1084 HEIDENHAIN Technical Manual iTNC 530 HSCI
Switching the PWM frequency depending on the speed
This function is used with high-speed spindle drives. This switchover is only
possible for double-speed control loops.
In MP2186.x and MP2188.x, a speed-dependent hysteresis for switching the
PWM frequency is specified. It only takes effect if the value in MP2188 is less
than the value in MP2186.
This function is connected with MP2182.x and MP2180.x. Only if MP2182.x
has the value 2 and MP2180.x is £ 5 kHz, does the switching of the PWM
frequency take effect. Please note that the adjustment of the current
controller (MP2420, MP2430) is based on the lower PWM frequency £ 5 kHz.
Adaptation of the current-controller parameters and consideration of the
power-module derating are performed automatically.
Using this function provides several benefits:
At lower speeds and therefore a lower PWM frequency, the power module
provides a comparatively high current. This results in a relatively high
maximum motor torque.
On the one hand, losses due to harmonics in the motor become more
important as the speed increases, and on the other hand, the relationship
between the electrical frequency and the PWM frequency worsens. These
two disadvantages can be counteracted by increasing the PWM frequency.
The resulting reduction of the current normally is insignificant, since in part
due to motor characteristics, very high motor currents are mostly no longer
possible or needed at higher speeds.
When determining the optimum switching speed for the PWM frequency, you
should consider that the maximum motor current decreases as the speed
rises, due to the finite DC-link voltage.
The current for the drive depends on the maximum permissible motor current
and power-module current. The smaller of the two values limits the current for
the drive. The value of the maximum power-module current is reduced by
approx. 30 % because of the derating when doubling the PWM frequency.
During derating the rated current I-N and the maximum current I-MAX are
reduced by the same factor.
The speed-dependent switching of the PWM frequency is only permitted
with power modules from HEIDENHAIN.
Speed-dependent switching of the PWM frequency with non-HEIDENHAIN
power modules can lead to malfunctions, and possibly to damage of the
power modules. Therefore, only use this function with power modules
from HEIDENHAIN.
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