Understanding stepper motor current draw

Question

I have the following motor:

Manufacturer Part Number: 23HE30-2804S
Number Of Phase: 2
Step Angle: 1.8 deg
Holding Torque: 1.85Nm (261.98oz.in)
Rated Current/phase: 2.8 A
Phase Resistance: 1.13 ohms± 10%
Inductance: 5.4 mH ± 20%(1KHz)

Driven by a 24V, 6A PSU

(Side note: Which doesn't make much sense to me 24V/1.13ohms = 21.23A?)

When a stepper is engaged in a holding state under no load - Both phases are passing current, which would mean a load of 5.6A being drawn?

My PSU however only shows 1A being drawn, why?

In this question "Understanding Stepper Power" a user states that for a motor with a phase current of 5.8A

If you are using say a 24 V power supply, then the average current flow will be 1.6 A ...the peak current flow (which comes from the output capacitor) will be 11 A peak

How did they arrive at 1.6 and 11A?

I have seen people calculate general load by multiplying rated phase current by 1.4

The rated current is the maximum current for one winding while the other winding carries no current. If both windings are active the sum of the squares of the two currents should be equal or less to the square of the rated current (quadrature drive).

Thus with full steps a 5.8A setting will send ~4A to both windings.

Which again conflicts with the "Understanding Stepper Power"

How can I confidently calculate and verify my stepper motor draw current?

Answer 1

You have to control the current with a stepper. A stepper motor is normally not run like other brushless designs; it "steps" from one rotor position to the next, starting and stopping at each step. This means that for each step, you must supply current for two components of torque; the torque of your load plus the torque to accelerate and decelerate the rotor's inertia. The torque is proportional to the current, but the stepper's windings are inductive, so it is common to use a higher voltage to get the current flowing in the inductors. Once the maximum desired current is reached, the voltage is pulse-width modulated to keep the current in the desired range.

The current for an unloaded motor can be zero, but if you need more holding torque, you will have to supply some current. The holding torque is the amount of external torque it would take to force the shaft to the next step, overcoming the motor's magnetic field. This torque is also proportional to the applied current. In the data sheet, holding torque is measured at the rated current. In your case, since you are applying only one ampere, your holding torque will be reduced proportionally.

The rated current is the maximum continuous current that keeps the windings from getting too hot. Your motor specifies an 80 degree Celsius rise at this current.

The torque and speed curves can be a little misleading, because they are often specified under optimum conditions. If run with feedback, a stepper motor can be switched in a manner so that the rotor inertia is maintained, which will give speed/load ratings that are difficult to achieve when operated in a normal stepping mode.