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AC Drives and EMI/RFI Mitigation

Copyright © 2001 Francis J. Martino

A variable frequency drive (VFD) will generate radio frequency interference
(RFI) in the range of 0.5 MHz to 1.7 MHz, and electromagnetic interference
frequencies (EMI) in the range of 1.7 MHz to 30 MHz. The high frequency
generation is caused by the high carrier frequencies of the pulse-width
modulation, the associated short rise times of the IGBT output devices, and the
reflected waves from the motor terminals. EMI is also produced by the harmonics
which are generated by the carrier frequencies, rise times and reflected waves.
"Reflected waves" are caused by the capacitive effects of long motor leads and
the resulting impedance mis-match between the motor cables and the motor
windings. EMI/RFI is also referred to as electrical noise.

The EMI/RFI will travel to the motor along the motor leads and will be
transmitted to ground via the capacitive effect between the motor windings and
the motor frame, the capacitive effect between the line conductors and bond
wire, and the capacitive effect between the line conductors and conduit. The
EMI/RFI will then seek a return path to the source, that is, to the input of
the VFD.

The return path to the input power terminals of the VFD will be from the ground
grid, through the grounded neutral of the wye secondary of the upstream power
distribution transformer, and through that transformer's load lines. See Figure 1.


The return path to source of EMI/RFI along a conduit or other portions of the
facility ground grid will cause a voltage gradient on both the conduit and grid that
may effect the proper operation of other equipment.

The grounding grid normally consists of motor and electrical enclosure frames,
cable conduit, conduit straps, structural I-beams, water pipes, and grounding
rods. Thus, EMI/RFI and the associated voltage gradients may be spread around
the facility, allowing problems to be created along the path of return.

Solutions to Power Cable Problems

In older installations which utilize a conduit for a ground return, and in all new
installations, introduce a bond wire that will pass through the conduit. A bond
wire will appear as low impedance to low frequency EMI/RFI, causing the low
frequencies to pass on the bond wire and not on the conduit. Thus those
frequencies will be kept off the facility ground grid and will create a lower voltage
gradient than would otherwise be created if they traveled along the conduit.

However, due to "skin effect" of high frequencies passing on a wire, the bond
wire will appear as a high impedance path to those frequencies. The higher
frequencies will therefore continue to pass along the conduit rather than the
bond wire.

Cable with both a metallic shield and an outer insulating jacket is preferred over
conduit. The aluminum or copper braid of a shielded cable or the clad of a metal
clad cable will present a lower path of impedance for the higher frequencies than
does conduit, allowing a minimal voltage gradient to develop along the braid or
clad on the return path. The outer insulating jacket will eliminate the problem
created by conduit and conduit straps which pass the EMI/RFI to the structural
I-beams and water pipes with which they come in contact.

A continuous corrugated aluminum sheath is preferred over interlocked
aluminum or interlocked steel. The effectiveness of interlocked shielding will be
lost over time due to oxidation increasing the turn-to-turn contact resistance.

A cable with a "bond wire" consisting of three symmetrically placed grounding
conductors is preferred over a cable with a single grounding conductor.

A ferrite core (also referred to as a choke) may be used to attenuate common
mode noise (noise that is passed from line to neutral and line to ground) on AC
and DC circuits by passing all output power lines through the core. The
properties of the core are such that it provides an inductance to the fields created
by the noise.

If the bond wire is passed through a ferrite core along with the output power
conductors, then the bond wire will present a higher impedance to the EMI/RFI,
thus encouraging the lower frequencies to remain on the conduit.

Differential noise (noise that is passed from line to line) may be attenuated by
passing a single wire through a ferrite core. However, for an application with DC
or with with a low fundamental frequency AC power, the permeability of the core
will be reduced by the saturation effects of the DC component of the line current.
In those applications, the effective attenuation will be reduced and the
impedance offered to the EMI/RFI by the core must be de-rated. A typical
cylindrical ferrite core may have an internal diameter of 1.0 inch, an outside
diameter of 2.0 inches and a length of 2.0 inches. See Figure 2.

An EMI/RFI filter at the drive input terminals will provide a return path for noise
that will effectively reduce the distance of noise travel along the ground grid and
keep the noise away from the upstream transformer and incoming power lines.
See Figure 3.


