Back-EMF in bipolar stepper motor seems inconsistent with torque graph
In the AM2224-1000 datasheet for the '1000' model the following parameters are given:
-
Nominal voltage per phase: 1.4V
-
Back-EMF amplitude: 3.8V/k step/s
There is also torque/speed graph present for different voltages:
However, it seems that the graph does not correspond to the data from the table. If I want to use the stepper at 4000 steps/s, the back-EMF generated would presumably be:
$V_{\text{back-EMF}} = 3.8 \times 4 = 15.2V$
Whereas
$V_{\text{8x-nominal}} = 1.4 \times 8 = 11.2V$
The graph shows at this point that motor is still generating torque. If the back-EMF really is this high, would it not be the case that the motor never reaches 4k steps/s, because as the back-EMF reached supply voltage, no more current can be pushed into the windings and it would reach max speed and 0 torque condition?
The 4x and 1x nominal graphs also seem to exceed what is possible, which leads me to believe I have misinterpreted the datasheet or that I am missing something fundamental.
2 answers
This isn't really an answer since I can't reconcile the discrepancy either. Things to try:
- Measure the open circuit output when the motor is turned externally. That may be hard to set up, but should give you a definitive answer as to what the back EMF looks like over a whole cycle.
- Maybe there is a discrepancy between peak voltage and average or RMS?
- Try driving an unloaded motor with a fixed voltage and ramp up the frequency until it fails. However, you have to consider the effect of the winding inductance too. That will decrease the current at higher frequencies at the same drive voltage, which will be in addition to the effective voltage loss due to back EMF.
- Ask the manufacturer. It's perfectly valid to bother them with questions when their datasheet seems to be inconsistent with itself.
When you do find out what is going on, please post an update to your question.
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I got an answer regarding the back-EMF number in the datasheet. The back-EMF voltage waveform is a sine and the listed value is the peak value. What this means in practice is that energy can still be pushed into the winding before and after the peak, once the back-EMF is below the driving voltage. The graphs are derived empirically.
To be clear, I have not received the motor and verified this, but it seems to make sense.
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