Power Supply Build for Router CNC

[Wobblycogs]

As I've got bits starting to arrive for my CNC build I thought it was about time I started looking into build a power supply. I've read until my head hurts and I think I've come up with a power supply that would work. It's certainly not the finished design, I would like to incorporate the eStop to cut the mains power, but I'd like to know if I'm heading in the right direction with the calculations and design. So without further ado...

The items this supply will be powering are

• 4 * CNC4YOU 3.1Nm Steppers
• 4 * AM882 Drivers

The steppers will be wired parallel which the datasheet gives as 4.2A giving a combined draw of 16.8A. Using an approximate real draw of 66% because of the phase shift gives 11.2A which I'll round up to 12A to account for the drivers and any losses.

I've read a ton of stuff on the correct voltage to run the steppers at, the calculation based on inductance [32*sqrt(3.2)] gives 57.2V and one based on the coil voltage rating [20*2.73] gives 54.6V. There's no end of people reporting that these steppers will run happily on 70V though so that's my target.

Working backwards from my 70VDC target gives a secondary voltage of (70/1.4) 50VAC which I plan on getting by wiring a 25+25 toroidal transformer in parallel. The transformer will need to be able to supply 70*12=840W so I was planning on using a 1000VA transformer (this one). There will be a 7A type T fuse on the primary side as well as a DPDT switch.

The bridge needs to be a bit of a beast so I was going to go with this 400V 35A part. At full tilt it will need some serious cooling (assuming my calculation is correct)...

1.1V * 20A * 2 = 44W of heat
(130 degC - 30 degC) / 44 = 2.27 K/W
2.27 K/W - 1.4 K/W = 0.87K/W

so I'm planning on attaching a CPU heat sink to it. I assumed a 30 degC ambient since it will be in a case, 20A is the transformer limit. In reality it will almost certainly never be going flat out so I don't think it'll be a problem. After rectification it looks like I'll be getting 67.8V.

For smoothing I'll be using some cheap and nasty 100V 10000uF capacitors I've found on eBay. My calculation shows I need 28,288uF of capacitance to achieve 7V of ripple:

C = (20 * 0.01) /7.07 = 0.028288F

As the capacitors don't list a ripple current I was just going to stick 4 in parallel to give me 40,000uF of smoothing goodness.

I originally designed the system with always connected bleed resistors but I changed my mind to have a relay system. I'm in two minds which is best but I've got the relay version drawn up at the moment. I chose 30 seconds as a bleed time because it would take longer than that to get into the case. The calculation for the bleed resistor is:

W = 4 * ((CV^2) / 2) = 4 * ((0.01 * 70 * 70) / 2) = 98J
90J / 30s = 3.3W
R = (70 * 70) / 3.3 = 1484Ω

So I've got a 5W, 1k5 resistor in there at the moment. I've had a look on RS and I can seem to find a suitable relay. I've never used a relay so hopefully what I've draw is the correct way to wire it up. If I understand correctly when power is applied it energizes the coil and opens the relay preventing power going though the bleed resistor.

And finally the circuit diagram. Keep in mind this is the first circuit I've ever designed so there could be any number of brain dead things in there. If any of the symbols / terminology are wrong please point it out as I'd like to try and get it right. I suspect the power supply is a bit powerful for what I need but I think the machine will grow another axis at some point so I'd rather go large than build a second power supply.



Cheers