Power Supplies

CNC machine actuators need a lot of power, and its the power supplies job to be able to keep up with powering all the actuators driving the CNC machine. There are two types of power supplies you will see in use with CNC’s and they are:

Regulated Power Supply

These power supplies are usually switched mode types and provide a constant regulated output of power. These work very well when used with CNC drives that have a narrow band for input voltage and need a very steady voltage that does not cross a certain threshold.

Regulated power supplies are available in a huge variety of different voltage and amp ratings. These voltage and amp ratings will tell you exactly how much voltage the power supply will constantly put out and the maximum amount of amps it will be able to deliver at that voltage.

Pros:

  • Steady regulated output
  • Strong output and reliable output when used within its range
  • Good price for smaller voltage and amp units

Cons:

  • Complicated devices with many components (more likely to fail)
  • Can be expensive for larger power units

Unregulated Power Supply

These power supplies are like the name says, unregulated. This means unlike the regulated power supplies they won’t provide an exact steady voltage under load. Unregulated power supplies have quite a basic setup, the transformer and usually one to three small circuit boards which all provide a different voltage.

Pros:

  • Simplistic devices with much fewer components than a regulated power supply
  • By tapping off different locations of transformer, one power supply can offer multiple different voltages (example: 70V, 12V and 5V)
  • Good for high power applications (Big CNC machines)

Cons:

  • No steady output voltage like a regulated power supply
  • Could destroy drives that have narrow voltage requirement

So Which Power Supply Should I Choose?

Well as always it depends on the application. If you are running smaller drive electronics (around 50 volts) that have a small threshold for voltage and won’t tolerate over-voltage then running a regulated power supply would be the best choice.

For much larger power hungry systems an unregulated power supply would suit. Bigger drives have much larger tolerances for input voltage so an unregulated voltage supply will work fine even if the voltage does jump around a bit.


Calculation

You should determine your power supply by the power needs of your motors. Here are some good calculations for finding the correct power supply for your set up:

Voltage = 32 * sqrt(Inductance in mH)
Supply voltage should be based off your motors inductance:

Current = Amps needed per drive * Number of drives * 0.7
Current needed should be 70% of the individual amp requirement of one drive times the number of drives in your setup. Generally it is possible to run your drives from even down to 50% of the total amp requirement, but for good measure stick with 70%.


Example

For this example I will be using a stepper motor with the following specifications:

Inductance / Phase = 3.6 mH (milli Henries)
Current / Phase = 2.8 Amps

Lets first calculate our voltage requirement:

Voltage = 32 * sqrt(3.6)
Voltage = 60.7 V

Alright so now we know we need around 60 volts for our motors to run at their recommended specification. Now we will calculate our voltage requirement for lets say three motors (for a 3 axis CNC machine):

Current = 2.8 (amps) * 3 (motors* 0.7 (70%)
Current = 5.88 A

So now we know to get the best performance from our three stepper motor’s we’ll need a power supply that can supply around 60 volts and 6 amps.

This example was based on my own stepper motors. I ended up choosing a 48V 7.5A switch mode regulated power supply as it was a good price and while the voltage isn’t quite 60V I was happy give up getting the full specified speed from the motors in exchange for paying less.