my apologies to mamouito for the long posts on his thread.
he seems to have a good set of parts so he should get results .
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in answer the why voltage ratio in key, and voltage increases motor power in steppers :
Except with motors, it seems we expect Ohm’s law applies to everything we do.
For motors, most hobbiests don’t think resistance applies and you can just ignore voltage and how it applies and just use more. More = better.
If you get nothing else from this, you should understand that it is the RATIO of motor voltage to power supply voltage that is important.
The corner point is NOT the goal or even desired, it is the point of failure and should be avoided at all cost. You should re-read about steppers and understand that the thin red line is separation between failure and safe operation. The graph is for motors. Not anything to do with drivers. It is for the motor alone.
With a driver, the current is limited. So it is ‘safe’ to use under the corner point as the runaway failure cannot happen due to the added current limiting circuitry.
Next is motor power. The problem with high speed is the greater and greater effect of the permanent magnets and the detent loss as stepper motor speed increases. So speed is limited by the increasing effect the permanent magnets on the motor. To get higher speed from a motor requires that the motor has more power. You cannot have more power at one point and not at other points.
If you look at the corner speed, you see that 2x voltage moves the point at low speed. Clearly the motor gains more power when you move the voltage to 2x nameplate. Again, ratio of voltage to nameplate.
A motor is comprised of sets of coils. So, we then look at why the exact same motor can be had with 1.5 volt nameplate, 5v, 12 volt, etc. The coil is what is different.
What is needed to be understood is that nameplate is the voltage that can be applied without damage. If you were to connect 12v to a motor rated for 1.5volt, you would damage it. ADD the control chip and things change. Understanding why the LM298 is a proper and very workable stepper driver is misunderstood by many. YES, there is newer technology so there are other choices, but for driving a motor, the fundamentals don’t change because silicon products change. The Atmel328 chip has been superseded by many processors, but we do not tell people to throw them away and get a ‘proper’ micro-controller.
Back to the coil.
If you have a 5v coil and feed it with a 5 volt power supply with a simple H-Bridge transistor, the coil will become fully charged at some point in time. let’s say 10.2ms. Now, let’s use a FET H-bridge, with the lower resistance, it still works, it might (for the purpose of this discussion) allow the coil to become fully charged in 10ms. There is a desire for newer technology but not any requirement.
The 5 volt could be left on the coil forever and no problems.
The following bit is what is not understood when discussing steppers and voltages.
If one changes the power supply voltage, then a new family of speed-torque curves result. As an example, if the power supply voltage is doubled then a new curve is generated; the curve now has twice the torque at any given speed above corner speed. Since power equals torque times speed, the motor now generates twice as much power as well.
Now, we create a chopper circuit like the L298 uses, with the needed resistor feedback to limit the current at the level the motor needs. Yes, other workable stepper drivers chips can be used and they too have resistors for setting the current limit like a proper stepper driver such as the L298 has.
A note about the L298, it is a perfectly workable stepper driver when connected properly as a stepper driver. Newer drivers have more features, microstepping and also use FET’s which offer much less resistance which results in less heating and more power delivered. The application did not change nor did the circuit, just newer technology with added features. It is not the recommended device, but it is perfectly workable.
But, with the pieces in place for a proper stepper driver, valves and feedback, we can use higher voltages. The reason that this is so commonly misunderstood and why it seems some are under the impression that Ohm’s law does not apply to stepper motors when it comes to voltages.
When you feed the coil with twice the motor rating, you can charge the coil in less time. The feedback will tell the driver chip to stop dumping power and start to current limit. That is the sole function of a proper stepper driver. L298 et al.
What happens in the coil ? During low speed, middle or high speed, does not make any difference. The coil becomes fully charged faster and the current reached the maximum point faster. This means each coil spends more time at full power. This is much more important at low speed where the motor delivers more power. At low speed the motor delivers more power where we use it in our applications.
Higher speeds is not important to 99% of applications as steppers are extremely poor choice for high speed applications. Low speed power is the goal of serious CNC machines and why we power our steppers at 70 volts or more and with power supply voltages of 25 times nameplate, If possible.
The correct reply to ‘what voltage’ should be 2-5 times nameplate or more.
They question of ‘why’ is : The motor delivers more power at low speed where we use the steppers and where they are best suited for applications.
The question of heat does come into play. Every motor has a data sheet so one should check the data sheet. The rule of thumb is that if you touch it, and you get burnt, it is over powered. If you cannot lay your hand on it comfortably for over 5 seconds, it is but too warm, reduce the power.
There are a lot of tutorials for steppers and most are full of errors and incorrect information, but in the hobby arena, we see that if you select devices and use them in the middle of their abilities, you can get things to work. So, like most Instructables, most tutorials get things to work and get results so, even though they contain bad or incorrect information, they do get people to get things working so have value to the community.
