Hi
I think e-stop is always going to be a compromise. You do want the kinetic energy to be zeroed on the machine, but without damaging the machine. You don't want someone hesitating to hit e-stop because they are worried about breaking the machine.
I did some testing today.
I connected a multi-meter across the braking resistor. It is definitely taking load. It can also be seen in the data recordings attached.
The aim of the testing was to identify a signal that can be used to regulate the bus voltage. The prime candidate is output frequency. If the output frequency approaches the actual motor frequency to reduce slip, the reverse power that is raising the bus voltage will stop rising. Zero slip angle = zero reverse power.
When the motor is stopped, the output frequency ramps down to zero. At the same time the bus voltage ramps up until the braking resistor is engaged, and flattens the peak. The resistor is 56ohms but the vfd controls the resistance with switching. If the switch conducts to the resistor 50% of the time, the resistor will look like it is 56 / 50% = 112ohms. Switching makes a fixed resistor look like a variable one.
Only a couple of channels of data are recorded because the data is obtained in real time over a 9600bit/s serial port. The sample rate is well below the Nyquist sampling frequency, so the data is likely to be missing key information. For example, the braking resistor switching transients are not shown. It is also difficult to identify the precise moment the over-voltage threshold was exceeded.
This shows the results of a normal run fwd, stop, then an e-stop. The e-stop tripped the drive.
This shows a normal run and stop at high rpm and high inertia. The drive close to tripping after the stop command.
At 67.5Hz, the e-stop works as expected, as shown in this report. If the frequency is raised to 70Hz, the e-stop trips the drive. At 67.5Hz, the drive is just managing to complete an e-stop.
The charts show there isn't much difference between a normal stop and a tripped e-stop. I suspect it will be difficult to implement a PLC type control function to reliably complete a fast shutdown. Part of the problem is the noise not shown in the charts.
The control algorithm for the braking resistor switch will know when it can't cope with the reverse power. Making use of that would require Yaskawa to change the firmware. Unlikely to happen any time soon.'
I plan to experiment with the PLC functions but at present, I am not confident I can make it work based on the test results attached. Any advice/feedback welcomed.
Dazz