Mitsubishi Electric FR-A720-00030-NA Instruction Manual page 300

(2) Soft-PWM control (Pr. 240)
 Soft-PWM control is a control method that changes the motor noise from a metallic tone into an unoffending
complex tone.
(3) PWM carrier frequency automatic reduction function (Pr. 260)
The carrier frequency is automatically lowered to as low as 2kHz in the following conditions. Set Pr. 260 to enable/
disable the automatic reduction of the carrier frequency under the LD rating.
Multiple rating
PWM carrier frequency automatic reduction
(Pr. 570)
0 SLD Valid
Pr. 260 = "0" : invalid
1 LD
Pr. 260 = "1" (initial setting) : valid
2 (initial value) ND Valid for
FR-A720-02150 or lower,
3 HD
FR-A740-01100 or lower
 Acoustic noise from the motor increases while the carrier frequency is automatically lowered. This is not a fault.
 When Pr. 260 is set to "0" under the LD rating, the carrier frequency becomes constant (Pr. 72 setting)
independently of the load, making the motor sound uniform.
Note that continuous operation should be performed at less than 85% of the inverter rating.
CAUTION
 For the FR-A720-02880 (FR-A740-01440) or more inverter, reduce the rated inverter output current to use when the carrier
frequency is 3kHz or more. (Refer to page 450)
 Decreasing the PWM carrier frequency effect on EMI measures and on leakage current reduction, but increases motor noise.
 When PWM carrier frequency is set to 1kHz or less (
operation due to increase in harmonic currents depending on the motor, resulting in insufficient torque. In such case, set fast
response current limit operation invalid using
Parameters referred to 
Pr. 156 Stall prevention operation selection
Pr. 570 Multiple rating setting
Invalid for
FR-A720-02880 or higher,
FR-A740-01440 or higher
1), fast response current limit may function prior to stall prevention
Pr. 72
Pr. 156 Stall prevention operation selection
Refer to page 152
Refer to page 157
Motor noise, EMI measures
Conditions for Automatic Reduction
Continuous operation with 85% of the rated
inverter current (the rated currents in
parentheses on page 450) or higher.
Continuous operation with 85% of the rated
inverter current (rated currents in parenthesis on
page 450) or higher while Pr. 260 = "1 (initial
setting) ".
Overloaded operation at a low speed (output
frequency lower than 3Hz)
.
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