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Output Noise Filter: 

Inverter (AC drive) Rated Current should be larger than or equal to Motor Rated Current x 1.1 (at continuous operation) 

The PWM controller converts the AC power source to a fixed DC power by a full-wave rectifier. The DC voltage is smoothed by a filter network and applied to a pulse width modulated (PWM) inverter using high power transistors. The speed reference command is directed to the microprocessor which simultaneously optimizes the carrier (chopping) frequency and inverter output frequency to maintain a proper volts/Hz ratio and high efficiency throughout the normal speed range. See Block Diagram below.
 

To compensate for this extra motor heat, consider derating a 1.15 service factor motor to a 1.0 service factor when operating on VDF power. 

But fan-cooled motors with centrifugal loads present less of a problem of motor heating.  For example, pumps and fans generally do not require full torque at reduced speeds. Thus in these applications, there is less thermal stress on the motors. Figure 4 below shows how temperature-rise values relate to torque requirements at various speeds. On the basis of this figure, the load reduction for operating a centrifugal load does not cause the motor to exceed thermal limits defined by the insulation system. 

 But two situations affect motor heating and cooling. First, variable-frequency power affects fan-cooled motors during speed reduction. Second, maintaining full torque at reduced speeds also affects the motors.

 One method of reducing motor noise is full-spectrum switching. Drive manufacturers accomplish full-spectrum switching by an algorithm within the drive regulator. The algorithm optimizes motor performance by evaluating motor characteristics, including motor current, voltage and the desired output frequency. The resulting frequency band, though audible to humans, produces a family of tones across a wide frequency band. Thus the perceived motor noise is considerably less than it would be with a single carrier frequency.

 In some installations, placing a variable speed drive (VSD, VFD) on a motor increases the motor's acoustical noise level. 

 AC drives (variable speed drives, VFDs) on the market consist of three main power sections, including an AC-to-DC converter, a DC filter and a DC-to-AC inverter. 

 Improving the electrical system power factor is another benefit of applying variable frequency drives (VFDs, AC drives).

Figure 3 shows a typical power factor triangle.