Siemens 6RA70 Faq

A&D LD CS 31 October 2007
FAQ for 6RA70 DC MASTER and SIMOREG CM
Question:
How can I manually record the field characteristic?
Answer:
The automatic optimization run P051 = 27 supports recording of the field characteristic. If this
routine cannot be used, the field characteristic needs to be recorded manually as described
below.
A field characteristic is required for motors which will also operate in the field-weakening
range. It must not be recorded for motors which will operate up to rated speed only.
Relationships:
The magnetic flux Φ is determined by the excitation current of the motor. When the excitation
current increases (up to its rated value), however, the flux Φ does not increase linearly
because the stator core becomes saturated. The relationship between the excitation current
and the flux is not linear and is represented by the field characteristic (magnetization
characteristic).
The induced motor voltage is the EMF of the motor.
EMF[V] = k * Φ * n[RPM]; the EMF reaches its rated value when the motor is operating at
rated speed. To prevent the motor EMF from reaching excessive values at speeds above the
motor rated speed, the flux Φ is reduced in this range and the EMF is regulated to a constant
value, i.e. the rated EMF value.
The following relation applies in steady-state motor operation: Ud – Id * Ra = EMF
Ud: Motor armature voltage, Id: Motor armature current, Ra: Armature resistance of motor
The maximum speed adjustment must be made before the field characteristic is recorded.
P741 for an analog tachometer or P143 for a pulse encoder. The maximum speed
adjustment must be made at a speed lower than rated speed.
The current controller and speed controller must then be optimized (Section 7.5 of Operating
Instructions). The field characteristic can then be recorded.
Recording of a motor field characteristic (magnetization characteristic):
To record the field characteristic, the magnetization characteristic is measured. This is done
by setting the field current setpoint , starting at 100.0 % of the rated excitation current of
I
fset
the motor , e.g. in the following increments: 91.0 %, 83.0 %, 76.0 %, 70.0 %,
I
field rated motor
65.0 %, 60.5 %, 56.5 %, 53.0 %, 50.0 %, 47.0 %, 44.0 %, 41.0 %, 38.0 %, 35.0 %, 32.0 %,
29.0 %, 26.0 %, 23.0 %, 20.0 %, 17.0 %, 14.0 %, 11.0 %, 8.0 %.
At a given speed, therefore, a specific actual EMF value is measured at each actual
EMF
act
value of the field current. The actual speed value at the individual measuring points is
n
act
not decisive, but it is important that the speed is maintained at a constant value for each
single measuring point.
The EMF value for each measuring point is displayed in parameter r037. The actual
EMF
act
speed value is displayed in parameter r025.
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