How to maximize the availability of your drive system
Availability is one of the key criteria for the end users according to numerous surveys and direct feedback. Unplanned downtime is simply too expensive and shall be avoided. At the same time multiple redundancies may not be economically viable (high investment cost, larger footprint, aging of unused equipment, overall complexity etc). A good compromise shall be found. Ideally, the compromise has a solid foundation based on practical experience. Let’s explore several things that do not cost a fortune and help to increase the uptime of the drive system. This article is about how to maximize availability of a drive system. It is a free continuation of our reliability & availability series [1].
The end user is the master of his production process. VFD manufacturer knows the strengths and weaknesses of his products. When combining the available knowledge a great optimization can be done.
Balanced specification
From our experience this is a very important part. You want to specify items and considerations that are important from your perspective. You also want previous lessons learned to flow into the specification. That is perfectly OK. In fact, it is great to do so. However, the level shall be balanced. The core technology and manufacturing processes shall be responsibility of the manufacturer.
Avoid forcing the manufacturer to design and fabricate the equipment in a way that is not in line with his design rules, best practice and past experience [2]. Who is finally responsible if the system does not work?
Talking to a very knowledgeable customer (end user) he said a wise sentence: We would never force the supplier to design and manufacture something that he does not feel comfortable with.
Optimize on system level
When optimizing always have the whole system in mind. Optimum technical solution often requires certain compromises. Looking as individual components isolated from the system might sometimes be contra-productive.
Consider the complexity of the system, impact of various options on overall availability and performance. Find the best ratio for your case.
Few examples to think about:
– Air cooled VFD is sometimes perceived as more reliable than liquid cooled VFD. The air cooling is rather simple and there is no issue with leakage. However, larger air cooled VFDs dissipate significant losses into the ambient resulting in bulky air conditioning of the electric room. Considering the reliability of the air conditioning system is the air cooled drive system overall really much better from reliability standpoint?
– VFD efficiency can be increased by lowering the switching frequency. However, if it results into more distorted current supplied to the motor and increased motor losses, does it really help for the drive system?
– Dry transformers reduce the fire hazard. When they are located outdoors and there are strict limits on noise the customer might wish natural cooling (AN). However, what if the transformer manufacturer does not feel comfortable using natural cooling for a complex multi-winding transformer of rather high power?
Correct specification
This item may sound very trivial. Of course, the specification shall be correct. However, what we mean are the details (the devil is in detail). Most people think of design margins and safety factors. What they often forget are these little details associated with the application and the way how it is operated.
Example: It is good when you specify the duty cycle incl. possible overload, add some margin etc. However, are there also idle periods? If so, maybe this fact shall be stated somewhere. It can have an impact on the design, resp. accessories.
If you already are in possession of VFDs and have your own service maintenance team, let your service technicians comment the specification and incorporate their experience. These guys are very valuable source of information. In the end they will service the new VFDs. So let’s make sure that these humans and the new machines will be good friends.
Role of a technical dialogue
Don’t underestimate the importance of open dialogue between you and perspective supplier. Such technical exchange is extremely valuable for both sides: supplier and end user.
– The supplier can better understand the application requirements and eventually also site specific limitations. it helps tremendously to jointly develop a technical concept that fits customer’s needs and turn it into a great performing drive solution.
– The customer can better understand the basic philosophy of the manufacturer. Maybe he has a different concept than what customer had in mind when writing the specification. And maybe this concept suits very well the needs. Having a chance to explain the principle and share the field experience the customer might be more convinced to purchase such solution.
We can testify from our experience that the best performing and most reliable VFDs out in the field are those where the design requirements were openly discussed and mutually agreed between manufacturer and end user.
The best thing you can do is to develop a sound technical solution together with VFD manufacturer. Something that both parties will be happy with.
Factory visit
Continuing the thoughts from previous paragraph we highly recommend (if possible) to visit the manufacturing premises and talk to the technical experts right in front of such equipment. In some cases it can be a big eye opener.
Once we were talking to a customer from US concerning VFDs for a new chemical plant to be built in Texas. The customer already knew the manufacturer quite well and had many VFDs of that brand installed worldwide. They were all running smooth without any major issues. However, the project engineer was very concerned using liquid cooled drives as he was afraid of possible leakage issues or process chemicals entering the cooling circuit . In his view air cooling was simpler and ultimately more reliable. Few emails were exchanged, but customer kept being reluctant towards having a liquid cooling for his new VFDs. After visiting the factory his view dramatically changed. He could see multiple high power liquid cooled VFDs on the shop floor in different stage of assembly or testing. Many of them were for marine propulsion. Seeing that the marine industry is relying since years on large liquid cooled VFDs and seeing the VFD design itself with redundant cooling pumps, high quality piping, leakage detectors and maintenance friendly concept the customer was open to install such drives inside his new plant as well.
