Transcript 7154 VFD - Cincinnati State

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
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 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
Which harmonics
Only even number harmonics are present
in VFD systems
Most important harmonics



3rd
5th
7th
Harmonic properties
3rd - also known as a “triplen” – Zero
sequence harmonic
5th - Negative sequence harmonic
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
5th 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
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
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