Beckhoff EL7037 User Manual download pdf (Page 23)

Product overview

Represented by magnitude and angle:

I(φ

) = I

ABS

· e

jφe

It follows that a rotation of the electrical angle φe is equivalent to four full steps. (A stepper motor with 200

full steps therefore has 50 pole pairs).

The shaft aligns itself if a constant current is set with no load at the motor shaft. Within a pole pairs the shaft

points in the direction of the active stator field.

If an external load is applied to the motor shaft, the shaft is turned out of the field direction, resulting in a load

angle (also referred to as angular displacement) (relative to an electric rotation of the angle φe). The load

angle depends on the design of the stepper motor itself, the motor current and the torque acting on the shaft.

The relationship is non-linear!

If the load angle exceeds a motor-dependent maximum value (i.e. if the maximum machine torque under

these boundary conditions is exceeded), the load torque can no longer be maintained by the motor. If the

shaft is turned further out of the rotary field, it "tips", resulting in one or more step losses. The "tip angle" may

vary between motor types. Often, it lies between around 45° and 65°.

Fig.12: Behaviour of the rotor under load

The load angle is of interest for the user, because it allows conclusions about the load on the shaft. It is

measured by evaluating the induced countervoltage* and can be used to optimise the drive system.

2.3.3 Field-oriented control

In the Extended Operation Modes the stepper motor is operated like a servomotor, based on the principle of

field-oriented control.

Function

The operating behaviour of the motor corresponds to that of a traditional DC motor, with commutation via a

mechanical commutator. With a constant exciter field, the torque of the DC machine is directly proportional to

the stator current and can be directly influenced by it. The exciter field is generated, depending on the

machine type, by permanent magnets or, with a separately excited DC machine, for example, via a separate

excitation winding.

Fig.13: Coordinate transformation of field-oriented control

For servomotors and also hybrid stepper motors, initially there is no direct link between the phase currents

and the torque. Field and torque are decoupled mathematically via Park's transformation. Two current

components, "d" for "direct" in field direction and "q" for "quadrature" in torque-forming direction, are

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Beckhoff EL7037 User Manual Page 23