Delta Tau GEO PMAC User Manual Page 92

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Geo PMAC Drive User Manual
82 Calculating Minimum PWM Frequency
CALCULATING MINIMUM PWM FREQUENCY
The minimum PWM frequency requirement for a system is based on the time constant of the motor.
Calculate the minimum PWM frequency to determine if the amplifier will properly close the current loop.
Systems with very low time constants need the addition of chokes or in-line inductive loads to allow the
PMAC to properly close the current loop of the system. In general, the lower the time constant of the
system, the higher the PWM frequency must be.
Calculate the motor time constant by dividing the motor inductance by the resistance of the phases.
motor
R
motor
L
motor
=
τ
The relationship used to determine the minimum PWM frequency is based on the following equation:
πτ
π
τ
2
20
)(
)(2
20
>
×
>
HzPWM
HzPWM
Example:
L
moto r
= 5.80 mH
R
motor
= 11.50
secm504.0
50.11
mH80.5
motor
==
τ
Therefore,
=
×
=
sec)m504.0(2
20
)Hz(PWM
π
6,316 Hz
Based on this calculation, set the PWM frequency to at least 6.32kHz.
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