Joe90's BIG RAT Build.

**AndyUK** · 31-05-2020

Wow that's some serious brewing activity...

Don't worry about phases in your calculation. Just take that single phase value, 4A or so and go with it.

If your Calcs end up needing more than 1000kVA you're probably not quite right, or you're building a beast. Most builds here use around abouts 500kVA.

I won't comment on the information you've already seen from my build log - no point hearing the same persons opinion twice. Although I will add that I think I was a little conservative with my secondary voltages in retrospect, and could have gone higher.

**Neale** · 31-05-2020

It's easy to get too wound up about these things. Whether you should use 60%, 70%, or 80% really doesn't make a bit of difference - if you are using a linear (i.e. toroidal transformer, etc) anyway, in practice, the average load will be way lower. Doesn't matter to the transformer - as long as it's rated for the average load with a bit left over, its thermal mass is so great that a few seconds a bit over rated current won't worry it. The rectifier is coping with short high-current pulses anyway as it charges the capacitors towards the peaks of the cycle. And capacitors aren't that critical as regards value as this is a CNC machine, not an audio amplifier. Ripple is not an issue. My drivers, for example, are rated for something like 20-80V input. A few volts more or less ripple ain't going to bother them any. I use a 650VA transformer that gives me around 67V and it doesn't even get warm.

Different case if you are using a switched-mode, stabilised, power supply. They are much less tolerant of peak loads and you might be safer over-specifying if you use one of those. Which is why linear PSUs are generally recommended!

**JAZZCNC** · 31-05-2020

As the others say don't get stressed over it and just build to give the required amps plus a few spare. Work on 60% of the total Motors rated amps and it will be fine.

Don't go crazy OTT with the amps because you will end up with a large Va rating and the inrush will bite you. 500Va does most 3 axis machines and 625Va for 4 axis running typical 4.2a steppers with 60-70Vdc. These are tried and tested values on 100's of machines.

**Clive S** · 31-05-2020

Originally Posted by AndyUK

Wow that's some serious brewing activity...

Don't worry about phases in your calculation. Just take that single phase value, 4A or so and go with it.

If your Calcs end up needing more than 1000kVA you're probably not quite right, or you're building a beast. Most builds here use around abouts 500kVA.

I won't comment on the information you've already seen from my build log - no point hearing the same persons opinion twice. Although I will add that I think I was a little conservative with my secondary voltages in retrospect, and could have gone higher.

That would be some big machine

**Muzzer** · 31-05-2020

Yes, 1000kVA would be about 750 hp. You'd need your own substation to be installed. Even 500kVA / 380hp would be pushing it. You can safely drop the k....

**joe.ninety** · 31-05-2020

Originally Posted by Clive S

That would be some big machine

Yes, I think there's been a of 'k' inserted where it shouldn't be. Lol

Can you imagine the size of the donut for a 1000kVA toroidal... Probably about the size of a truck tyre inner tube blown up to its max.

**Doddy** · 31-05-2020

I think you're all missing the important part of this thread. How's the beer? (if you don't mind I might PM you over this - I've a similar interest, or at least an interest in getting the missus interested).

**Kitwn** · 01-06-2020

I'm with you Doddy! There's a definite new social possibility in this thread... "come round for a free beer and have a look at my home-made CNC machine". I might be abe to make some friends!

It is something of a minefield trying to understand what the ratings of various components mean and how they need to be interpreted when designing a machine from scratch. Best thing is to just do what Dean says, as usual!

Kit

**JAZZCNC** · 01-06-2020

Originally Posted by Kitwn

Best thing is to just do what Dean says, as usual!

Kit

Not quite sure how to take that.? I just offer advise based on experience and people are free to take it or leave it.!

**Kitwn** · 01-06-2020

Meant as a compliment! Your range of experience is unique among forum members and as a professional machine builder your customers would have told you loud and clear about anything that didn't work effectively and reliably.