The link I posted for stepper motors might give you a better idea. Also there's this data sheet which I've found has a good description on how micro-stepping works. How the current in one phase is set with respect to the other for each micro-step. https://www.ti.com/lit/ds/symlink/lmd18245.pdf
>From the perspective of current draw think about what an inductor does to >current flow. If you apply a voltage across a resistor the current starts >flowing pretty well instantly. When you put a voltage across an inductor it >takes time for the current to reach maximum. So what is maximum? Well if the current rating of the motor is say 3A and the winding resistance is 1 ohm then Ohms law say the motor voltage to create 3A must be 3V. But it doesn't happen immediately because the inductance slows it down. And since it's not at 3A right away the motor doesn't develop full torque right away. One way to get the current to max faster is to use a higher voltage. Let's use 30V instead. 30V/1 Ohm is 30A. Oops. Magic smoke just came out of the motor. So what he drivers do is sense the current and when it reaches 3A turn off the voltage. When it drops below 3A which also takes time because of the stored energy in the coil, the controller switches on the voltage again. It's called chopping. Now instead of say taking 2.5% of the step pulse to reach max current it takes 0.25% so max torque is reached and held for much longer. Look at page 23 of the data sheet I linked and you can see from the table that the one big feature about stepper motors is the winding current changes direction every step. When you start micro-stepping current through one winding is 100% while the other is 0% So half the torque if you use the T=Amp x Turns rule and reversing the current also takes time. Again, 30V for a 3V coil or even 60V for a 3V coil means virtually zero time to reach full current. Let me take a little side trip here. If you've ever played with slot cars or small dc motors and increased the voltage to make them turn faster you have, as a kid, learned the simple rule. More voltage makes the motor spin faster. But why? With brushed DC motors as the windings that are not powered move through the magnetic field, just like a generator a voltage is induced in the windings. Simply put this induced voltage equals the applied voltage at the speed created by the applied voltage. So if it turns 1000 RPM at 6V and you apply 12V it may will reach 2000 RPM (for example) but will go no faster. Change it to 18V and now we get 3000 RPM. How does relate to stepper motors? To change the direction of the current through the winding the controller has to exceed this generated voltage caused by the motor turning through the magnets. When the induced voltage reaches 3V below the applied voltage; 27V induced and 30V applied the time for the current to reach that 3A is back up to 2.5mS. Trouble is the step pulses are 1.25mS long. So again with lots of simplification (and a linear viewpoint), the current only reaches 1.5A before it's time to change the direction of the current again. That means only 50% of the desired torque. It's actually less because the actual math will talk about the area under that triangular waveform and as Robin mentioned, voltage * current = power and remains steady. But it doesn't help us for the torque. Double the voltage, the time to change direction of current decreases, there're more current in the windings for a longer time and the torque increases. And yes, 2x voltage * current under the curve equals a doubling of power. But again, we don't care. Unless we know that within a given step period that the 'average' current has increased we don't have more torque. So indirectly power might well remain the same but generally the one _fixed_ parameter in a stepper drive system is the power supply voltage. We don't change that. So the average current through the windings during a specific step determines the motor torque. Power as a number just isn't important once you've bought the power supply and the stepper driver that can set 1A to 8A for different motors. Now you choose the motor that has a torque verses rpm that matches your system. There are very few, if any stepper drivers that use 200V. Most are max 80V or so. So where is the flat part of the graph? At a very low speed if the stepper driver voltage is 3V. Much longer and at a higher speed with 24V and really a lot longer at 80V. But unlike DC motors, it's the reversing of the current through the windings every step that is the Achilles heel of the stepper motor architecture. So although the power part of the curve might look flat the drivers don't keep the average current the same for faster step rates by allowing the winding current to increase above the rated value so the area under the curve remains the same regardless of step pulse width. And that's because too much winding current and induced magnetic field will damage the magnets in the motor. So I stand by my original position that the torque curve verses RPM is way more important that power which no one publishes because it's not useful. John > -----Original Message----- > From: Thaddeus Waldner [mailto:thadw...@gmail.com] > Sent: February-05-22 8:26 PM > To: Enhanced Machine Controller (EMC) > Subject: Re: [Emc-users] What Would You Suggest? > > > > > On Feb 5, 2022, at 6:48 PM, John Dammeyer <jo...@autoartisans.com> wrote: > > > > No. The motors are designed to handle N amperes although they get quite > > warm that should be a 24/7 rating. Because they get so > warm many drivers back the current off when the motors have been idle for a > period of time. > > Yes, N amperes. Is that not the flat part of the graph? > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
