AN3327 Datasheet, PDF (8/22 Pages) STMicroelectronics – L9942 back EMF stall detection algorithm

Universal motor concepts
AN3327
Where:
• N is the number of coil turns
• P is the number of poles
• θ is the flux
Note again that current (I) and torque (T) are directly proportional to each other. There are
other factors that affect current like the temperature and resistivity of copper. Copper
resistivity improves the torque potential (at cold) or limit the torque potential (at hot) but it
does not change the torque-to-current relationship.
2.1
Universal motor concepts applied to stepper motors
A stepper motor is typically a fixed current system. That is, the controller feeds a fixed set of
currents at a rotational velocity that is directly reflected by the rotor. If a fixed current into a
motor produce a fixed torque how can a stepper motor have a fixed current and rotate at a
fixed speed for a wide range of loads or torques? The answer is in the phasing,
automatically, of the back EMF with respect to the drive current.
The current generates the torque based on the aforementioned equations. What direction
that torque are applied depend on the external load. A lightly loaded stepper motor has a
small portion of the torque actually driving the load. A good portion of the motor torque is
used trying to slow the motor down. To never go above the commanded rotational speed the
current is first driving the motor to go faster and then braking it to go slower. The overall
torque coming out the output shaft is zero for an unloaded motor.
In Figure 3 the back EMF is representing Rotor position. Rotor position is also an indication
of where the rotor magnetic field is in relation to the stator magnetic field. The rotor magnetic
field is fixed to the rotor and rotates with it. The stator field is related to the current in the
stator. A "positive" current creates a "positive" field and vice versa.
Figure 3. Phase shift in an unloaded motor
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As the motor is loaded the Back EMF shifts to convert more of the torque to forward motion.
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