MartinC wrote: ↑Wed Feb 12, 2025 3:18 pm I'll use AC servos as described lately by Uwe and I hope they handle braking properly.
Check the manual for the drives you will be using. It should say what which models come with a braking resistor installed or if an external resistor should be provided. Most AC servos systems I have seen between 400 W and several kW include the braking resistor.
The A6 Servos are also with a small internal braking resistor, perfectly ok for axis drive.
For spindle use with more inertia an external resistor can be used for less than 0.2s deceleration / 1000rpm
Uwe
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You can get a pretty good idea how fast you'd need to turn your motor before the resulting voltage caused a problem by considering that at normal supply voltage (220-240Vac for most of these servo drives), the motor is specified for operation up to something like 2000-3000rpm. As the back emf will always be less than the internal bus voltage, you can infer that you'd need to manually spin the motor faster than that. The only exception would be if the servo drive operated in the field weakened region ie above base speed (where the back emf equals the dc bus voltage). I'm not aware of any "normal" servo drives that do this.
If the driver is unpowered, the switching transistors will be turned off, so any back ac emf from the motor would be peak rectified by the flywheel diodes. The only load on the motor would be due to the bleed resistors on the internal dc bus which would give a fairly modest drag to the motor. Try it if you are not sure - as long as you don't exceed the max rated speed of the motor you should be fine.
Speaking from 30+ years experience developing motor drive products.
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MartinC wrote: ↑Wed Feb 12, 2025 3:18 pm
I guess this is independent on Acorn.
Turning any PM motor (regardless if PM servo or stepper) will generate a back EMF voltage (proportional to speed) that goes back into the driver. Whether the driver could be damaged or not depends on the driver. I'd guess mostly not, but you are generating uncontrolled voltages that go into the bus voltage. In the extreme case (probably impossible by hand cranking, but very real by decelerating a rotating axis) the generated voltage could exceed the max. voltage of the driver (and the acorn if connected to the same e.g. 48 V). For that reason higher power drives have brake resistor control. In the stepper world, system builders mostly seem to ignore this (so it can't be that much of a problem). But I have killed a lab PSU once by braking a small motor from full speed to zero. I'll use AC servos as described lately by Uwe and I hope they handle braking properly.
Besides the (probably not so likely) potential damage to the driver, the main reason why I would avoid turning motors with power switched off is that the current you induce by turning the motors create a (roughly velocity proportional) counter torque, which feels like someone has filled your motors with honey. So if you want to turn the motors by hand, apply power to the system, but set the driver enable to false. This way the driver power stage has bus power (like 48 V) and unless the generated back EMF of the motors is larger (which would happen if you rotate the fast enough, unlikely by hand), no current will flow (through the power stage free-wheeling diodes): No honey-like feeling and no danger to drivers or other connected electronics.
Martin
Thank you Martin
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MartinC wrote: ↑Wed Feb 12, 2025 3:18 pm I'll use AC servos as described lately by Uwe and I hope they handle braking properly.
Check the manual for the drives you will be using. It should say what which models come with a braking resistor installed or if an external resistor should be provided. Most AC servos systems I have seen between 400 W and several kW include the braking resistor.
TY Centroid
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suntravel wrote: ↑Mon Feb 10, 2025 3:23 am
With Acorn Six you can have full closed loop with scales or rotary encoders, you can also have the scale position on the DRO.
But turning an axis by hand is no option, even with disabled servos, only moving the axis with buttons/mpg ect.
I also considered using an analog joystick, but for me, it is on the save side to have one MPG for each axis with 1x 10x 100x buttons.
The s curve acceleration is also active with MPG or jogging with the VCP.
Uwe
Interesting thread. I'm just rereading the milling machine manual. There is an option to disable servos to allow manual machining. Power Feed, then F3, Free X and Y. Would this not work?
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suntravel wrote: ↑Mon Jun 16, 2025 3:43 am
Maybe, but it why?
Moving with Jog/MPGs is fast and precise
Uwe
Just a question / idea. There is arguably more control using a larger handwheel but I'm more curious. Looks like my 5 Axis Mill running UCCNC might be sold. I'm looking at options, keen to try the Acorn 6 board and might try it on my 2ton concrete mill (replaces CS Labs, running SIM CNC at the moment).
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