Motors, Drives

Four risks to consider when choosing a medium-voltage drive

A rigorous specification can help minimize downtime, safety and financial risks during a drive’s typical 20-year lifecycle. Examine the details.
By Pat Lemmon June 6, 2019
Courtesy: Intermountain Electronics

Too often, drive specifications are recycled from previous projects without any real thought given to an application’s unique needs. Drives also often are chosen based purely on price, without consideration for how different drives can meet different operational needs.

However, medium-voltage drives are big-ticket items and large-scale operations rely on them. The right drive can help companies meet their application’s unique requirements and reduce potential downtime and safety and financial risks.

That’s why it’s crucial to consider these four key risk factors when buying or specifying drives.

1. Project risks

A one-size-fits-all approach to drives can create long-term challenges for a company’s operations.

Yes, all drives may provide similar functionality to control a motor’s speed, torque and direction, but they can vary in their topology, semiconductor design and principles of operation. As a result, each drive will have its own unique structural, design and reliability benefits.

In other words, different drives are better suited for different industries and applications. Those involved need to identify the drive that’s right for each individual use case.

For example, a downhill conveyor application will benefit from the use of a drive with regenerative braking. This capability offers a more efficient motor-stopping method and the ability to return power back to the utility. A platform-based electro-submersible pump should specify a drive that supports long cable lengths. Some drives can control motors as far as 15 km away, which can reduce the need for multiple electrical houses and drives and lower project costs. However, the needs of less stringent general-purpose applications, like typical pumps, fans and compressors, can be addressed by most drive topologies.

Companies should bring in a vendor partner to help plan, specify and design a drive application. They can help companies avoid common pitfalls during the specification process, and help find the right drive based on specific application criteria such as harmonics, motor compatibility, regeneration capability and dynamic performance.

This raises another important consideration for reducing project risk: choosing the right vendor. Companies don’t want their operations to be a testing ground for an unproven vendor’s products. Stick with vendors that have at least 10 years of drive-application experience and hundreds, if not thousands, of field-installed units. A large installed base often translates into a robust service and support network, which is better equipped to support a company’s application.

2. Downtime risks

Downtime is always a concern in industrial operations, which is why it should be factored into process-critical equipment like medium-voltage drives.

First, determine the mean time between failure (MTBF) needed from a drive. Considering most heavy operations expect to rely on drives for decades, a good target is a minimum of 100,000 hours. That pans out to more than 10 years before a predicted failure and can be extended by following the manufacturer’s recommended preventive maintenance program.

Next, determine the drive’s mean time to repair (MTTR). Given the high cost of downtime heavy industries often face, MTTR should be a matter of minutes, not hours. Fifteen to 30 minutes is common, assuming spare parts available.

There are two primary factors that influence a drive’s MTTR.

The first is the drive’s design. Maintenance workers should be able to access components without needing to disassemble the drive. In some cases, drives may require a lifting cart to handle larger assemblies. Finally, component failures shouldn’t cause collateral damage, which can lead to costly cascade failures and longer repair time.

Some drive designs also can use redundancies to help shorten MTTR. For example, an automatic bypass capability allows a drive to continue running following a device failure. This gives companies time to plan a shutdown instead of dealing with an immediate shutdown after the failure occurs.

Fans, pumps and compressors, new or retrofit, are among applications for Allen-Bradley PowerFlex 6000 medium-voltage ac drives from Rockwell Automation. These include variable torque and constant torque applications. Courtesy: Intermountain Electronics

Fans, pumps and compressors, new or retrofit, are among applications for Allen-Bradley PowerFlex 6000 medium-voltage ac drives from Rockwell Automation. These include variable torque and constant torque applications. Courtesy: Intermountain Electronics

The second factor that affects MTTR is the company’s support structure. Companies need technicians with the right expertise to monitor, maintain and repair the drives. It’s also a good idea to have a solid spare-parts inventory to repair or replace drives as fast as possible.

A vendor can help companies that can’t manage these responsibilities. However, the company needs to specify support requirements that meet the stated MTTR goal. For example, the vendor’s trained service personnel should be located within a set distance of the facility or be able to arrive within a set period of time. Some vendors also offer remote monitoring and can notify the company of any detected faults, warnings or performance anomalies with the drive.

3. Safety risks

A medium-voltage drive can help manage safety risks in a few key ways.

First, some drives offer arc-resistant enclosures as an option to enhance personnel protection. These drives redirect the energy created from an arc-flash event away from personnel, even when the low-voltage control cabinet door is opened for maintenance purposes. This can help companies meet a range of global arc-resistant standards.

Some drives also offer optional functional safety features, where safety is designed into the drive. Safe torque off (STO) is one example; it allows workers to remove power from a motor without removing power to the drive. This can protect people and property from preventable accidents and help restart a system after a safe state is reached.

Finally, drives with remote connectivity can deliver diagnostic data straight to workers at their desks. This can minimize the need for them to enter electrical rooms to access the data, thus reducing exposure to potential hazards.

4. Efficiency risks

The drive a company selects may be able to meet the application’s requirements, but how efficient is its performance?

Different drives can have small variations in their operating efficiency. However, what seems like a negligible difference today can add up to big expenditures over a drive’s lifecycle.

Consider a drive that’s 1-2% less energy efficient than a competitor’s offering. The less-efficient drive may cost less up front, but it will generate more heat. This added heat could require the company to purchase a higher-rated air-conditioning system or air-to-air heat exchanger. Companies also must pay for the energy to power those systems over the life of the drive. This can translate to hundreds of thousands, or even millions, of dollars in added costs over a period of 20 years.

When specifying a drive’s operating efficiency, a good rule of thumb is to target a minimum of 96.5% at full load and full speed.

Worry less, do more

On the surface, medium-voltage drives may seem like just another piece of a company’s operations. However, they can play an important role in helping manage risks to a company’s personnel, production and profitability. By considering these four risks in each drive application and addressing them with rigorous specifications, companies can worry less about unexpected downtime or costs and focus more on staying productive.

Pat Lemmon is director of technical support, Intermountain Electronics. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, cvavra@cfemedia.com.

MORE ANSWERS

Keywords: medium-voltage drives, machine safety

Medium-voltage drives are responsible for many large-scale manufacturing operations.

A one-size-fits-all approach to choosing a medium-voltage drive can create long-term challenges for a company.

Safety and efficiency risks are also major concerns that should be addressed when choosing a medium-voltage drive.

Consider this

What is your biggest risk concern when choosing a medium-voltage drive?

ONLINE extra

For more about the drives, see the New Products for Engineers database listing.

See parts 1 and 2 of this series by the author linked below.

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Pat Lemmon
Author Bio: Pat Lemmon, director of technical support, Intermountain Electronics.