Troubleshooting 460-Volt Delta Transformer Issues: Seeking Insights on VFD Failures

Instead of purchasing input reactors, consider investing in 480/460 transformers featuring a 460/277 wye secondary configuration. Connect the output windings in a wye (or star) arrangement and ground the center of the wye. This approach will help you avoid future complications. To clarify the underlying issues, it's important to note that the delta secondary of the distribution transformer is ungrounded, which means any leakage from other voltage sources can elevate the voltage levels to potentially hazardous degrees. I have personally recorded delta networks reaching 1575VAC to ground in one instance and 800VDC to ground in another facility. Furthermore, if a ground fault occurs on one phase, the other two phases may experience a surge up to 460VAC to ground. Many brands of AC drives, including those from AB, feature input protection MOVs intended for balanced phase-to-ground voltages around 400VAC. It's critical that manufacturer guidelines clearly specify that input MOVs must be disconnected when used with floating delta systems. Failing to do so leaves the drive inputs vulnerable to surges and spikes. By using the suggested input transformers, you can convert ungrounded delta power into grounded wye power, effectively providing an excellent transient filter through the transformer—superior to a line reactor. Typically, you will find similar precautionary measures detailed in the instruction manuals within the section covering connections to the input power terminals.

• 14-01-2025
• DickDV

Instead of purchasing input reactors, consider acquiring 480/460 transformers equipped with a 460/277 wye secondary configuration. Arrange the output windings in a wye (or star) formation and ensure that the center point of the wye is grounded. This setup will eliminate potential issues. To clarify some key concerns, it’s important to note that in a floating delta secondary distribution transformer, the lack of grounding means that any leakage from other voltage sources could elevate the network to extremely hazardous levels. For instance, I've personally recorded delta networks reaching 1575VAC to ground and, in a different facility, 800VDC to ground. Additionally, as previously mentioned, a ground fault on one phase can cause the other two phases to spike to 460VAC relative to ground. Many brands of AC drives, including Allen-Bradley (AB), come with built-in input protection Metal Oxide Varistors (MOVs) designed to handle balanced phase-to-ground voltages around 400VAC. However, it is crucial to follow the manufacturer’s guidelines, which typically advise disconnecting input MOVs when integrating with floating delta systems. Failing to do so leaves the inputs of the drives vulnerable to surges and voltage spikes. By implementing the recommended input transformers, you effectively convert delta power to a grounded wye power configuration while simultaneously benefiting from superior transient filtering provided by the transformer—more efficient than a traditional line reactor. Generally, you can expect to find similar safety precautions outlined in the instruction manuals when reviewing the connections to the input power terminals. This enhances reliability and protects your equipment, ensuring safe and stable operations.

• 14-01-2025
• DickDV

Floating power distribution systems can generate ground voltages in the range of thousands of volts, which can potentially damage insulation and lead to capacitor failure. To mitigate this risk, implementing a grounding transformer is essential. For more information on grounding transformers, including zig-zag and grounded wye-delta configurations, visit http://electrical-engineering-portal.com/grounding-transformer-zig-zag-grounded-wye-delta.

• 14-01-2025
• lfe

User "AustralIan" inquired, "Have the MOV jumpers been removed from the defective drives?" Click to reveal more information... That’s absolutely right! For further details, please consult the AB manual regarding ungrounded service procedures.

• 15-01-2025
• DSMR

Thank you for your assistance, everyone! We’ve recently discovered that the MOV's jumper was still connected. We are currently in the process of removing it. Your support is greatly appreciated! Best, Eli

