Over the past few years, we have received an increasing number of calls related to failures of inverter-based electric machinery and driven equipment. Some of the problems extend beyond these, as we are also finding VFD failures, PLC and electronic failures, lighting failures, transformer failures, and other increased electrical and electronic nuisances in commercial, industrial, and utility spaces.
As we have been involved in extensive research into newly recognized conditions, such as supra-harmonics and ground/neutral currents, we have published a significant amount of information in these areas. However, most often the problem is far more basic, even when the damage appears to be far more complex. In this article, we will cover two of these conditions.
Behind many costly VFD and equipment failures are simple wiring and grounding errors.
The call often starts as ‘we installed a new drive,’ or ‘we’ve noticed an increase in failures that no one can figure out.’ When we hear that the motor is over 25 horsepower or 20 kW, other conditions often include that ground fault protection has been jumped out in different areas. Sometimes, we’ll request images of the inside of the VFD cabinet before determining if an on-site visit is required.
Figure 1: While a small common-mode choke is installed, note the loose conductors and interspersed ground leads.
The problem shown in Figure 1 is nothing too new. To keep costs down, the installation manual is often ignored, and individual unshielded conductors are put into conduit along with ground conductors.
The result in larger applications, such as this one, is high ground currents feeding the surface of the motor, impacting the reference between the motor and VFD, and putting large high harmonic current into ground.
This is in addition to noise being induced between phases, resulting in damaging inter-harmonics on the motor winding, ground, bearings, the VFD, and other components. Figure 2 is an example of a ground lead current on the output of the VFD on one of three ground leads.
Figure 2: Ground current on one of 3 ground leads from Figure 1.
Figure 3 is the three-phase going to the motor, with each of the three phases going into its own conduit along with the ground lead. This would be the total for all three phases, including noise added to the output currents.
Figure 3: Phase current output total from Figure 1.
The high ground current and related noise, even with the common mode choke, are found across the surface of the motor and its surrounding structure. We were literally able to read high current on the ladder rungs up to the motor before testing. This is not unusual to the point where we will often demonstrate this type of circulating ground currents, knowing what will be found.
Conductors between the drive and the motor need to be inverter cabling in which grounds are balanced, conductors are shielded, or any combination of the above, as well as cable shielding that is grounded at both ends. This provides a Faraday cage that prevents inducing noise and currents into other cabling and ground/neutral systems.
Improper cable routing turns drives into noise generators, compromising reliability across systems.
Another common condition is that control and ground leads may be placed in parallel in the box, as well, which results in noise and other conditions that will affect the operation and reliability of the machine, drive, and driven equipment. Ground and controls are required to be perpendicular to power leads, especially the output leads from the drive to the electric motor.
The second condition is filtering and not just outputs to the motor. In an effort to save money, we will often see either buyers or sellers decide to leave out input and output filters as unnecessary, or they will be added if there is an issue, which, of course, never happens.
Installation can also be an issue with output and input conductors being brought together, bypassing the filter systems. The result, in addition to the kinds of outputs seen above, is as shown in Figure 4 for the voltage and current at the VFD that is feeding back to transformers and other loads.
Figure 4: Supply current to the VFD in Figure 1.
The result of Figure 4 is a high harmonic condition, as shown in Figure 5. When multiple systems like this run in series or parallel, depending on their phase, inductance, and capacitance related to the circuit, the conditions can be quite dramatic at the primary transformer, as shown in Figure 6, which is measured on the utility side and exceeds IEEE 519 requirements.
Figure 5: Current harmonics at the incoming power side of the VFD.
Figure 6: Example of inverters at the primary side of a 13.8 kV, 5000 kVA transformer at the utility point of common connection (PCC).
The additional concern with these harmonics, in addition to the electrical and mechanical reliability issues, is the increase in energy costs associated with these extremes. For instance, a potential reduction between 5 and 10% of the total energy consumption associated with the system associated with the Figure 6 switchgear could be substantial.
At a potential reduction of 270 kW, or 2,365 Megawatt-hours per year, at $0.20/kWh and 0.699 metric tons CO2/MWh, this could result in an impact of $473,000/year in energy costs and 1655 metric tons of CO2 per year. The energy savings alone can justify the corrective actions at the facility, excluding the reduction in equipment failures and reduced equipment life, even if we consider it half this value (5% reduction).
The lesson? Read the instructions. None of this is hidden from the equipment owner, and the instructions are typically available in the installation manual or supporting documentation. In fact, in the application in Figure 1, the manual explicitly states not to do this in the application.
Instead, vendors, repair companies, and the equipment owner were blaming each other for the low reliability, when the installation was the actual issue. How are your VFDs and electronic systems being installed?
