QSH4218 Manual (V1.06/2011-APR-12)

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Copyright © 2011, TRINAMIC Motion Control GmbH & Co. KG

6.3.1 Determining if the given driver voltage is sufficient

Try to brake the motor and listen to it at different velocities. Does the sound of the motor get

raucous or harsh when exceeding some velocity? Then the motor gets into a resonance area. The

reason is that the motor back-EMF voltage reaches the supply voltage. Thus, the driver cannot bring

the full current into the motor any more. This is typically a sign, that the motor velocity should not

be further increased, because resonances and reduced current affect motor torque.

Measure the motor coil current at maximum desired velocity

For microstepping:

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

sufficient.

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:

The voltage is valid as RMS voltage per coil, thus the nominal current I

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.

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A

I

Nm

ngTorque

MotorHoldi

s

rad

V

U

NOM

BEMF

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