Plant equipment: steering the way to motor efficiency

Machine drives, which are primarily electric motors, pumps and fans, account for about 8% of the manufacturing sector’s total fuel consumption, according to the US Energy Information Administration.

The electricity that machine drives consume varies considerably as a percentage of delivered energy between industrial sectors – from about 25% for plastics and rubber to less than 10% for food and chemicals and under 5% for petroleum and coal.

Moreover, since rotating machinery is at the beating heart of most industrial processes, it’s not just energy efficiency that determines how successfully drives and motors contribute to a profitable business. Maintenance costs and the costs of not running motors during unplanned stoppages can also be a significant drain on the operation if they’re not optimised.

According to Dave Hawley, ABB’s local business unit manager, motors & generators, it all adds up to a scenario in which the upfront cost of a new motor is probably the last thing you should be considering: “To see the real cost of any electric motor, you have to look beyond the purchase price. The capital cost of acquiring a new motor is normally less than 3% of the total life cycle costs of the motor. The other costs are far more significant and must be taken into account when making purchasing decisions.”

High efficiency and reliability and low maintenance costs will help bring down your cost of ownership and ensure many years of profitable productivity

Dave Hawley, ABB’s local business unit manager, motors & generators

Hawley divides these other costs into running costs and the costs of not running. The cost of running the motor depends on its efficiency and the cost of electricity. Over a typical lifespan of two decades, running costs are by far the biggest contributor to the overall cost of running a motor.

“The cost of running is about 70 to 95% of total costs, and is based on the energy price per kilowatt hour and the annual running hours,” says Hawley.

Price of downtime

And when it comes to the costs of not running, he adds that just one unplanned production stop per year can equate to between 2% and 30% of total lifetime costs.

“Downtime for a motor depends on the industry. In the oil and gas sector, for instance, unplanned downtime can cost an estimated £150,000 every hour. In metal and mining this is typically £55,000 per hour, while for power utilities it can be £35,000 per hour,” says Hawley.

An effective programme of preventive maintenance is therefore a key tool for reducing lifetime costs by mitigating unplanned downtime owing to motor failure.

In terms of energy efficiency, there are many factors that can contribute to optimising consumption and keeping a lid on costs. For example, making sure the motor runs cool is one simple measure that will help minimise heat losses. It can also prolong the intervals between services, thereby reducing maintenance and the costs of not running.

“High efficiency and reliability and low maintenance costs will help bring down your cost of ownership and ensure many years of profitable productivity,” says Hawley.

Bearing up

Bearings, which literally bear the brunt of much of the wear and tear experienced by motors in the process industries, play a key role in delivering energy efficiency, as well as preventing unplanned stoppages through bearing failure.

“The choice of bearing will be determined by a number of factors. These include: motor size, speed and load rating, environmental conditions (temperature, humidity, local contamination), required service life, noise levels and maintenance,” says Paul Dysiewicz, engineering manager at bearing specialist SKF.

“Most electric motors are designed with a locating and non-locating bearing arrangement. The locating bearing positions the shaft axially. The non-locating bearing accommodates thermal expansion of the shaft, to prevent excessive axial forces being induced on the bearing arrangement.”

The choice of bearing will be determined by a number of factors. These include: motor size, speed and load rating, environmental conditions (temperature, humidity, local contamination), required service life, noise levels and maintenance

Paul Dysiewicz, engineering manager at bearing specialist SKF

He explains that most medium and large motors use a locating bearing that is a deep groove ball bearing, while the non-locating bearing is typically a deep groove ball bearing, a cylindrical roller bearing or a CARB bearing. In contrast, smaller motors are generally fitted with two deep groove ball bearings mounted on a short shaft and often have a cross-locating bearing arrangement.

Finding the optimum bearing solution can be a balancing act, depending on which factors the user chooses to prioritise. For instance, selection is often driven by the need for increased reliability and service life, in which case, a product such as SKF’s hybrid bearings is a good option.

“These use balls or rollers made of silicon nitride, which is ideal for demanding applications requiring high stiffness and excellent electrical insulation properties,” says Dysiewicz.

“Alternatively, if the requirement is for high speed and low noise, then SKF Explorer bearings offer optimised internal geometry that minimises friction, wear and heat generation, while withstanding heavier axial and radial loads.”

Monitoring the situation

As mentioned previously, preventive maintenance is a key tool in avoiding the costs associated with unplanned shutdowns. So Dysiewicz says that one of the most significant recent developments is the arrival of bearings with built-in condition monitoring.

“These [types of] products incorporate self-powered, intelligent wireless sensors, embedded within the structure of each bearing, to provide instant condition monitoring data via the internet. SKF Insight monitors dynamic parameters such as vibration, temperature, lubrication condition and load, and informs the user when conditions are abnormal and can threaten to cause bearing damage.”

With all these factors in mind, the initial cost of bearings can pale in comparison to ongoing costs and savings, leading to a ‘total cost’ argument for bearings that’s strikingly similar to the case for overall motor systems.

“Although the initial cost of bearings will always be an issue, especially to procurement teams, of greater importance is lifetime operating cost. Operating costs in terms of energy usage and re-lubrication, together with the costs of downtime from unreliable bearings, or bearings that wear faster than expected due to incorrect specification, can rapidly outweigh the original purchase cost,” concludes Dysiewicz.

Small advancements

Meanwhile, variable speed drives (VSDs) are also a major component of motor efficiency.

These bits of kit match the speed of the motor to the load, rather than having the motor on a pump or fan running at a constant speed and using valves or dampers to choke the resulting flow, for instance.

In fact, ABB says VSDs, which are also referred to as inverters, can reduce energy consumption by as much as 60%.

Improving energy efficiency for an already efficient product is all about making small advancements on many fronts

Matt Handley, product manager at Mitsubishi Electric

However, this is all well-established, so suppliers have more recently been turning their attention to fine-tuning the performance of VSDs.

“Improving energy efficiency for an already efficient product is all about making small advancements on many fronts and the Mitsubishi F800 has a wide range of dedicated features and functions that create efficient operation,” says Matt Handley, product manager at Mitsubishi Electric.

Taken together, the various features achieve energy efficiency of 98%, Handley says.

Other advantages arise from making the inverter ‘smarter’ with a built-in programmable logic controller (PLC). This enables the drive to react appropriately to non-critical load variations, reducing false alarms that might otherwise trip the motor, for example, he says.

“Far fewer false alarms mean more uptime, fewer maintenance trips and faster responses to genuine issues such as jammed pumps, broken impellers and snapped belts that can now be more accurately detected by the drive.”