7154 VFD Presentation #2 May 2002 Paul Weingartner
Inverter types VVICSI 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 This can cause the drive to fault due to a bus overvoltage
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)
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
Vector drive benefits Speed regulation to 0.01% High torque at zero speed Torque linearity Fast response to load changes
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
Installation issues of VFDs Harmonics SWR / Cable length issues Bearing currents Carrier frequency Radio Frequency Interference (RFI)
Harmonics In the United States, 60 Hertz power is the standard In Europe, 50 Hz power is used
Terminology 60 Hz is the fundamental frequency Also known as the 1 st harmonic Also known as the 1 st harmonic The frequency of any harmonic can be calculated by the number of the harmonic multiplied by the fundamental frequency Example: The 5 th harmonic would be 5 x 60 Hz = 300 Hz Example: The 5 th harmonic would be 5 x 60 Hz = 300 Hz
Which harmonics Only even number harmonics are present in VFD systems Most important harmonics 3 rd 3 rd 5 th 5 th 7 th 7 th
Harmonic properties 3 rd - also known as a “triplen” – Zero sequence harmonic 5 th - Negative sequence harmonic
3 rd 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
5 th Harmonic Counter-rotating (i.e. BAD!)
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
Mitigating harmonic problems Installing line reactors and/or filters Reactor – is another term for inductor Filter – implies RC, RL or LC components
Standing Wave Ratio (SWR) Transmission line effects due to high frequency pulses Maximum power transfer theorem Creates high voltages at the motor terminals
What can be done Minimize the cabling length Use inverter rated motors Don’t use a high carrier frequency Use reactors
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 This causes the grease to break down Worse it can cause arc pitting of the bearings and introduce metal fragments Worse it can cause arc pitting of the bearings and introduce metal fragments
Mitigating bearing current problems Lower the carrier frequency Use insultated bearings Ground the shaft with a slip ring
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
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 Avoid ground loops
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