User Manual Instruction Manual

Application Notes
13-77
13.29 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
will 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, increasing downtime and repair costs while decreasing productivity.
In Flying Start mode, the drive’s response to a start command will be to identify the
motor’s speed and apply a voltage that is synchronized in frequency, amplitude and
phase to the back emf of the spinning motor. 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 motor is “picked
up “smoothly at its rotating speed and ramped to the proper speed, little or no
mechanical stress is present.
13.29.1 Configuration
Flying Start is activated by setting the Flying Start En (169) parameter to “Enable.
The gain can be adjusted to increase responsiveness. Increasing the value in Flying
StartGain (170) increases the responsiveness of the Flying Start Feature.
13.29.2 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.