AC30 Zero Speed Threshold and Zero Speed Stop Delay

Hello Dusan.  Yes, both of these are used for a similar purpose.  When the AC30 decelerates a motor to zero speed, it needs a way to determine when to report that zero speed has been achieved.  Depending on the mode in which the drive is operating, different approaches will be used:

• If the drive is operating in V/Hz mode, then the speed of the motor is never really known by the drive.  The drive assumes that once it reaches 0 Hz on its output, the motor will be at 0 RPM.  This is unlikely to be the case since the motor and load may still coast a little after 0 Hz drive output is reached.
• In Sensorless Vector (SLV) mode, the drive does know roughly how fast the motor is spinning most of the time.  One of the key limitations of SLV is that the voltage feedback from the motor will not provide a very good indication of speed when the speed is below ~10%.  So, like in V/Hz mode, the motor may coast a little after the drive thinks zero speed has been achieved.
• If the drive is running in Closed Loop Vector (CLV) mode, then an encoder or resolver is used as speed feedback.  Zero speed is reported when the speed feedback reads as zero to the drive.  In most cases, this is fine.  However, some low-resolution encoders are coarse enough that they are subject to noticeable quantization error when the drive uses pulses/second to calculate RPM.  Like other speed calculation methods, this one is most vulnerable near 0 RPM.

The Zero Speed Threshold allows the user to specify a speed below which the drive should assume it has successfully stopped the motor.  This is especially important with drives in SLV or CLV since their feedback may not ever truly report zero speed.  The default is 0.1% and this is fine for most applications.  You may consider increasing it if you are having trouble getting the drive to report zero speed.

The Zero Speed Stop Delay works together with the Zero Speed Threshold.  Once the motor decelerates below the Zero Speed Threshold, the drive waits the amount of time specified by the Zero Speed Stop Delay before signalling that the motor has stopped.  The idea is that this delay should be long enough so that the motor will definitely have stopped.  The default is 0.5 seconds.  You might consider increasing it a little if you have a very large inertial load and very little friction in your machine (e.g. flywheel, centrifuge).  You could also decrease it to reduce cycle time.

For most applications, you can leave these parameters at default.  I would only change them under one of the following conditions:

• The drive is reporting that the motor has stopped, but the motor is actually still moving a little.
• You want to reduce cycle time.

I hope this helps.  Please let us know if you have any questions.