Stepper motor question. Low voltage, high current vs low current, high voltage

[Gibson486]

I have a question about stepper motors. Some manufacturers give you the option to use a high voltage, lower current stepper. This is at the expense of a higher inductor.  Is it best to keep everything at a low voltage so we make the inductor smaller? The way I see it, the bigger the inductor, the more time it will take for energy to build up and drain. This means the higher voltage cannot run as fast as a lower voltage one. Is this correct? is there any other reason to not use higher voltage steppers?

[Benta]

Makes no difference at all. The lower inductance just means you have a lower driving voltage and need to reach a higher current. Same time constant. Even the lower resistance cancels out due to the higher current demand. The relationship between voltage, current and inductance is linear, only copper fill factor can make a small difference.

[langwadt]

Makes no difference at all. The lower inductance just means you have a lower driving voltage and need to reach a higher current. Same time constant. Even the lower resistance cancels out due to the higher current demand. The relationship between voltage, current and inductance is linear, only copper fill factor can make a small difference.

inductance is turns squared, field is turn*current i.e. a stepper that can be wired in series or parallel, parallel will have
1/4 the inductance and resistance and only need 2x the current

[Gibson486]

So, in essence, that means the voltage rating on a stepper is kind of useless. It is only applicable for L/R Drivers or any other drivers you cannot adjust the current.

[T3sl4co1l]

The voltage rating is continuous, only.

Typically you'll have a quite high supply voltage, to achieve reasonable torque at high RPM.  A stepper motor driver/controller acts as a crude VFD, delivering only the voltage necessary to change current from positive to negative (at whatever the current limit is set to).  Because the windings have considerable inductance, this voltage is approximately proportional to frequency.

If you don't need fast operation, you can run from a supply of the rated voltage.  Torque drops off at a frequency around DCR / (2*pi*L), or (freq * 60 / (steps per rev)) RPM.  Otherwise, to maintain torque, voltage must rise ~proportionally above this point.

Tim

[langwadt]

So, in essence, that means the voltage rating on a stepper is kind of useless. It is only applicable for L/R Drivers or any other drivers you cannot adjust the current.

I won't say it is useless, the higher voltage steppers will usually can't be run as fast

[Doctorandus_P]

Benta is correct in saying that the "copper fill factor"has influence, but that is something you have no controll of, unless you wind your own stepper motors.

The Voltage and current ratings on stepper motors are for DC, and For low speed applications there is little difference.
If you have for example an Nema 23 motor with a 5V 3A DC rating, it is quite common to use these with a stepper motor driver and a 24V or even 48V power supply.

The overal goal of the stepper motor driver is to generate 2 Sine wave currents with a 90 degree phase difference (for a 2-phase motor). For this it puts the supply voltage on a stepper motor winding untill the target current is reached, and from there it modulates the current and uses the motor inductance as part of a SMPS current regulated circuit.

At higher RPM of a stepper motor it becomes difficult to push enough current fast enough through the stepper motor winding.
Increasing the power supply voltage helps to achieve higher RPM, (or to maintain torque at high RPM).
With lower self inductance of the windings, it also becomes easier to push more current through a stepper motor winding, which also results in higher achievable RPM before the torque drops to a low value, or in extreme cases the motor simply stalls.

Fetch some random datasheets of stepper motors. Most have a graph which plots available torque versus RPM and they often plot curves for different supply voltages in the same graph.
Some of the motors have each winding separated into 2 physical coils, which can either be put in series, or parallel.

When the windings are parallel, each winding has the same (maximum) current, so the total current will be doubled, but the indutance is only 1/4 compared to windings in series.
If you find a datasheet of such a motor it will have the above mentioned graphs for both configurations.

Putting the windings in series has some advantages for low speed applications. Losses in the stepper motor drivers increase with motor current, so this configuration will have a bit better efficiency with low RPM.

A problem with some stepper motor drivers is that they need a minimum motor inductance to work properly.

In the last few years the "closed loop" stepper motors are becoming more popular. (Nema23 and bigger).
These have a quadrature encoder which measures motor position, and normally regulate the motor current to a (much) lower value than the maximum. This results in higher efficiency and in much quiter and smoother operation of the motor.

I've collected some links to open source projects which make use of this principle:
https://hackaday.com/2016/06/01/mechaduino-closed-loop-stepper-servos-for-everyone/
https://hackaday.io/project/20980-ananasstepper-20
https://www.crowdsupply.com/citrus-cnc/tarocco
https://trmm.net/ServoStep#Testing
https://github.com/neuroprod/ClosedLoopDriver