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
VFD Efficiency (%)
Speed 100% Torque 100% (Load 100%)
Speed 75% Torque 56% (Load 42%)
Speed 50% Torque 25% (Load 13%)
Speed 25% Torque 6% (Load 2%)
10,0 VFD Nominal Output (kW)
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.
Line-current and switching-frequency (nearly proportional to motor power). Line-current (nearly proportional to motor power).