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7154 VFD

7154 VFD. Presentation #2 May 2002 Paul Weingartner. Inverter types. VVI CSI PWM – current technology. Acceleration. Linear ramp S-curve. Deacceleration. If the drive is ramped down too quickly, the motor becomes a generator and will drive the DC bus voltage up

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7154 VFD

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  1. 7154 VFD Presentation #2 May 2002 Paul Weingartner

  2. Inverter types • VVI • CSI • PWM – current technology

  3. Acceleration • Linear ramp • S-curve

  4. Deacceleration • If the drive is ramped down too quickly, the motor becomes a generator and will drive the DC bus voltage up • This can cause the drive to fault due to a bus overvoltage

  5. Skip frequencies • Every mechanical system has a resonant frequency(s). • Drive operation at that resonant frequency can cause problems and potentially damage the system • VFDs can be programmed to skip over these frequencies (i.e. not dwell on them)

  6. Flux vector drives • Use encoder feedback to obtain exact information on what the motor is doing • Adding an encoder and feedback board to a drive will typically add $1000 in cost

  7. Vector drive benefits • Speed regulation to 0.01% • High torque at zero speed • Torque linearity • Fast response to load changes

  8. Sensorless vector drives • Do not have the encoder for external feedback • Feedback is derived from the motor terminals • Better control than a standard VFD • Needs to go thru an auto-tune procedure • Needs to be given motor characteristics in order to achieve the superior control

  9. Installation issues of VFDs • Harmonics • SWR / Cable length issues • Bearing currents • Carrier frequency • Radio Frequency Interference (RFI)

  10. Harmonics • In the United States, 60 Hertz power is the standard • In Europe, 50 Hz power is used

  11. Terminology • 60 Hz is the fundamental frequency • Also known as the 1st harmonic • The frequency of any harmonic can be calculated by the number of the harmonic multiplied by the fundamental frequency • Example: The 5th harmonic would be 5 x 60 Hz = 300 Hz

  12. Which harmonics • Only even number harmonics are present in VFD systems • Most important harmonics • 3rd • 5th • 7th

  13. Harmonic properties • 3rd - also known as a “triplen” – Zero sequence harmonic • 5th - Negative sequence harmonic

  14. 3rd Harmonic • Causes neutral currents to flow • NEC specifies that the neutral should be either one wire gauge larger than the gauge of the phase line or two lines should be pulled for the neutral • Can cause overheating in transformers and premature failure

  15. 5th Harmonic • Counter-rotating (i.e. BAD!)

  16. Harmonic distortion • Measure of the sum of all of the harmonic components compared to the fundamental 60 Hz current • IEEE-519 • Total Harmonic Distortion (THD) • Total Demand Distortion (TDD) • Typically, a THD of less than 5% is considered good

  17. Mitigating harmonic problems • Installing line reactors and/or filters • Reactor – is another term for inductor • Filter – implies RC, RL or LC components

  18. Standing Wave Ratio (SWR) • Transmission line effects due to high frequency pulses • Maximum power transfer theorem • Creates high voltages at the motor terminals

  19. What can be done • Minimize the cabling length • Use inverter rated motors • Don’t use a high carrier frequency • Use reactors

  20. Bearing currents • Using VFDs to drive motors create common mode voltages that often will reach ground through the motor shaft • This causes the grease to break down • Worse it can cause arc pitting of the bearings and introduce metal fragments

  21. Mitigating bearing current problems • Lower the carrier frequency • Use insultated bearings • Ground the shaft with a slip ring

  22. RFI • Anytime there is distortion in a waveform, harmonics are present. • Since harmonics are multiples of the fundamental frequency and these harmonics exist in meaningful amplitudes at over 100x the fundamental frequency, RF energy is released • This RF energy can cause problems for sensitive equipment and sensors

  23. Mitigating RFI problems • Use shielded cable • Don’t run signal wires in the same conduit as power wires • Use proper grounding methods • Avoid ground loops

  24. Ventilation • Ventilation for the enclosure for a VFD (and any other equipment) should be considered due to the heat generated • Cooling fans – also need to be checked for proper air flow and if filters are present, need to be cleaned periodically

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