The Operating Principle Of The Vfd; The Problem With Harmonics; Figure 15 - Power Absorbed By The Compressor Depending On The Load - Daikin EWYD-BZ Series Installation, Maintenance & Operation Manual

Figure 15 - Power absorbed by the compressor depending on the load

The operating principle of the VFD

The VFD (also known as an "inverter") is an electronic power device designed to vary the speed of rotation of induction
motors.
The motors revolve at a practically fixed rpm speed which depends only on the frequency of the power supply (f) and on
the number of poles (p), as per the following formula:

f 60

rpm
p
(In fact, for the motor to produce torque, the rotation speed, known as the speed of synchronism, must be slightly less
than that calculated above.)
To vary the speed of rotation of an induction motor, the supply frequency of the same therefore needs to vary.
The VFD does this, starting with a fixed grid frequency (50 Hz for European power grid, 60 Hz for the US) operating in
three steps:
step one involves a rectifier to transform the alternating current into direct current, which is typically achieved using a
diode rectifier bridge (leading solutions use bridges with SCR)
step two involves charging the capacitors (direct current bus, also known as a DC-Link)
step three involves the reconstruction of the alternating current (a genuine inverter) by means of a transistor bridge
(normally IGBT) with variable voltage and frequency values, set by the control system. The voltage is in fact the result of
a high-frequency PWM modulation (in the range of a few kHz) from which the fundamental variable frequency
component is taken (typically 0-100 Hz).

The problem with harmonics

The rectifier bridge of a VFD requires current from the grid that is not purely sinusoidal. Indeed, due to the presence of
diodes, which are non-linear components, the current absorbed by a rectifier bridge has a higher frequency than the
frequency of the power grid. Such components are known as harmonics: in the case of a power supply at 50 Hz, the
component at 50 Hz is defined as the fundamental harmonic, while the second harmonic is the component at 100 Hz, the
third harmonic is the component at 150 Hz, and so on. (In the case of a power supply at 60 Hz, the fundamental
component is that at 60 Hz, the second that at 120 Hz, the third is that at 180 Hz, and so on.)
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