• Set current - the higher the current, the stronger the motor, but the same is true for the generated BEMF
• Speed/mechanical load - of course, BEMF will increase as the load increases. This is the working principle of the sensorless homing of the Trinamic StallGuard - it measures BEMF!
Actual impact of BEMF:
In the following measurement, we can see acceleration and close-up of two areas - low speed/high speed
When the speed is still very low, the motor controller still has enough margin to adjust the current well, so it can be considered that the sine wave is ideal. But if we zoom in later, we can see that the current looks more like a triangle, and the applied voltage is not very accurate. That is because the controller has no voltage margin to correctly adjust the current. In fact, although the motor is still running, the sine wave will be distorted.
Now that we know how to control the stepping motor, we can go to the next point and answer the last question - what happens when the BEMF is so high that it is close to the power supply voltage? You may guess that the motor will start to lose step - indeed, but it will not appear immediately! To be honest, I was surprised at the ability of the drives and motors to handle extreme speeds. Let‘s see:
This is the current flowing through the motor coil during a complete operation using a 24V power supply. The printer starts at a standstill, then accelerates to 900 mm/s at a speed of 9000 mm/s2, and finally stops. So, what actually happened? At first, the driver can maintain a sine wave, but later, when the BEMF approaches the power supply voltage, the waveform will deteriorate, as we can see above. But at this time, the printer still does not reach the required speed - soon the back EMF voltage generated by the motor is too high to reach the set current value, and it drops until it reaches the required speed, and then the amplitude becomes stable, but we no longer see the sine wave - in this point, it is closer to the square wave.
These results look bad, but in fact - the results are good! The machine will not have any problems after running for more than one year under this setting. This is normal in high-speed applications. Of course, the torque is greatly reduced, and the accuracy may not be perfect, but after the deceleration, the motor returns to the nominal torque, and the position is accurate. 900mm/s is the maximum speed I thought was safe before I started to lose step.
I also tried to use the raw data from the oscilloscope to calculate and display the average "voltage consumption" during the operation.
It turns out that this is more difficult than I expected, so the results are only indicative - that is why no figures are provided. Anyway:
The two figures show voltage and current with "Local RMS", which is more or less the average effective value.
We can see that as the speed increases, we need to apply more and more voltage until the limit is reached, at which time the current will drop a little. Two important conclusions can be drawn from these charts:
• We can never provide 100% power supply voltage because we need to change the current ->we need some time to let it drop.
• At high speed, we cannot provide full power for the motor.
Benefits of higher supply voltage
Maybe some people have realized that in most measurements, I use 32V instead of 24V power supply. Indeed - I upgraded my machine to 32V, which is why I decided to play with my oscilloscope and compare the two options.
Is it worth it? really
Using the previous setting parameters, the waveform shape looks much better, and the current amplitude is about 60% higher than before, which means that there is better stability and higher margin before the motor starts to lose step. On the other hand, I can print at a fairly high acceleration, even up to 1200 mm/s, rather than a higher safety margin! This is not to say how much it means for FDM printers But I am very satisfied with the result.
Summary and suggestions!
Even a few volts difference will improve the operation of our stepper motor driver or let us reach a higher speed. Sometimes higher printing speed will lead to lower print quality, but this is usually not a big problem. At least we can improve the travel speed, which will not only reduce printing time, but also help shrink adjustment.
With all the knowledge we have gained, we can now more confidently choose motors for our machines. So:
Ensure that the rated inductance and resistance of the motor are as low as possible
For drivers like TMC2208 or TMC2130, the motor with rated current of 1.5-1.7A should be the best
For TMC2209, TMC2660 and TMC51X0, the rated current is 2.0 – 2.5A
Select the motor power voltage as high as possible, but carefully check the rating of your drive and motherboard!
Personally, I think that in the next few years, we will see more and more 36V and higher versions of 48V motherboards used for Reprap/commercial 3D printers, so our machines will become better and better, and the speed available will be improved. The only disadvantage is that the heater is usually designed for 24V - but maybe this will also change!
Instruments used:
Silent SDS 1104X-E oscilloscope
HANTEK CC65 current probe
150W Mingwei power supply
CoreXY 3D printer
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