Best Practices for VFD Grounding
Minimize the damage caused by common mode noise.
by Jay Marchi
01/15/2016
According to the U.S. National Electric Code NFPA70, Article 100B, an effectively grounded
scheme is one that is intentionally connected to earth through a ground connection or
connections of sufficiently low impedance and that has sufficient current-carrying capacity to
prevent the buildup of voltages that may result in undue hazard to connected equipment or to
persons.
Grounding of a non-linear drive system is done for two basic reasons: safety and common
mode noise containment or reduction. With the advent of power electronics used in products
ranging from personal computers to variable frequency drives (VFDs), harmonic noise
containment has become even more important.
Grounding Systems
A good grounding system is an integral part of any electrical system. Older, linear systems
were designed around the linear relationships between voltage and current. However, the
proliferation of solid-state power supplies that take in incoming alternating current (AC) power,
rectify it to direct current (DC), then reconstitute it at varying voltage and current ratios at
different frequencies, called nonlinear systems, presents some new challenges for a typical
ground scheme. The byproducts of these nonlinear systems are the production of significant
harmonic distortions, voltages and currents on a system and, when referenced to ground, are
described as common mode noise (CMN). CMN can have deleterious effects on some
mechanical components such as bearings and electrical and control systems on the same
ground bus.
Figure 1. Comparison of the sinusoidal current wave output of BJT (top) vs IGBT output (Graphics
courtesy of ProPump Services)
Figure 2. General schematic of a PWM VFD drive
VFDs are becoming more common in pumping systems. They offer a means of controlling
pump output more efficiently by controlling the operating speed of the electric motor driving
the pump. Before VFD technologies, matching pump speed to system load conditions was
accomplished using turbines, fluid couplings, multi-ratio gearboxes or variable speed
combustion engines.
These reliable methods are still in use. But with the growing emphasis on improving operating
efficiency, the inherent inefficiency of traditional methods is driving users toward VFDs.
Because the head and flow rate of the pump can be regulated by controlling its rotating
speed, the VFD controlling the drive motor speed gives operators a wider operating range,
allowing the tailoring of the pump's output to the system's requirements, which can increase
the system's overall pumping efficiency.
The VFD Evolution
Developed by Nikola Tesla in 1924, the induction motor can be directly connected to an AC
three-phase power source. The motor speed is directly proportional to the applied frequency.
For example, a two-pole pair motor with no load operating on a 60-Hertz (Hz) system would
rotate at 3,600 revolutions per minute (rpm). However, the same two-pole motor operating on
a 50-Hz system would rotate at 3,000 rpm. Therefore, by controlling the frequency of the
supplied power to the motor, the motor's speed can be controlled.
Traditionally, the frequency of the power system is determined by the speed of the generator
and number of pole pairs. Like the motor, a two-pole pair generator operating at 3,600 rpm
will produce AC power at a frequency of 60 Hz. Operating the same generator at 3,000 rpm
will cause the output power to oscillate at 50 Hz. To change the power supply frequency, the
speed of the generator must be changed.
Until 1968, when Danfoss introduced their VLT5 frequency converter, the most widely used
method of providing an AC induction motor with variable frequency was using a speedcontrolled DC motor turning an AC generator to produce the desired AC frequencies. Although
this system is effective enough to control the speed of the motor within a few rpm, it is
expensive to operate and has inherent inefficiencies. By comparison, the VFD can vary the
power and frequency to the motor, but with greater flexibility and fewer moving parts.
Because of the continuously increasing capabilities of solid-state power electronics, the pulse
width modulation (PWM) VFD is becoming the drive of choice. It can work with motors that
range in size from fractional to thousands of horsepower. It can also couple reliably, offer
affordability in cost per horsepower and deliver measurable electrical energy savings.
PWM VFD Basics
The PWM-VFD is the current state-of-the-art method used to control frequency and power.
VFDs generate AC current by sequentially switching the DC current in alternating directions.
The latest solid-state electronic switches, insulated gate bipolar transistor (IGBT) and the
newer integrated gate-commutated thyristors (IGCT), have increased the switching speeds five
to 10 times over the older bipolar junction transistor (BJT). The faster the switching, the
smoother the current sine waves to the motor. That switching generates significant amounts of
CMN that must be contained and dissipated by the power and ground systems. To keep these
stray currents from damaging or interfering with system components, the ground grid must
provide a low impedance path for CMN currents.
Figure 3. Common mode current paths in a VFD drive system
Figure 4. Recommended grounding practices
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Power Equipment Ground Loop
A power equipment (PE) ground (protective earth) terminal usually serves as safety equipment
ground for AC and DC drives. The CMN currents are replete throughout the entire VFD circuit
(shown in green in Figure 3), and any ungrounded components can accumulate electrical
charges because of current leakage that results in voltages greater than the recognized safe
touch potential of 50 volts (V). The direct path of CMN currents between the drive and motor
frame is shown in red in Figure 3.
The lack of direct connection between the drive and motor frame is typically the reason for
the inducted voltages in excess of the acceptable National Electrical Manufacturer's
Association (NEMA) standard of 300 millivolt voltage on the motor shafts. This problem is also
the primary cause of bearing damage in motors that are not equipped with means to resist or
shunt the CMN currents from the motor.
True Earth Ground Loop
The true earth (TE) ground loop is the most familiar ground system. All structures have it
primarily for safety because a short circuit in an ungrounded system could be fatal.
Commercial building structure steel is usually the best connection for zero voltage TE
potential because girders are connected in a low impedance grid pattern that has multiple
column paths into ground. In most private homes, ground connections are established by
bonding to copper water piping or embedded ground rods.
However, installing a VFD system and relying solely on the TE ground may be problematic.
Depending on the impedance (resistance to AC current), some higher-frequency CMN
currents may not pass back to the drive and build up voltages above the safe touch potential
of 50 V. The stray CMN currents can cause damage to anything in the VFD circuit.
References
1. Wiring and Grounding Guidelines for Pulse Width Modulated (PWM) AC Drives,
Installation Instructions Rockwell Automation Publications DRIVES-1N001M-EN-PMarch 2014
2. Bill Howe, Avoiding the Hazards of Pulse-Width Modulated Adjustable-Speed Drives, E
Source Inc. Tech Memo TM-93-6, 1993
3. David Bezesky and Scott Kreitzer, NEMA Application Guide for AC Adjustable Speed
Drive Systems, IEEE Paper PCIC-2001-7
To read more Motors & Drives articles, go here.
Jay Marchi is a 1985 graduate of Pennsylvania State University with a Bachelor of Science
degree in electrical design engineering technology. Marchi is employed by ProPump Services,
an engineering and diagnostic consulting firm located in Bethlehem, Pennsylvania. Marchi is a