Download 7154 VFD - Cincinnati State

7154 VFD
Presentation #3
May 2002
Paul Weingartner
Motor insulation info
The Ten Degree Rule indicates that a ten
degree temperature increase cuts
insulation life span approximately in half
and vice-versa.
Insulation life span
Material taken from
RC snubber circuits
RC Snubbers
The RC snubber is the simplest and lowest cost of
methods employed. In its simplest form it consists of
resistors and capacitors configured as shown in Fig. 3.1.
The RC snubber is typically installed at the motor
terminals and acts as an impedance matching network.
The snubber components are carefully selected to cause
the load impedance to match the characteristic
impedance of the motor cables. When the motor surge
impedance is equal to the line characteristic impedance,
then voltage reflection does not occur and excessive
voltage will not be experienced at the motor terminals.
RC snubber circuit
Before and after waveforms
LC filters
The LC filter combines a load reactor and a capacitor
network to form a low pass filter as illustrated in Figure
4.1 . The basic concept is that the filter network has a
resonant frequency of approximately 1 to 1.5 kHz and
frequencies higher than that will be absorbed by the filter
and not passed on to the motor. Of course it is important
that the inverter switching frequency be set to 5khz to
prevent excessive filter current and drive malfunction. In
fact, it has been found that the performance of this basic
LC filter network, while very good at 5 kHz, actually
improves as the switching frequency is increased.
LC filter schematic
Before and after waveforms
Line reactor ratings
Line reactors
Reactor specs (from MTE)
Each variable frequency drive must be equipped with an input
reactor offering noless than 4.5% effective impedance at rated motor
amps (the fundamentalcurrent).
They must be harmonic compensated and be UL-506 and UL-508
approved. Nema 1 enclosed units must be UL Listed. The
continuous current rating of the reactor must be equal to or greater
than the rms input current rating of the drive.
Reactors must be copper wound with a UL class H (180 C)
insulation system. They must be suitable for an ambient temperature
of 45 C and a have a maximum temperature rise of 115 C. Their
watts loss must be less than 1% of the rated load. Box lug type
terminals must be provided on all reactors rated from 2 amps thru
400 amps. Higher current reactors may be supplied with copper tab
type terminals. Reactor must be MTE Corporation type “RL” series.
Motor efficiency
Because harmonic currents and frequencies cause
additional watts loss in both the copper windings and the
iron of a motor, the actual mechanical ability of the motor
is reduced. These watts are expended as heat instead of
as mechanical power. When a harmonic compensated
reactor is added to the VFD output, harmonics are
reduced, causing motor watts loss to be reduced. The
motor is able to deliver more power to the load at greater
efficiency. Utility tests conducted on VFD's with and
without output reactors have documented efficiency
increases of as much as eight percent (at 75% load)
when the harmonic compensated reactors were used.
Even greater efficiency improvements are realized as the
load is increased.
Motor noise
Because the carrier frequency and harmonic spectrum of many
Pulse Width Modulated (PWM) drives is in the human audible range,
we can actually hear the higher frequencies in motors which are
being operated by these drives. A five percent impedance harmonic
compensated reactor will virtually eliminate the higher order
harmonics (11th & up) and will substantially reduce the lower order
harmonics (5th & 7th). By reducing these harmonics, the presence
of higher frequencies is diminished and thus the audible noise is
reduced. Depending on motor size, load, speed, and construction
the audible noise can typically be reduced from 3 - 6 dB when a five
percent impedance harmonic compensated reactor is installed on
the output of a PWM drive. Because we humans hear
logarithmically, every 3dB cuts the noise in half to our ears. This
means the motor is quieter and the remaining noise will not travel as
far.
Info from
ANSI 519 Current
ANSI 519 Voltage
Sizing the reactor
What do they look like?
VFD issues
Catching a spinning load
V/Hz boost on big loads