232 Rockwell Automation Publication MOTION-RM003I-EN-P - February 2018
would increase rapidly. Active control of motor current is lost whenever the power
structure is disabled. So the concern is when the power structure is disabled with
the motor still spinning. This can be the case when the drive executes a Category 0
Stop due to a control initiated Disable Request, a Major Fault action, or a Safe
Torque Off activation.
Three speed values are shown in the preceding figure. As defined in the first figure,
the rated speed (S
r
) corresponds to operation at rated voltage and continuous
torque. This is the maximum continuous torque that can be achieved without field
weakening. S
OV
is the speed at which the CEMF voltage from the nominal Ke
would be equivalent to the maximum DC Bus Voltage rating of the drive, or the
DC Bus Overvoltage Limit.
If active motor current control is removed while the motor is operating at speeds
above S
OV
, the CEMF voltage results in a DC Bus overvoltage condition that can
damage the drive and, possibly, other drives sharing the same DC Bus.
S
m
is the absolute maximum operating speed of the motor based on mechanical
constraints. For a rotary motor, S
m
would be given by the Rotary Motor Max
Speed attribute.
Major damage to devices connected to the DC Bus can occur when the PM motor
is allowed to run between S
OV
and S
m
and the drive's power structure is disabled.
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