VFD APPLICATION GUIDELINES

By Colmac Coil

Background

The application of Variable Frequency Drives (VFDs) in heating and cooling units is becoming more common every day.  VFDs work by converting an AC voltage to DC voltage and then pulsing the DC voltage to simulate an AC sine wave at the required frequency to control motor speed. VFDs are an effective way to reduce energy usage but Colmac’s experience has shown that there are several design considerations to be accounted for in order to produce a successful application. These considerations are particularly important for extending motor life by reducing the potential causes of bearing currents and insulation breakdown that may arise with the use of VFDs.

Failure Modes

Bearing Current Failure – Improperly configured VFD systems can contribute to high shaft voltages which may result in Electric Discharge Machining (EDM). EDM occurs when voltage levels on the rotor/shaft exceeds the dielectric rating of the bearing lubrication and an arc is drawn across the bearings to ground. Every time this arc occurs a pit is created in the bearing race which, over time, will cause a fluted pattern in the bearing race. As the EDM continues to deteriorate the bearing surfaces the motor will experience vibration, increased noise levels, overheating, hard starts, overloads, and eventually, bearing failure.

There are several ways to mitigate the effects of EDM, however, these do not address the root cause. Such solutions include insulated or ceramic bearings and shaft grounding systems. While effective for mitigating EDM, these can have a high initial cost and represent an ongoing maintenance burden for the system.

Insulation Failure – The DC voltage pulses produced by VFDs travel down the conductors to the motor and can be reflected back to the drive. The reflected wave can increase in magnitude to the point where a partial discharge can occur (corona). This corona effect falls short of an actual insulation breakdown but can act to produce ozone which leads to carbon tracing and insulation degradation. Left uncorrected, the corona effect will eventually result in insulation failure and equipment damage.

Application Recommendations

System – When utilizing VFDs care must be taken to coordinate all components and to confirm their compatibility. The system designer and installing electrical contractor are responsible for designing the VFD electrical system to protect the specific motor(s) being controlled by the VFD.

Cabling – Conductors supplying motors should be rated and sized appropriately for the motor load, voltage drop, and environmental conditions. Line lengths between the VFD and motors(s) should be minimized wherever possible. Shorter line lengths will reduce the magnitude of reflected waves and, in general, benefit the longevity of the installation. Always follow the VFD and motor manufacturer’s specifications when selecting and installing conductors for a VFD installation.

Motors – Colmac motors are specified to comply with the National Electrical Manufacturers Association (NEMA) standard MG1 part 31 requirements for inverter duty motors.

Grounding – It is essential the electrical system, building steel, motor and VFD be properly grounded. The National Electric Code (NEC) describes the minimum requirements for grounding and bonding an electrical system for safe operation. In addition to providing a ground from the drive chassis and motor frame to earth ground, Colmac recommends a separate ground conductor from the motor frame to the VFD ground bus. Proper grounding is a critically important means of mitigating reflected waves and bearing current failures.

Carrier Frequencies – Colmac recommends setting the drive carrier frequency as low as possible (typically 2 kHz). Lower carrier frequencies result in higher levels of audible VFD noise but will help to reduce destructive bearing currents.

Motor Speed – Generally it is not recommended to over-speed motors or to operate motors at less than 25% of the motor rated speed.

Filtering – The VFD’s DC output waveform is typically jagged and can be strongly influenced by the electrical equipment it supplies and the length and type of cables used to supply that equipment. It is important to keep this waveform within a safe range to protect both the VFD and the supplied equipment. External filters can be applied to the line and load side of the VFD to smooth out the DC waveform and protect the system from damage. In general, line side filters protect the VFD while load side filters protect the motor. Load side filters can extend motor life by decreasing bearing wear and by lowering the motor operating temperature. Three common types of load side filters are available today. These are Load Reactors, dV/dT filters, and Sine Wave filters. dV/dT and Sine Wave filters are more effective than Load Reactors at reducing reflected waves and voltage spikes. The use of dV/dT or Sine Wave filters ensures the longevity of the installation by ENG00020282 Rev 0, 04‐13‐18 ‐ Page 3 of 3 mitigating reflected waves, voltage peaks, and other potentially damaging transient effects. Colmac requires load side dV/dT filters or Sine Wave filters on all VFD applications.

Conclusions

There are many factors that can contribute to the success or failure of VFDs applied to Colmac equipment, most of which are the direct responsibility of the system designer and installing electrical contractor. The general design requirements listed above represent the minimum criteria for proper VFD system design. Care should be taken to follow all the drive manufacturer’s recommendations and all applicable electrical codes and standards.

References

(1) http://www.greenheck.com/library/articles/58

(2) http://literature.rockwellautomation.com/idc/groups/literature/documents/in/drives-in001_-en-p.pdf

(3) http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/drives-wp016_-en-p.pdf

(4) http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/drives-wp019_-en-p.pdf

(5) https://library.e.abb.com/public/fec1a7b62d273351c12571b60056a0fd/voltstress.pdf

(6) http://www.eaton.com/ecm/idcplg?IdcService=GET_FILE&allowInterrupt=1&RevisionSelectionMethod=LatestReleased&noSaveAs=0&Rendition=Primary&dDocName=AP043001EN

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Four Ways to Maximize Energy Procurement Savings

By Drew Fellon, business leader for energy supply services, Trane

 

It’s well-known that energy costs are always one of the most significant portions of a commercial building budget. Finding ways to reduce that spend can be tricky, but there are ways you can impact it: reduce the amount of energy consumed, change the time of day energy is used, or reduce how much you pay for that energy.

