Flying Start - Allen-Bradley PowerFlex 70 Reference Manual

Flying Start

Figure 22 Flux Up Current versus Flux Up Time
Flux Up Current = Maximum DC Current
Rated Flux
Rated Motor Flux
Current
Motor Flux
T1
T2
T3
T4
Flux Up Time
[Flux Up Time]
Once rated flux is reached in the motor, normal operation begins and the desired
acceleration profile is achieved.
Figure 23 Rated Flux Reached
Ir Voltage - SVC
Greater of IR Voltage or
Voltage Boost - V/Hz
Stator Voltage
Flux Up
Rotor Speed
Voltage
Motor Flux
Motor Flux
Stator Freq
Normal
Flux Up
Operation
Time
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.
Rockwell Automation Publication PFLEX-RM001H-EN-P - June 2013 105