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Introduction
1.1.2
High Power Factor and Nearly Perfect Sinusoidal Input Currents
1
Power factor is a measure of the fraction of current that produces real power to the load. Typically, power factor is
given as a percentage. A high power factor VFD (e.g., 95%) makes much better use of its input line current demand
in producing real power to the motor than a VFD operating at a low power factor (e.g., 30%). VFDs having a low
operating power factor often generate square-wave-shaped line currents. This can lead to harmonics and other
associated resonance problems.
The Perfect Harmony series draws nearly perfect sinusoidal input currents having a power factor that exceeds 95%
throughout the entire speed range without the use of external power factor correction capacitors. This eliminates
utility penalties for power factor and demand charges, and improves voltage regulation. In addition, feeders, breakers,
and transformers are not overloaded with reactive power. Low speed applications specifically benefit from the
Perfect Harmony series because a high and stable power factor is maintained throughout the entire speed range using
standard induction motors. Figure 1-2 compares graphs of power factor versus percent speed for the Perfect Harmony
series and a typical phase-controlled SCR drive.
100
90
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70
60
50
40
30
20
10
Figure 1-2: Comparison of Perfect Harmony and a Typical Phase-Controlled SCR Drive
1.1.3
Nearly Perfect Sinusoidal Output Voltages
The design of the Perfect Harmony series of variable frequency drives inherently provides a sinusoidal output without
the use of external output filters. This means that the drive provides a low-distortion output voltage waveform that
generates no appreciable audible motor noise. In addition, there is no need to derate motors (the drive can be applied
to new or existing 1.0 service factor motors). In fact, Perfect Harmony drives eliminate harmful VFD-induced
harmonics that cause motor heating. Similarly, VFD-induced torque pulsations are eliminated (even at low speeds),
thereby reducing the stress on mechanical equipment. Common mode voltage stress and dV/dt stress are also
minimized. A typical graph of the output current from a Perfect Harmony drive is illustrated in Figure 1-3.
Figure 1-3: Nearly Sinusoidal Wave Form of the Output Current from a Perfect Harmony Drive
1-2
Perfect Harmony
Series Drive
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33
40
47
53
s s s s
Perfect Harmony GENIII/e User's Manual
60
67
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80
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100
Phase - controlled
SCR Drive
Percent Speed
A1A19000405A: Version 1.3
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