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  • IEC 60034-31:2009

  • 19 –

2/1554/CD

5.12

Frequency Converter

Efficiency

Frequency converters generally have a high level of energy-efficiency. As with motors, their efficiency drops at partial load (see Figure 7).

VFD Efficiency in Partial Load

100%

98%

96%

94%

VFD Efficiency (%)

92%

90%

88%

86%

Speed 100% Torque 100% (Load 100%)

Speed 75% Torque 56% (Load 42%)

Speed 50% Torque 25% (Load 13%)

84%

Speed 25% Torque 6% (Load 2%)

82%

80%

0,1

1,0

10,0 VFD Nominal Output (kW)

100,0

1000,0

Figure 7

Typical efficiency of indirect three-phase voltage source type converters with a passive front-end

Table 2 gives the frequency converter loss components for the most common industrial converter type (low voltage indirect frequency converters of the voltage source type with uncontrolled three-phase diode rectifiers as line side converter) in the output power range from 1 to 100 kW:

Switching losses (output stage) Line-rectifier losses

Forward losses (output stage) Internal control circuit losses (microcontroller, internal power supply, display, keyboard, bus- communication, digital and analogue ins/outs…) Switching losses (line-side converter / active front-end only) Compound losses (line-side converter / active front-end only)

Typical percent of losses for passive front-end converters 30 to 50

20 to 25

15 to 20

5 to 20

-

-

Factors affecting these losses

Motor-current and switching-frequency. Line-current (nearly proportional to motor power). Motor current.

Nearly constant.

Line-current and switching-frequency (nearly proportional to motor power). Line-current (nearly proportional to motor power).

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