Rockwell Automation 1557 MEDIUM VOLTAGE AC DRIVE - Manual (Page 54)

5-36 DESCRIPTION OF OPERATION
1557-UM050F-EN-P – June 2013
Flying Start
The 1557 drive is capable, with certain restrictions, of restarting a motor that is not stationary but is
already rotating. This is a so-called flying start. In normal operation, the output of the drive is
synchronized with the motor flux, which is derived from the stator voltage and current feedback. If there
is no motor current, then whether the motor is rotating or stationary it does not generate any significant
voltage and it is not possible to determine the stator frequency. If there is no detectable stator voltage the
drive assumes that the motor is stationary, because this is the most likely case. Therefore, when the
drive is started the frequency starts from an initial value of zero and ramps up until motor flux is detected.
Significant flux is created in the motor only when the slip frequency (the difference between the stator
frequency and rotor frequency) is small. When the drive is started with the motor stationary, the initial slip
frequency is small and the motor flux builds up fairly quickly. But, if the motor is already spinning then
very little flux will be induced until the stator frequency is quite close to the rotor frequency, at which time
the motor flux will suddenly rise to a level sufficient for the drive to detect and synchronize itself to. If the
drive reaches the commanded speed without detecting any motor flux then it will trip on a motor stall fault.
There are four possible causes of a motor stall when starting:
1.
The motor has pulled out and stalled during starting due to insufficient torque. The remedy for this is
to increase the
value of some or all of the parameters “Torque command 0”, “Torque command 1”,
and “Acceleration time 1”.
2.
The motor was already rotating but the flying start failed because the drive passed through the low
slip region too quickly to allow the motor flux to build up. The solution to this problem is to increase
the value of parameter “Acceleration time 1”. Most medium voltage motors have a rotor time
constant in the range of 1 to 5 seconds and it can take a few seconds for the flux to rise to a
detectable level. Until flux is detected, the drive does not use the normal speed ramp but continues
to accelerate at the rate defined by parameters “Acceleration time 1” and “Ramp speed 1”.
3. The motor is rotating in the direction opposite to the commanded direction of rotation. The slip
frequency will increase instead of decrease as the drive accelerates and no flux will be induced in
the motor.
4.
The motor is rotating in the commanded direction but at a speed higher than the commanded speed.
The drive will reach the commanded speed and trip before the slip frequency is low enough to
induce flux in the motor.
If the motor is coasting at a high enough speed (above about 40 Hz) and the output contactor is closed,
then the motor will self excite with the drive motor filter capacitors and generate a high stator voltage that
the drive can detect.
If the optional tachometer feedback is installed, then the drive knows the speed of the motor at all times
and can perform a flying start for any speed or direction of rotation.
With a synchronous motor, flying start is much quicker and more reliable because a detectable stator
voltage is produced whenever the field is applied and the motor is rotating, even with zero stator current.
When the drive is started, rated field current is applied to the motor but the stator current remains at zero
until the end of the ramp start delay to allow the rotor flux to build up. If the stator frequency is greater
than about 2 Hz, sufficient stator voltage is generated to allow the drive to detect the speed and direction
of the motor and synchronize itself to the motor flux. If the flux feedback does not reach the level
specified by parameter Flux Minimum, the drive assumes that the motor is stationary and starts from zero
frequency. If the motor is not in fact stationary but is rotating very slowly, it may fail to start. If the
optional position encoder is installed, a flying start can be performed for any speed or direction of rotation.