# How to calculate power supply requirements for Closed Loop Stepper Motors?

I have a CNC Router table that has 4 Nema 23 stepper motor (open loop). I want to upgrade the machine and use 4 Neam 34 Closed Loop Steppers. These are the motors and drivers I'm considering:

Steppers: Nema 34 Closed Loop Stepper

Motor Type: Bipolar Stepper
Rated Current/phase: 5A
Voltage: 5V
Phase Resistance: 1ohms

Output Peak Current: 0~8.2A
Input Voltage: +24~80VDC (Typical 48VDC)
Logic Signal Current: 7~16mA (Typical 10mA)
Pulse Input Frequency: 0~200kHz
Isolation Resistance: 500MΩ


My questions are:

1. How should I calculate/choose the power supply for these 4 steppers? I have looked around for this and found many things, like measuring the voltage on the motors while running, some multiply the Rated Current/phase for the number of motors, and so on. I think part of my confusion is that I don't have a lot of experience with electricity (voltage and amps).

These are the ones this company offers.

Is it as easy as selecting one (350W 60V 5.9A) that is close to 48V?

1. Does it make sense to find an unregulated power supply for my CNC? It looks like none from this website is.

2. Does it make sense to find a power supply that is more powerful and uses only one for all motors instead of 4 of them?

So there's a couple of things you need to go through when choosing the power supply for this application. Number one you need to differentiate between the driver capabilities and the motors requirements.

Now the motor you have listed is no longer available on the website, but I found several motors on the website that are similar to what you have listed. The data sheets are a bit sparse, but the current requirements and recommended running voltages for the motor are what you're looking for. The MTBF (Mean time between failures) for the motor I found is measured at 24v(though Nema34 are typically driven at at least 48v) and the current per phase is listed at 6A.

If you have 4 of these motors working at full torque capacity that means you'll need 48v at 24A. though you probably want to give yourself some overhead on current. Most of the time a 20% overhead margin is recommended. That would mean you'd need a 48v ~30A power supply if you were planning on fully loading all 4 motors.

Now, since this is a CNC machine, and it is very unlikely that you'll ever have more than 2 motors working fully at any given time we have a bit of wiggle room. So if we assume the maximum load we're ever likely to see will be the equivalent of 2.5 motors working at full torque capacity, that means we're down to about 15A, so with a margin of error a 48v 18A power supply should do nicely for your application. The closest thing they have on the website you have listed is a 48v 1000W (21A) supply which would work well. It's always better to overshoot on the capacity of your power supply than to under-spec it.

Now this is another component of this to deal with. You mentioned a 60v power supply. If you are familiar with Ohm's law you might be inclined to think you could get away with a 60v power supply with equivalent wattage. That's not quite the case though, as the way stepper motors and drivers work, they are designed to hold a constant current regardless of input voltage. If you go up in voltage you'll need a higher wattage power supply to still be able to deliver the necessary current.

As far as unregulated, I would definitely not consider an unregulated power supply for any sensitive operations due to the high voltage ripple, and I don't think you'll be able to find one that would deliver the necessary wattage at a reasonable price. Typically they are limited to low current linear supplies. Unregulated supplies that could deliver the necessary current are extremely expensive, and you would still have the issues with the ripple voltage. The higher the load on the supply the larger the ripple in the voltage, so with a potential draw of 15A or more, you'll see tremendous fluctuations in the output voltage. This would be pretty disastrous on a CNC machine.