Rockwell Automation 20A PowerFlex 70EC/700VC - Manual (Page 51)

Flying Start 47
Flying Start The Flying Start feature is used to start into a rotating motor, as quick as possible,
and resume normal operation with a minimal impact on load or speed.
When a drive is started in its normal mode it initially applies a frequency of 0 Hz
and ramps to the desired frequency. If the drive is started in this mode with the
motor already spinning, large currents will be generated. An overcurrent trip may
result if the current limiter cannot react quickly enough. The likelihood of an
overcurrent trip is further increased if there is a residual flux (back emf) on the
spinning motor when the drive starts. Even if the current limiter is fast enough to
prevent an overcurrent trip, it may take an unacceptable amount of time for
synchronization to occur and for the motor to reach its desired frequency. In
addition, larger mechanical stress is placed on the application.
In Flying Start mode, the drive’s response to a start command is to synchronize with
the motors speed (frequency and phase) and voltage. The motor will then accelerate
to the desired frequency. This process will prevent an overcurrent trip and
significantly reduce the time for the motor to reach its desired frequency. Since the
drive synchronizes with the motor at its rotating speed and ramps to the proper
speed, little or no mechanical stress is present.
Configuration
Flying Start is activated by setting the [Flying Start En] parameter to “Enable”
The gain can be adjusted to increase responsiveness. Increasing the value in [Flying
StartGain] increases the responsiveness of the Flying Start Feature
Application Example
In some applications, such as large fans, wind or drafts may rotate the fan in the
reverse direction when the drive is stopped. If the drive were started in the normal
manner, its output would begin at zero Hz, acting as a brake to bring the reverse
rotating fan to a stop and then accelerating it in the correct direction.
This operation can be very hard on the mechanics of the system including fans,
belts and other coupling devices.
Cooling Tower Fans
Draft/wind blows idle fans in reverse direction. Restart at zero damages fans,
breaks belts. Flying start alleviates the problem
70EC 700VC 700H
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