Stepper driving sanity check

[Infraviolet]

When buying stepper motors and drivers separately, for use together, is ensuring compatibility as simple as ensuring the current ratings match up, or is there more to it?

Considering only 4-wired bipolar steppers of the typical NEMA sizes (yes, I know NEMA is a face-plate size not a full performance spec, but for any given NEMA size one has a typical range on each performance parameter), and ignoring fancier considerations like closed loop feedback...

Is there anything which needs to be matched up other than current rating when picking a stepper motor and a driver "unit" to go with it.

And then to select a higher voltage (12V, 24V, 48V) DC supply to power the driver unit, one just picks one with a current rating high enough to given sufficient wattage? Again, no other paramter that needs to be matched?

It seems so simple that I feel I have missed something?

By "unit" I refer to the large (usually black with green screw terminals) box shaped modules which can be bought as "microstepping driver units" from all manner of official, semi-official and unpronounce-able chinese-name suppliers for a wide range of prices. I'm considering those, rather than single IC solutions or directly using H bridges.

Stepper's usually have a rated voltage, but this is virtually meaningless in practice? It simply indicates what voltage would have to be applied to make the rated current flow in a long-term DC scenario with inductance ignored?

So one just has to pick an appropriate stepper for one's face-plate-sizing limits, torque requirements, and step angle, then one selects any driver which has, when selected by switches atleast one current setting close to the motor's rated current?

The trouble is in many cases the driver's documentation, particularly from cheaper sources, doesn't distinguish between the total current and the per-phase current. Such "documentation" often lists peak and RMS currents, but doesn't seem to indicate whether this is what it can supply to either phase of the motor, or what it can supply overall. Peak surely means a brief peak in time, like the peak ratings in the absolute maximums section of a chip's datasheet, and RMS as long-term time averaged. Whereas motors from just about all suppliers explictly give their current requirements as per-phase.

And those sort of stepper driver units are designed for a constant current output? So if you have a driver which is slightly underpowered for a stepper you'll use it with, then you'll simply be running the stepper below it's usual torque level, so long as the driver is still providing enough current to overcome inertia and frictional effects. Whereas a driver set for too high a current would risk damaging the stpper by overheating, if in use for sufficiently long? This is the opposite way round from the situation if you were driving a stepper using a dual h-bridge chip and a constant voltage supply, in which case a stepper with a current rating higher than the driver's would be the situation which would lead to overheating (of the IC instead of the motor).

And as it is giving a constant current output at, given typical bipolar steppers' rated voltages, a much lower voltage than the 12V or 24V one is probably powering the driver unit from... The 12V or 24V supply will only need to supply a fraction of the current that the stepper is using? A microstepping driver unit giving 4 amps output to a "3.6 volt" stepper, will be happy with a 12V or 24V supply capable of >14.4Watts, rather than needing a supply able to match the stepper's current? Or do microstepping drivers often have such inefficient stuff inside that they'd actually need a hugely powerful 12V or 24V supply with a current rating high enough to match their peak?

Thanks

[thm_w]

For the most part yes, unless you get into oddball steppers or need very specific performance characteristics the universal drivers should be fine.

And those sort of stepper driver units are designed for a constant current output? So if you have a driver which is slightly underpowered for a stepper you'll use it with, then you'll simply be running the stepper below it's usual torque level, so long as the driver is still providing enough current to overcome inertia and frictional effects. Whereas a driver set for too high a current would risk damaging the stpper by overheating, if in use for sufficiently long? This is the opposite way round from the situation if you were driving a stepper using a dual h-bridge chip and a constant voltage supply, in which case a stepper with a current rating higher than the driver's would be the situation which would lead to overheating (of the IC instead of the motor).

Yes overheating the motor is the concern, so you can set the driver to full step, and use a thermocouple or IR gun to measure the case temperature and see if its reasonable, adjust if not.

And as it is giving a constant current output at, given typical bipolar steppers' rated voltages, a much lower voltage than the 12V or 24V one is probably powering the driver unit from... The 12V or 24V supply will only need to supply a fraction of the current that the stepper is using? A microstepping driver unit giving 4 amps output to a "3.6 volt" stepper, will be happy with a 12V or 24V supply capable of >14.4Watts, rather than needing a supply able to match the stepper's current? Or do microstepping drivers often have such inefficient stuff inside that they'd actually need a hugely powerful 12V or 24V supply with a current rating high enough to match their peak?

It depends on the driver, ancient ones are going to be insufficient transistor based units. Modern drivers will be FET based, have low on resistance, and minimal losses.

https://electronics.stackexchange.com/questions/318283/what-is-the-efficiency-of-a-stepper-motor-driver
https://www.omc-stepperonline.com/support/how-to-choose-a-power-supply-for-my-stepper-motor
https://engineering.stackexchange.com/questions/49839/how-to-select-the-right-power-supply-for-a-stepper-motor

[mawyatt]

For stepper driver/controllers look at Trinamic, they have some really good chips and boards that work well.

Best,

[Infraviolet]

The stepperonline link gives a supply power example:
P=n*I*V*1.2

But then discusses V as being the driving voltage. For a typical bipolar stepper this would be much lower than their example though? You'd drive the stepper driver unit at say 36V, but the stepper itself, to operate at an appropriate current given its resistance, is only being driven at the equivalent (time averaged atleast) of a few volts? So in reality a 3 motor, 3 amps per motor system would need a lot less than 400W, unless the steppers were high resistance ones taking high voltages directly at their coils?

[Doctorandus_P]

If you want stepper motors of size Nema 23 or bigger, then I highly recommend to buy closed loop stepper motors. These run quieter, stay cooler, and (in general) deliver more torque. With the bigger stepper motors, 3-phase motors are also a bit more common, and you need a matching driver for those.

[thm_w]

The stepperonline link gives a supply power example:
P=n*I*V*1.2

But then discusses V as being the driving voltage. For a typical bipolar stepper this would be much lower than their example though? You'd drive the stepper driver unit at say 36V, but the stepper itself, to operate at an appropriate current given its resistance, is only being driven at the equivalent (time averaged atleast) of a few volts? So in reality a 3 motor, 3 amps per motor system would need a lot less than 400W, unless the steppers were high resistance ones taking high voltages directly at their coils?

Yeah in reality the average power consumption will be less. Maybe its a worst case calculation.

Here is an excerpt from the DM860I datasheet:

Power Supply
When using regulated power supplies, it is important to have large current output rating to avoid problems like current clamp, for example using 4A supply for 3A motor-drive operation. On the other hand, if unregulated supply is used, one may use a power supply of lower current rating than that of motor (typically 50%～ 70% of motor current). The reason is that the drive draws current from the power supply capacitor of the unregulated supply only during the ON duration of the PWM cycle, but not during the OFF duration. Therefore, the average current withdrawn from power supply is considerably less than motor current. For example, two 3A motors can be well supplied by one power supply of 4A rating

If I set the DM860I stepper driver to:
- 2A = 2.3A meas = 7W PSU draw
- 4.3A = 5A meas = 23W PSU draw
- 6A = 7A meas = 60W PSU draw

Power draw is near constant as supply voltage is varied (18-60V).
Motor used: 2.32V, 2.8A, 0.83 ohm, 2.2mH
Current measured on one phase with UT210e, the measured current matches up with the peak current rating on the spec table.

So roughly Iset avg² * phase resistance * 2 is the power draw seen here.