Typically, reducing energy consumption is the go-to solution many building owners and energy managers use in an effort to reduce costs and improve sustainability. While this is an impactful way to decrease energy spend, there’s another, less-conventional strategy, that is often overlooked — even though it can yield significant savings — energy purchasing.

The obvious, though less common, solution is gaining traction in the industry as owners and managers explore ways to reduce their energy spend budgets. Developing a strategy for examining and assessing your energy procurement options can help you make the most of your commercial building budget.  

Consider these energy procurement best practices to help streamline the energy supply process, maximize savings and optimize efficiency in your next commercial building project.

Tip No. 1: Engage a knowledgeable partner

Energy procurement is a complicated process, compounded by the many factors that impact energy rates. Working with a reputable and knowledgeable partner makes the process easier — and can save your company money.

Energy procurement and management companies are powerful resources that can help you avoid spending too much on energy. These companies have long-standing supplier relationships and market intelligence capabilities that they leverage as they work with you through the procurement process and can assist you in getting the best deal possible. Using energy price forecasting, regulatory and legislative monitoring, and years of energy industry experience and contacts, an energy procurement and management firm can help you make insightful decisions to reach your company’s goals.

When choosing to engage an energy procurement and management company, select an experienced partner with insight into the full supply chain and industry pricing structure. This organization can provide visibility to the tens of thousands energy transactions taking place annually. Consider working with an independent advisor without ties to a specific energy company or market. This independence helps create loyalty to you, not the energy supply company, and will provide transparency throughout the procurement process.

The right procurement and management partner can help you navigate the energy purchasing waters — and prepare your company for trends that may impact the industry down the road.

Tip No. 2: Consider your goals

Saving money is typically the top priority for most energy and supply chain managers examining their energy procurement options. But as customers demand more efficient, environmentally conscious solutions for their buildings, many energy and supply chain managers are prioritizing sustainability and the use of renewable energy. When it comes to reaching these types of goals, you need to look beyond price in the purchasing process. It all comes down to what you’re trying to achieve.

If the goal is to reduce energy costs by 10 percent, then purchasing renewable energy is likely not the solution. And if you are hoping to reduce the company’s carbon footprint, you’ll need to buy a certain type of energy that may not always have the lowest price tag. All of which is to say, that putting sustainability first, can look very different than your typical energy procurement process and sources.

Because many energy contracts are long term (between 10 – 20 years), it’s important to take a comprehensive, enterprise-wide approach to buying renewable energy solutions. If your enterprise has buildings or operations in many states, you can achieve more buying power when you coordinate efforts.

Make sure you’re considering the total end cost of an energy contract. There are many factors that affect the final cost of an energy contract, and this may not be clear in every situation. An energy procurement and management firm can help you understand the total cost of the contract structure — not just the initially quoted supply price.

Tip No. 3: Link supply with demand

Before entering into a supply contract, you should fully understand all the factors that influence energy demand in your building — including the supply and demand relationship. Pay close attention to how much energy your building or enterprise uses, taking time to look at both prior and forecasted usage, to determine how much energy to buy.

It is imperative that the supply chain, which drives purchasing, communicates with the operations staff, who oversees day-to-day use of building facilities. Otherwise, the energy manager or supply side personnel may assume that because X amount of energy was used last year, X amount of energy will be needed again next year. When in reality, operations may plan to install a new production line or more energy efficient equipment that will impact the facility’s energy demand.

If your organization is going to take measures to reduce peak demand, you need to make sure your energy supply contract allows you to benefit financially — rather than potentially being penalized by your supplier. Demand-side reduction efforts should be negotiated into your supply-side contract. It’s also important to run a competitive bid process every time you seek a new supply contract, rather than allowing your current supplier to set the price. Doing so will help you secure the best deal for each new contract.

Tip No. 4: Plan ahead

Another common pitfall in energy procurement is waiting until just before the current contract expires to start looking for a new one. The energy market is seasonally cyclical and impactful market events can drive prices up or down for short periods. To avoid paying too much for your next contract, get an early start and take time to really dig into your options and review pricing. This is where working with an experienced energy procurement and management firm can yield big returns. Tracking the energy market daily and understanding how certain events will impact pricing over time are both critical to long-term buying success.

If your organization requires stakeholder approval of energy contracts, be aware of that and secure that approval in advance. This allows you to move quickly when the time is right, so you can lock in the best price. Not having the approval process in place could translate into delays and lost opportunities in terms of pricing, as prices are often only valid for a short period of time.

Purchasing with strategy

Your energy purchasing strategy is dependent on many things, most of which are out of your control. Weather, government regulations, demand charges, and new energy developments are all issues that can impact pricing for commercial customers. Taking control of your procurement options with thoughtful research, meaningful partnerships, conscious goal setting and extensive planning can help you make the right purchasing decision for your building. And remember that you don’t have to do it alone. Working with an experienced energy consultant can help you streamline the energy supply process to increase your savings.

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