HARSFEN0602
VL[N] Command low limits for position and speed
Table 14-1: Commands relevant to limits, protection, and diagnosis
CAN users have an added protection level. The Harmonica issues Emergency objects in the
case of errors or applied protections – refer the CAN manual.
14.1 Current limiting
The Amplifier protects the motor from over current.
The current protection is applied in two stages:
The motor maximum peak current (Given by PL[1]) is available for the peak duration time
(specified by PL[2]). For long time periods, the current is limited to its continuous limit
CL[1].
The current limiting process is dynamic. If the current has been close to its continuous limit,
then the time allowed for the peak current reduces.
The next paragraphs detail the mathematics of the current limiting process.
The absolute value of the measured motor current (in vector servo drives this is 22 DQ II +) is
applied to a first order low-pass filter. The state of the filter is compared to two thresholds.
When the state of the filter is above the upper threshold, the continuous limit is activated.
When the state of the filter is below the lower threshold, the peak limit is activated.
The time constant of the low-pass filter is
−
−
=
MC
CL
PL
]1[
1log
]2[
τ, where MC is the maximum
servo drive current.
The maximum time, for which the peak current can be maintained, after the current demand
has been zero for a long time, is - τ⋅
−]1[
]1[
1log PL
CL seconds. If PL[1]=MC, then the
maximum time allowed for peak current is PL[2]. Otherwise, the servo drive can provide the
peak current for a longer time.
More generally, if a current demand of PL[1] has been placed after the current command is
stable at I1 < CL[1] for a long time, then the peak current PL[1] will be available for
τ⋅
−
−− ]1[
1]1[
1log PL
ICL seconds.
The resolution of PL[1] is about MC/1000, and the resolution of PL[2] is about 0.1 second
Example:
The graph below shows the signals concerned with the current command limiting process for MC=6,
PL[1]=6, PL[2]=3, and CL[1]=3.
The application motor current command is increases from zero to 6Amp at the time of 0, and is then
decreased to 0 at the time of 5sec.
The state of the filter increases until it reaches the continuous current limit of 3 at the time of 3 sec. At that
time, the LC flag is raised, and the motor current command is decreased to CL[1]=3Amp.
After the motor current command is set to zero, the state of the filter begins to drop. When the state of the
filter drops to 2.7Amp = 90% of 3Amp, the LC flag is reset and the torque command limiting is again to
6Amp.