A CNC machine without power is little more than a very expensive lump of inanimate metal but supplying power that is appropriate, clean and safe is not entirely trivial. It is certainly possible to just buy a power supply that will get the machine going but if you want to get the most out of your machine you will almost certainly have to custom build a power supply.
CAUTION: Building a power supply involves working with potentially deadly voltages and you do so entirely at your own risk. Do not just assume that anything you read here is correct, make sure that you understand where every number comes from.
Before you start thinking about building a power supply you need to know what you are powering so that you can build the optimum supply for your needs. This means providing the right voltages and an appropriate amount of current. Supplying the wrong voltages would be bad for your system with levels of bad ranging from not powering up to never powering up again. Supplying too little current will give poor (or no) performance and being able to supply far too much is just wasteful. For this example I’m going to assume that the items that will need power are (this is essentially the Yeti build):
- 4 * Stepper motor, model 60BYG301B wired in parallel.
- 4 * Stepper driver, AM882
- 1 * PMDX-127
- 1 * PMDX-107
- 1 * Water pump
- 1 * Driver fault
- 1 * Limit
- 1 * VFD
The motors and drivers require 70V DC, the VFD and PMDX (both parts) require 230V AC and everything else is 24V DC. This build will focus on the 70V DC supply since the 230V AC just comes from the mains and it is simplest to supply the 24V DC with an off the shelf unit as only a small current is required.
Required Current from the 70V Supply
Working out the required current is a little bit calculation and a little bit black art. The simplest method is to just add up the maximum current required by each stepper in the system. The specification for the steppers is:
So taking the above machine as the example we see it has four steppers which are wired in parallel and that gives a maximum current draw of:
4 * 4.2 = 16.8A
A power supply capable of delivering 17A would, therefore, power the system with a little room for other items. There’s a problem though, that’s quite a large (and therefore expensive) power supply. If the system isn’t going to be using all that power money (and space) could be saved by providing a smaller power supply. When wired in parallel a stepper will draw less than 70% (actually 1/√2%) of it’s rated current and when wired in series it will draw less than 33%. Going back to the calculation then the maximum current draw is:
4 * (4.2 * 0.707) = 11.9A
Reducing the required power by 5A has reduced the cost of the power supply but a 12A supply is still quite large. The largest linear power supply Zapp sell, for example, is only good for about 6A at 68V. Fortunately there are a couple of factors at play that mean a seemingly under sized power supply can often still be used. Firstly the machine is not going to be running at maximum power all the time. The calculated 11.9A requirement would be equivalent to all four motors turning slowly (as there is maximum current draw at low speeds). Secondly, while the situation may arise where the maximum current is required it is unlikely to last for very long and linear power supplies are quite good at handling short surges.
For the example machine I would aim to supply around the 12A mark if making the power supply. If you are going to the trouble of making the power supply then there’s little point in not providing ample power but if you are limiting yourself to something you can buy it may be worth considering a lower powered option.
Required Voltage from the 70V Supply
The might seem like a crazy heading but a power supply that is simple won’t necessarily give exactly 70V all the time. The aim is to build something that gives a range of acceptable voltages and correct calculation the voltage required from the power supply is more critical than correct calculation of the cutting as an incorrect voltage can lead to dead drives and or steppers. The introduction to stepper motors page gives a good high level view of how to calculate the desired voltage for a given stepper motor. Rather than repeat that calculation here I’ll take the results from that article and state that the desired voltage is in the range 65 to 75V.
The AM882 drives will run at up to 80V and the higher the voltage the faster the steppers can be run so why not build a power supply that can supply bang on 80V? Ideally you would supply exactly 80V as that would give the best performance but the real world, as always, conspires against us. The first problem is that if the supply voltage isn’t bang on the voltage assumed in the calculation the output voltage will be different to what is expected (unless a linear regulator is incorporated). Europe is nominally on a 230V supply but the allowable range of supply voltages is 230V +10% -6%. That means in theory the supply could be anywhere between 253V and 216V (the UK is typically around 240V). Secondly the steppers on the machine will generate back emf when the machine is slowing down and the stepper changes from being a power sink to a power source. That can cause the voltage in the system to rise above the design voltage.
Linear or Switch Mode Power Supply
There is really no competition here and only one choice if you are building your own power supply: linear. A switch mode power supply (SMPS) is great for situations where efficiency is of primary importance and where the load is well understood and almost static with no surges, an obvious example of where a SMPS would be used is a computer. Switch mode power supplies also tend to be smaller, lighter and cheaper than linear power supplies.
Linear power supplies on the other hand produce power with less noise and are much better at coping with power surges. Low line noise is important for CNC use as noise on the line could cause problems with accuracy. Power surges, as mentioned above, are a fact of life with steppers driving heavy equipment.
You should now read part two: Building a Power Supply – Parts