B091/B088: 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 B091 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 B088 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 B091=00. However, applications such as HVAC fan control will often use a free-run stop (B091=01). This practice decreases dynamic stress on system components, prolonging system life. In this case, you will typically set B088=01 in order to resume from the current speed after a free-run stop (see diagram below, right). Note that using the default setting, B088=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 and Phase Loss” on page 3–29), and inverter trip events in general (see “Miscellaneous Functions” on page 3–61). If all free-run stop behavior is important to your application (such as HVAC), be sure to configure each event accordingly.
Some additional parameters further configure all instances of a free-run stop. Parameter B003, Retry Wait Time Before Motor Restart, sets the minimum time the inverter will free-run. For example, if B003 = 4 seconds (and B091=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. Parameter B007, Restart Frequency Threshold, sets the motor frequency at which the inverter will no longer resume and accelerate, instead resuming from 0 Hz (complete stop).
| B091=01 | Stop Mode = free-run stop | | B091=01 | Stop Mode = free-run stop |
| | | Resume from 0Hz | | | | Resume from current speed |
| B088=00 | | B088=01 |
| | Zero-frequency start | | | | | | | | | | | | | | | | wait time |
Motor | | Motor | | | | | | | | | B003 |
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speed | | | | | | speed | | | | | | | | | | | | | | | |
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