Stepper torque

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Stepper motor datasheets often contain information about holding torque, and sometimes even graphs of torque at various rotational speeds. However, the manufacturers usually perform these measurements at 24V or higher operating voltages with motor drivers tuned to the particular motor. To obtain realistic values for a RepRap setup, we performed measurements at 12V with a Pololu A4988 driver board.

So far, only motors with a NEMA 17 mounting have been measured on Willy's test jig -- NEMA 17 Stepper motors. This is intended to help users, and not to be relied on as a motor test.

The test setup:


Data about the motors tested. Sources for those motors can be found by searching for the model number ("Typ"):


Glossary: Anbieter = Manufacturer, Typ = Model, Anbieter-Angaben = Manufacturer provided data, Messungen = measurements, Motorlänge = motor body length, Widerstand = resistance, Erwärmung = heat production, warm = warm, lauwarm = lukewarm, heiß = hot

I_mittel is the average current as measured by a laboratory power supply. V_ref is the reference voltage set at the potentiometer on the motor driver board. The heat production data is subjectively measured by touch after the test was performed.

The Pololu motors have 6 connecting wires, the white and yellow ones were left unconnected. The manufacturer data for voltage, resistance and inductance appear to refer to each half-coil (from one side to center tap). They must be doubled when comparing to the other motors.

Results of torque measurements:

M vs n.PNG

The results match the expectations fairly well: The lower the inductance and the higher the rated current, the higher the torque as speed increases.

With the exception of the Pololu-supplied motors, all measured torque values are significantly lower than the holding torque promised by the manufacturer. While the holding torque and the dynamic torque are not directly comparable, the major limiting factor appears to be the Pololu motor driver. At V_ref = 0.7V the theoretical current through each coild is 1.75A, which is close to the driver's limits.

However -- even with a higher-current driver -- torques above 30Ncm could not be sustained for long periods of time as the motor itself would likely overheat.

--Willy (talk) 03:57, 24 September 2013 (PDT)

Translation by Kliment

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