Thread: dc amps vs ac amps
Perhaps "starve" is a bit excessive :D
A 1 Farad capacitor would deliver one Amp for one second, so .00047F is good for about half an Amp for 1 millisecond.
Your stepper driver should have built in decoupling to handle fast transients. I think maybe you are getting bogged down worrying about things that aren't going to give you problems.
You can add fuses and fat caps later if you find a need, but I don't think you will.
While what you are proposing CM will work, good electrical working practice is to independently feed high current circuits directly from the power supply to minimise interaction between circuits.
The reasons are many:
1/ A good quality power supply is a low impedance which will absorb transients. Daisy-chained circuits potentially allow transients from one driver to impact on another, for example the back-emf from the driver into the PSU when the motor coil is switched. This can push the supply voltage up several 10's of volts and it may be that only the PSU's impedance and back-emf protection is protecting the other drivers.
2/ Starvation is possible, if for example the connection between motor 2 and 3 isnt as good as that between motor 1 and 2 (the feed being to #1) and motors 1 or 2 are drawing a high current then the voltage across motor three may drop several volts leading to poor torque regulation. However we only really need to consider this possibility where the resistance of the connection is likely to be significant in respect of the current drawn - traction systems for instance where the current is measured in tens or hundreds of amps.
3/ Step current demands are transients and the shape of the step is determined by the higher frequency components which by their nature travel in the outer skin of the conductor. Long conductors present a much higher impedance to transients and therefore can benefit from localised capacitive reservoirs. Without the right equipment (osclliscope for instance) its impossible to tell whether these are actually needed. One determining factor will be the regulation capability of the power supply... i.e. how the output voltage changes under repetitive step current loads. A 5% regulation is considered poor, 0.1% would be pretty good (thats 2.5v or 0.05v on a 50v supply).
You say your power supplies are rated at 7A typical, 9A peak and you are running them in parallel. What supplies are they and are they linear or switched mode regulated or unregulated? If unregulated or switched mode you may find extra capacitance at the motor beneficial (but again it can be hard to tell).
Make sure the wire connecting the supplies together is heavier than the motor wire - best practice is to wire both supplies to a common connector block from where the motor wiring is taken (in high current systems a copper bar (busbar) is even better). Failing this, use 2 or 3 pieces in parallel.
I would argue that fuses are essential. If they 'go wrong' and are correctly sized then its a sign there is a problem somewhere... better to blow a fuse than fry a driver...
Incidentally, I just re-read your original post re your 'electrician'... I reckon he was talking out of where the sun don't shine!
Last edited by irving2008; 05-09-2008 at 09:25 AM. Reason: added pic
The PS Power supplies are available as 40V and are non regulated switch mode power supply specificaly made for use with our range of stepper and servo drivers.
Supply Voltage: 180 to 250V AC
- PS407 - 40V DC at 0A and 38V DC at 7A
- PS407 - 260W (Continuous)
I'v just found this as well: http://www.mycncuk.com/pdf/wiring.pdf
If a fuse goes pop for no good reason, the part you are cutting becomes junk PDQ. Then the cutting tool ploughs into a holding clamp, the bed dances a jig and it can get really expensive really quickly.
You don't want to end up using both hands to stall it while trying to switch it off with your foot :D
By petejw in forum General ElectronicsReplies: 7Last Post: 11-08-2009, 10:40 PM