockwell Automation Publication 750-QS001A-EN-P - March 2015
Reference Section
Acceleration and Deceleration Time
Acceleration Time
Longer acceleration times can be an application consideration due to high inertia of the load and motor rotating mass. A
typical application example that requires this consideration is a centrifugal fan load. Issues that can occur include overload
conditions of the motor and/or drive or over current conditions of the drive. Results of these issues can be the drive
exhibiting a fault condition in either motor or drive overload fault (F7 and F64 respectively) or can cause a hardware over
current fault (F12). If these faults are present and in order to prevent these faults as part of startup, acceleration time must
be set to the capabilities of the drive based on the load and application requirements. The normal solution will be to set
parameter 535 [Accel Time 1] to longer time period so that an overload or over current condition of the drive does not
occur.
An approach to perform this configuration is to set acceleration
times to incremental 30 second acceleration times above
the last setting and restart the drive until the load is able to start without a fault condition. The maximum acceleration time
for most applications is up to 5 minutes, however very high inertia loads could require higher acceleration times (for
example, 30 minutes may be common for a centrifuge). If faster acceleration time is required for the load, contact your
Allen-Bradley distributor or Allen-Bradley technical support for further application review or potential drive sizing
considerations.
Recommendation:
If the load has a large inertia value and acceleration time is not an application concern, set the current limit of the drive in
amps to the continuous current rating of the drive in amps. The overload current setting is configured in parameter 422
[Current Limit 1]. This lets the drive apply its rated current to the motor on a continuous basis until the application
reaches full speed. Default values for this parameter that are time limited equal 110% of the Normal Duty (ND) or 150% of
the Heavy Duty (HD) rating of the drive that may have induced a hardware over current fault. To obtain the continuous
current rating of the drive, see parameter 21 [Rated Amps].
Deceleration Time
If the drive faults on either an overload or an over voltage condition during deceleration, it is likely due to the deceleration
time being too short for the system, which results in the fault condition. Either the drive can be set to coast to rest
(parameter 370 [Stop Mode A] is set to “0”) or parameter 537 [Decel Time 1] can be extended to a longer period of time to
eliminate the fault condition. Set parameter 537 [Decel Time 1] to the longest necessary decel time allowed by the system.
If drive faults or over load conditions continue to exist, dynamic braking can be required. Refer to PowerFlex Dynamic
Braking Resistor Calculator, publication
PFLEX-AT001, in order to apply a dynamic brake to the application.
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