Variable Frequency Drives Harmonics Overview Siemens Building Technologies

Harmonics Overview What causes harmonics ? Non-linear load devices create harmonics when they convert ac to dc, dc to ac, and ac to ac. This can produce high levels of harmonic voltage and current distortion. Siemens Building Technologies

Harmonics Overview • Harmonics operate at a frequency that is a multiple of the fundamental • Harmonics operate at a magnitude that is inversely proportional to the magnitude of the fundamental • In other words. . . the higher the harmonic order, the less effect it has on the waveform. • In most cases, harmonic analysis is limited to the 13 th harmonic. Siemens Building Technologies

Harmonics Overview • Frequency is defined as the number of times a given waveform repeats itself every second. • Example: the typical household line current impresses 115 volts AC, 60 times per second. Repeated 60 times every second 0 166 msec 1 cycle Siemens Building Technologies

Harmonics Overview • Harmonic currents operate at the same time as the fundamental, but at faster rates • Harmonic currents are additive, producing a “distorted” sine wave. Fundamental = 60 Hz 5 th Harmonic = 300 Hz Siemens Building Technologies

Harmonics Overview • Harmonic Currents are currents that flow at the same time as a fundamental current, 60 Hz frequency, but at multiples of the fundamental. • For a typical 6 pulse inverter, these multiples are: – Fundamental = 60 Hz – 5 th Harmonic = 60 x 5 = 300 Hz – 7 th Harmonic = 60 x 7 = 420 Hz – 11 th Harmonic = 60 x 11 = 660 Hz – 13 th Harmonic = 60 x 13 = 780 Hz – 17 th Harmonic = 60 x 17 = 1, 020 Hz Siemens Building Technologies

Harmonics Overview • A harmonic current operates in “harmony” with the fundamental but faster; i. e. the 5 th harmonic of the 60 Hz fundamental repeats itself 300 (60 x 5) times per second. Repeated 300 times every second 0 30 msec 1 cycle Siemens Building Technologies

Harmonics Overview Harmonics have several harmful side effects: Can interfere with electronic equipment and people • • – through radiated (RFI) interference through the air – emitted (EMI) interference over power and control wires. Can cause an increase in supplied power (KVA) over what is really needed (k. W or HP) – Entire AC supply line must be sized accordingly – End user transformers, switchgear, wires, contactors, and fuses have to be bigger. – The power company generator • Can cause Poor Power Siemens Building Technologies

Harmonics Overview Poor Power. . . Reduces the Load Carrying Capacity of Your Electrical System! • Transformers & wiring are sized by current capacity. If additional current is required due to low power factor, capacity for work producing current is displaced. Everything in your building is oversized to make up for power factor. Siemens Building Technologies

Harmonics – DC Capacitance Link • A DC capacitance link provides voltage to the inverter between the peaks of the mains voltage waveform. Current only flows from the mains into the DC link when the mains voltage exceeds that of the capacitor. Current only flows when the mains voltage is near its peak, as shown. • The larger the DC link capacitor, the shorter the period of current flow, and the higher the peak current. Single or V + Three Phase Power C C Current Rectifier Siemens Building Technologies - D C Link Inverter Building Technologies

Harmonic - Current Wave Form Ideal current wave but not attainable with a 6 -pulse drive What the current wave form looks like with a 6 -pulse drive Approaching a perfect current wave Siemens Building Technologies

Typical Drive Input Current Waveform MM 4 45 k. W Line Current Waveform (low impedance mains) I RMS = 75 A Power = 12 k. W (x 3) If current is sinusoidal Only 52 A is required for the Same power. Siemens Building Technologies

SED 2 Typical Waveform SED 2 11 k. W Line Current Waveform I RMS = 53 A Power = 12 k. W (x 3) If current is sinusoidal Only 52 A is required for the same power. Siemens Building Technologies

DC Link Design Results 15 HP Typical VFD Line Current Harmonics Spectrum 15 HP SED 2 Line Current Harmonics Spectrum Meter deflection Siemens Building Technologies

Harmonics Competitive Analysis How do other VFD manufacturers address Harmonics? • Some build in DC link chokes • – This adds cost, weight and current losses Some offer line reactors – Rarely built-in to VFD - often offered as a separate option – Commonly mounted in bypass cabinet, or supplied in its – • • own enclosure Adds cost, weight and can cause nuisance tripping of drive on undervoltage Some build special 12 and 18 pulse drives – Extremely expense and not practical for most application under 100 HP Some just ignore the problem Siemens Building Technologies

Harmonics - The SED 2 Advantage • SED 2 is specifically designed for HVAC applications • Sophisticated Siemens control technology enables the SED 2 to significantly reduce current harmonics by dramatically reducing the size (capacitance) of the DC link. • In lieu of large (high u. F) electrolytic capacitors used in most VFDs, SED 2 uses much lower value (low u. F) plastic capacitors. – Fans and pumps with variable torque load characteristics • Typical 15 Hp VFD: DC link capacitance of 1000 u. F • SED 2 15 HP VFD: DC link capacitance of 20 u. F Siemens Building Technologies

Siemens SED 2 VFD Advantages • Low harmonics design • Small enclosure size • Many standard features • Reduces operating cost • Low power losses/high efficiency • Minimal effect on customer power • Wide power range – from ½ to 125 HP Siemens Building Technologies