A drive isolation transformer with a solidly grounded neutral will also provide a
return path that will keep the noise away from the upstream transformer and
incoming power lines. See Figure 4.

Both a load reactor or a dv/dt filter will reduce the high rate of change of output
voltage which is caused by the rapid switching of the IGBT devices. The dv/dt
filter derives its name from the high rate of change (d) of voltage (v) with respect
to a small change in time (t).

The reduction of the rate of change of voltage will reduce the capacitive
effects which cause EMI/RFI. Thus, a load reactor or dv/dt filter wired directly
to the output terminals of a VFD will reduce both common mode noise and
differential noise.

In addition to reducing the rate of change of output voltage, the filtering
of the high carrier frequencies via a dv/dt filter will also yield a reduction
of the impedance mis-match at the motor terminals and, consequently, a
reduction in the reflected wave. A dv/dt filter consists of a load reactor with a
parallel capacitor.

Keep in mind that a load reactor or dv/dt filter with 1 1/2% impedance will
cause a reduction in output voltage to the motor of 1 1/2%. A 5% impedance
will cause a corresponding 5% reduction in voltage to the motor.

Solutions to Signal Problems


When building a control panel, if the incoming power cables, mechanical bonds
and wire bonds enter to the right, then mount all sensitive electronics on the left.
Also install the exiting power cables, mechanical bonds and wire bonds on the
right. That configuration will cause the bond path on which EMI/RFI flows to
remain on the right side of both the enclosure and the enclosure panel and will
keep the EMI/RFI away from the portion of the panel on which the electronic
equipment is mounted.

Reduction of noise on signaling circuits may be achieved by using shielded
cable and a common mode ferrite core with both wires and the cable shield
passing through the core. It is generally preferred to ground the shield at the
source of the signal rather than at the receiving end, thus preventing the need
of noise returning to source via the ground grid.

The use of optical signal isolators will also reduce noise on signaling circuits.
Control signals transmitted on fiber optic cables will prevent noise problems
caused by long runs and, in some applications, may also allow a VFD to be
placed at the motor location, thus eliminating both long motor lead cables and the
need for dv/dt filters. In addition, short motor leads, when used with an EMI/RFI
filter or drive isolation transformer, will also shorten the return path along the
system grid.

For long lead lengths on run speed potentiometers and other signaling sources,
use #12 AWG EMC (Electromagnetic Compatibility) cable. The EMC cable has a
copper braided sheath which provides shielding from EMI/RFI and will offer a
lower resistive voltage drop per foot than the standard #18 AWG shielded cable.

For exceptionally long leads, signal conditioning is required at the potentiometer
to convert the potentiometer voltage output signal into a 4 to 20 mA signal.
A milliamp signal is less susceptible to EMI/RFI than a voltage signal.

Measurement of EMI/RFI


Common mode noise (line to neutral/ground) may be measured by connecting t
o the line leads three one-meg ohm resistors in a wye configuration. With an
oscilloscope, you may then observe and measure the noise that exists between
the neutral of the wye and common.

It is possible for a standing wave at the motor terminals of a 480 VAC system to
be as high as 2300 volts or more. If it is necessary to measure at the motor
terminals, use isolated leads, insulated gloves and safety glasses. Remove wrist
watches, bracelets and rings which might possibly get caught on the ground grid and
thereby restrict a hasty retreat in the event of flashover at the test leads.

A hazard also exists with long motor leads that are run in parallel with other
motor leads in a cable tray. When a VFD is operating, EMI/RFI will be coupled
from one cable into another. As a result, when replacing a motor, the motor cable
may be found to have a high potential at the end of its conductors even though
the motor's drive is disconnected from the line.


References:
Gary L. Skibinski, Russel J. Kerkman, Dave Schlegel, "EMI Emissions of Modern
PWM AC Drives," IEEE Industry Applications Magazine, Vol. 5, No. 6
November/December 1999, pp 47-81.

John M. Bentley, Patrick J. Link, "Evaluation of Motor Power Cables for PWM AC
Drives," IEEE Transactions on Industry Applications, Vol. 33, No. 2 March/April
1997, pp 342-358.

Fair-Rite Soft Ferrites, 14th Edition, pp162-184, Fair-Rite Products Corp.,
Wallkill, N.Y.

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