Datasheet

QSH5718-10000-AT Hardware Manual • Hardware Version V1.00 | Document Revision V1.00 • 22.02.2019

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For microstepping: If the waveform is still basically sinusoidal, the motor driver supply voltage is

suﬃcient.

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For Fullstepping: If the motor current still reaches a constant plateau, the driver voltage is suﬃcient.

If you determine, that the voltage is not suﬃcient, you could either increase the voltage or reduce the

current (and thus torque).

6.4 Back EMF (BEMF)

Within SI units, the numeric value of the BEMF constant has the same numeric value as the numeric

value of the torque constant. For example, a motor with a torque constant of 1 Nm/A would have a

BEMF constant of 1V/rad/s. Turning such a motor with 1 rps (1 rps = 1 revolution per second = 6.28 rad/s)

generates a BEMF voltage of 6.28V.

The Back EMF constant can be calculated as:

BEMF

MotorHoldingT orque

2∗I

N OM

The voltage is valid as RMS voltage per coil, thus the nominal current INOM is multiplied by 2 in this

formula, since the nominal current assumes a full step position, with two coils switched on. The torque is

in unit [Nm] where 1Nm = 100cNm = 1000mNm.

One can easily measure the BEMF constant of a two phase stepper motor with a (digital) scope. One just

has to measure the voltage of one coil (one phase) when turning the axis of the motor manually. With

this, one gets a voltage (amplitude) and a frequency of a periodic voltage signal (sine wave). The full step

frequency is 4 times the frequency the measured sine wave.

6.5 Choosing the Commutation Scheme

While the motor performance curves are depicted for fullstepping and halfstepping, most modern drivers

provide a microstepping scheme. Microstepping uses a discrete sine and a cosine wave to drive both coils

of the motor, and gives a very smooth motor behavior as well as an increased position resolution. The

amplitude of the waves is 1.41 times the nominal motor current, while the RMS values equal the nominal

motor current. The stepper motor does not make loud steps any more – it turns smoothly! Therefore,

16 microsteps or more are recommended for a smooth operation and the avoidance of resonances. To

operate the motor at fullstepping, some considerations should be taken into account.

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