• 16-01-2025
• Elijah58

I am employed at a manufacturing facility utilizing an ungrounded delta system, constructed in the 1950s. Throughout the plant, we have numerous transformers that convert delta configurations to wye, particularly in areas with motion control systems. Recently, I encountered an issue with a parts balancer experiencing intermittent jerkiness. Upon investigation, I discovered a heater in a parts washer on the opposite side of the facility that was grounding out. This grounding caused the jerkiness, which nearly led to a fault. The heater's cycling on and off disrupted the balancer's operation, causing it to function erratically. I'm currently preparing to install a transformer specifically to address this issue. In another instance, a rotary table that operated smoothly for several months suddenly began to malfunction. I replaced the inverter, which occasionally fixed the issue, but more often than not, it wouldn’t run properly. By feeding the inverter through a delta to wye transformer, it started functioning reliably and has continued to perform without failure since. In this scenario, I did not encounter a shorted (grounded) leg within the plant's electrical system. Wes

• 16-01-2025
• wes.s

Eliminating the jumpers can address one of the potential issues that arise when Metal Oxide Varistors (MOVs) could act like small explosives if the reference voltage to ground exceeds the MOV's rating. This risk is especially pronounced with ungrounded or corner-grounded delta power supplies. However, it’s important to note that removing the jumpers also negates the advantages provided by the line-side MOVs and nullifies the UL certification of the Variable Frequency Drive (VFD). This predicament isn't exclusive to PowerFlex drives; it applies universally to all VFDs available on the market, regardless of their age. All drives are engineered for solidly grounded wye systems, and it's worth mentioning that delta systems are primarily found in North America. Some manufacturers do not offer a method to eliminate this ground reference (the aforementioned jumpers) and simply state in their manuals that the drive must only be used with solidly grounded wye sources. Only a select few leading manufacturers, who recognize the significance of the North American market, provide jumpers or other means to remove the ground reference. However, solving this issue addresses only one aspect of the challenges you may face. I support DickDV's suggestion that the most effective solution is to incorporate a “drive isolation transformer” before each drive or for a group of drives located in the same area. This transformer setup typically involves a 480V input with a 480/277V wye output to the drive. These transformers also achieve some of the same benefits as line reactors, so consider avoiding line reactors for this application and save your budget. A side note: ungrounded delta systems have become a code violation unless you either ground one leg (corner ground), which undermines the original intent, or implement an expensive ground fault monitoring system for the entire service. While your facility may be “grandfathered” in, any permits pulled for modifications will likely require you to rectify the situation. It may be prudent for your management to address this issue proactively while the economy is favorable, rather than facing higher costs in the future due to ongoing expenses for replacing VFDs, power electronics, or needing to install numerous smaller transformers. In summary, ungrounded delta systems belong to a bygone era and hold little relevance in contemporary applications.

• 16-01-2025
• jraef

User jraef commented: "Ungrounded delta configurations are outdated and lack relevance in today's technology landscape. While we still find many of these systems in rural communities, much of the infrastructure has been transformed into 'corner ground' setups. Unfortunately, they remain unsuitable for the filters used in modern drive systems. Additionally, it's worth noting that nearly all of the 240/3ph setups utilize delta configurations, which is far from ideal!"

• 16-01-2025
• Gene Bond

Dear Jraef, could you please provide one or two references regarding the discontinuation of "floating delta" as a viable code? This information would be incredibly helpful and could put to rest my long-standing opposition to it. I’ve been advocating against this practice for 15 years, as it almost had devastating effects on my well-being. If this information is accurate, it would be a dream come true! I plan to include these references in my educational materials for the classroom. Thank you so much!

• 16-01-2025
• DickDV

According to NEC Article 250.20, all alternating current (AC) premise wiring systems must be properly grounded, with specific exceptions outlined in Section 250.21. Notably, subsections 250.21.A.1-4 identify particular scenarios in which grounding may be omitted. However, subsection 250.21.B stipulates that if grounding is bypassed, a ground detection system must be installed at the service point, accompanied by appropriate signage—a requirement often overlooked. Implementing a ground detection system can be more costly than simply corner grounding a transformer. It's important to note that in the context of Variable Frequency Drives (VFDs), a corner-grounded delta system may function similarly to an ungrounded system, which warrants caution when recommending such setups. I typically advocate for transitioning away from ungrounded delta configurations and suggest switching to a wye system or establishing one, especially when dealing with any power electronics.

• 16-01-2025
• jraef