PSU amp rating when overvolting steppers

[Novec]

Hi!

I'm kinda new to CNC, but I've understood that you can overvolt the steppers by atleast 10x as long as the amperage doesn't exceed the motors rating, and that the best way to do it is with a chopper circuit (PWM). That brings up the question about how much power I need.

Let's take the example of a 5V 1A motor. To keep it very simple, I could go with a 5V PSU supplying 1A. However, if I'd overvolt 10x to 50V, would I still need 1A, or would 0.1A do the trick? Although the current draw still is 1A at the motor, and input current is 50V, I'd figure the power required would be the same, in this case 5W.

Others say that I'll need to feed the chopper the full amperage regardless of input voltage, although the effective/average voltage over the engine coils would be unaffected by it.

So - what sayeth the learned?

Edit: I'm also wondering whether I need one chopper on each of the motor coils for a bipolar series setup, or if I can use just one for the whole thing.

[stuart76]

ive read alot of forums and asked alot of questions and still have minimal knowledge. everyone has a different answer. im pretty sure that anything over 4 x the rated voltage is overkill. I hope someone answers the question about what amperage is needed or affected by upping the voltage???

[ger21]

Current remains constant, regardless of the voltage. Torque is basically proprtional to current. If you only supply .1A to a 1A motor, you'll only get ~10% of the rated torque. Torque drops off as speed increases, because the current can't flow fast enough into the coils of the motor. What increasing the voltage does, is speeds up the current flow to the motor, which let's it maintain higher torque at higher speeds. A chopper type drive will regulate the current for you. Non chopper drives need to have resistors wired in line with the motors to regulate the current.

If you run a stepper at the rated voltage, the torque will most likely drop off so fast that it would be unuseable. You should try to use at least 5x the rated voltage, maybe more. Keep in mind that if your system works as expected at a given voltage, any higher voltage will just heat the motor more, with no benefits. Some systems may work fine with 5x the rated voltage, some may need 15x-20x. It depends on the application. Each machine is different.

[Novec]

In a way, your theory makes sense, but in another way, it doesn't. In essence, a chopper circuit is a PWM power supply, so if the resistance is constant, my theory whould be correct. That's easy to check by connecting a 60W lightbulb to a 12V PSU - it will draw 5A from the 12V line, but only 0.5A from the 120V in your wall (add PSU inefficiency, ofcourse).

There, however, is where my theory ends... A stationary stepper motor fits this bill pretty well, but as soon as it starts moving, the resistance won't be constant anymore, because as the current direction alternates for every step, it will have to fight the resistance generated by the coils inductance. Afterall, that is the reason for overvolting in the first place... This is also how a low-pass crossover in a speaker works - the higher the frequenzy, the higher the resistance caused by the coils inductance, thus the lower the volume (voltage) is on the other side.

All in all, that sounds pretty obvious too - higher speeds require higher power. Unfortunately, I don't know the necessary formulas to calculate it all, but I'll let you know my results when I've hooked it up.

[Al_The_Man]

Originally Posted by Novec In a way, your theory makes sense, but in another way, it doesn't. In essence, a chopper circuit is a PWM power supply, so if the resistance is constant, my theory whould be correct. That's easy to check by connecting a 60W lightbulb to a 12V PSU - it will draw 5A from the 12V line, but only 0.5A from the 120V in your wall (add PSU inefficiency, ofcourse).

That is not a good analogy as obviously you could not connect the same lamp up to 12 & 120 as the resistance are totaly different in order to provide 60w.
The difference between a PWM power supply and a PWM stepper drive is the power supply provides a sufficient pulse width in order to maintain a given voltage output, The stepper PWM amp varies the pulse width in order to maintain a mean current level equal to the current rating of the stepper motor.
The aim of a resistance fed amplifier and a PWM amplifier is to attempt to provide the rated motor current throughout the speed range in order to provide constant torque throughout. In both cases the impedance of the stepper motor changes with rpm, so the resistive method monitors the current passively and the PWM monitors actively.
Al.

[Novec]

Sorry about the lightbulb analogy, it was just a clumsy choice of words. Naturally, I ment two different lightbulbs, each consuming 60W at their rated voltage. But enough of that...

I think we agree on the power supply vs stepper drive issue - they're basically the same, but the feedback to control pulse width monitors different values. Where as the power supply only makes minor corrections to keep the voltage steady at varying load, the chopper constantly corrects the pulse width/voltage to match the varying resistance, in order to keep a steady current.

So, in essence, I could run my 1.85V 3.5A stepper on a 12V 2A PSU, and have full torque as long as I don't go full speed. Right? However, I'd need a 24V 3.5A PSU to run it at full torque and max rated rpm, when it's rated 24V 3.5A. Damn, I'm making things complicated here...

All I need now is a drawings for a 7A chopper, to run both phases

[H500]

One thing to keep in mind is that a motor is not just an inductor. Because it is spinning, it is doing mechanical work. The power input to the motor is equal to the motor's mechanical energy (the good stuff) plus the energy wasted in heating up the motor (the bad stuff).

That's why you can't simply do nameplate Volt X Amps calculation to determine your power supply requirements. The motor's nameplate value simply tell you what happens at standstill, where 100% of the power is wasted heating up the motor.

[Dave's_Not_Here]

Originally Posted by ger21 Some systems may work fine with 5x the rated voltage, some may need 15x-20x. It depends on the application. Each machine is different.

.... give some basic examples of scenarios where the above applies...

especially helpful to a novice like myself, that is trying to understand all the subtilities of stepper specifications and the interlationships with, power supplies, controllers and drives... while trying to plan the configuration of a router mill....

Nothing worse than finding that you just spent $300 on steppers that don't work well with your other components because they are not "matched" correctly...