Second example is a customer that intended to replace aging rotating frequency changers with new state of the art static frequency converters. Reliability was one key topic. Layout was another one as the space in existing building was limited. Visiting the factory, talking to service experts and seeing very similar frequency converter in the production the customer got convinced about the solution and made his decision.
The list of such experiences could go on and on… Therefore, if you can, take the chance and visit the manufacturing place. See similar equipment to the tendered one in reality, talk to project engineers, service experts and developers. Watch how the equipment is assembled, discover the testing. Make your own impression about the quality level. Talk to a project manager to sense how the project execution typically looks like. Let the product experts guide your through the factory. Maybe some of your concerns will be void at the end of the visit while completely new thoughts and inputs will come to your mind.
Avoid excessive customization
Certain applications of projects require higher level of customization and it is great if the VFD manufacturer supports you with the right level of flexibility. For example, a complex versatile test stand will require highly engineered drive solution. On the other hand, if your application is a standard one, do not customize too much.
A fully tailor made system that is “one-off” might be difficult to maintain over long period of time. It is a bit risky that the manufacturer himself would lose the overview what was actually delivered as the equipment was so much modified from its standard product. Also, sound VFD manufacturers guarantee availability of spare parts over a long period of time. However, a fully tailor made solution with non-standard components can be challenging also from this perspective.
In the end, it is also about finding the right level. Adjust what is necessary, but accept standard proven solutions when they are suitable.
Use good engineering practices
Some people seem to believe that the magic key to robustness and high reliability are design margins. The more reserve you have the better. This is partly true, but it is not the ultimate solution.
For example, loading of certain VFD components is not linked with the power that the VFD delivers to the motor. Similar thing is with the transformer. For example hot spots due to stray magnetic flux are not linked with transformer loading. They might actually be higher at partial load.
Good engineering practices shall be used for installation (cabling, grounding etc). It does not help for plant availability to have overdimensioned VFD when the drive system trips on earth fault (for example).
Service network
The VFD manufacturer shall have a solid service network with a reasonable proximity from the site where the drive system will be installed. Their service engineers shall be trained and certified for the product that you intend to purchase. This advise may sound very basic, but when something goes wrong the time is precious. You don’t want to wait couple of days or weeks until the service person arrives on site. Reasons for a slow reaction time might be various, e.g. need to apply for a visa or simply lack of service personnel on manufacturer side (the only experienced guy is on another site and you have to wait until he completes his mission there). Therefore, make sure that the manufacturer has some local service footprint and right level of local service competence.
Operation and maintenance training
Generally we recommend the user to send his personnel for Operation & Maintenance (O&M) training [4]. Alternatively, the VFD manufacturer can provide this training at your premises. Both options are good, each having certain pros and cons. But each of them provides your maintenance staff with valuable knowledge. The training shall have a good balance between theoretical and practical part with sufficient hands-on exercises.
Spare parts
It is recommended to have the most critical spare parts available on site. VFD manufacturer can guide you about what spares you shall have on stock.
Large amount of spare parts on stock is not good as some spares are subject to aging even when not used. Therefore, let the VFD manufacturer advise what spares to purchase. He knows the best the recommended replacement intervals and also has the statistics from the field about failure rates of different components.
Predictive maintenance
Use the possibilities of predictive maintenance. Utilize them fully. Most renowned VFD manufacturers support predictive maintenance since few years and the systems are continuously improving.
Predictive maintenance is especially powerful in prevention of failures that slowly develop over time and can be recognized by monitoring certain parameters or quantities.
Maximizing drive system availability: Summary
As a wrap-up we would like to stress following key points:
- Write balanced specifications. Define the most important requirements, but let the manufacturer to use his manufacturing technology and experience.
- Seek an open technical dialogue. Let both parties (supplier - customer) understand each other. Develop the solution together.
- Visit the factory. Have a peer discussion in front of the equipment.
- Let the manufacturer present an alternative solution for your requirement.
- Use suppliers with sound service capabilities in your region.
- Send your service crew for Operation and Maintenance training. Be able to perform basic maintenance activities yourself.
- Have critical spare parts on stock. At the same time avoid large stock of spares, especially those that are subject of aging.
- Follow the maintenance recommendations. Benefit from predictive maintenance.
- Use sound engineering practices.
Following above tips shall help you to maximize the availability of VFD drive systems, reduce cost, increase competitiveness and maximize the benefits that this technology offers.
References
[1] Reliability and availability of variable frequency drives, https://mb-drive-services.com/category/reliability-and-availability/
[2] Project specifications – Too little/too much? https://mb-drive-services.com/projspec-1/
[3] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
[4] Drives trainings, https://new.abb.com/drives/services/training
[5] MV drives – ABB University, https://new.abb.com/service/abb-university/ch/lcmvd
