“B” Group: Fine Tuning Functions
Configuring
Drive Parameters
3–32
B_91/B_88: Stop Mode / Restart Mode Configuration – You can configure how the
inverter performs a standard stop (each time Run FWD and REV signals turn OFF).
Setting B_91 determines whether the inverter will control the deceleration, or whether it
will perform a free-run stop (coast to a stop). When using the free-run stop selection, it is
imperative to also configure how you want the inverter to resume control of motor speed.
Setting B_88 determines whether the inverter will ensure the motor always resumes at
0 Hz, or whether the motor resumes from its current coasting speed (also called
frequency matching). The Run command may turn OFF briefly, allowing the motor to
coast to a slower speed from which normal operation can resume.
In most applications a controlled deceleration is desirable, corresponding to B_91=00.
However, applications such as HVAC fan control will often use a free-run stop
(B_91=01). This practice decreases dynamic stress on system components, prolonging
system life. In this case, you will typically set B_88=01 in order to resume from the
current speed after a free-run stop (see diagram below, right). Note that using the default
setting, B_88=00, can cause trip events when the inverter attempts to force the load
quickly to zero speed.
NOTE: Other events can cause (or be configured to cause) a free-run stop, such as
power loss (see “
Automatic Restart Mode” on page 3–24), or an intelligent input
terminal [FRS] signal. If all free-run stop behavior is important to your application (such
as HVAC), be sure to configure each event accordingly.
An additional parameter further configures all instances of a free-run stop. Parameter
B_03, Retry Wait Time Before Motor Restart, sets the minimum time the inverter will
free-run. For example, if B_03 = 4 seconds (and B_91=01) and the cause of the free-run-
stop lasts 10 seconds, the inverter will free-run (coast) for a total of 14 seconds before
driving the motor again.
Stop Mode = free-run stop
B_91
=
01 B_91
=
01
B_88
=
00 B_88
=
01
[FW, RV]
Motor
speed
t
Resume from 0Hz
Zero-frequency start
t
Stop Mode = free-run stop
Resume from current speed
[FW, RV]
Motor
speed
B03
Wait time
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