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3:16 pm
July 14, 2011
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Calculated Savings: Driving Energy Efficiency

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Inefficient motor-driven systems lead to the loss of power. That’s  the equivalent of losing money. Lots of money. Taking a little time to do the math can help your operations change this equation.

By Brent Oman, Gates Corporation

Tens of millions of electric motor-driven systems in operation at manufacturing facilities around the United States are not running at optimum efficiency. Why does that matter? These motor-driven systems consume 70% of all electricity used in the plants, and the energy costs companies billions of dollars.

Today, approximately one-third of the electric motors in the industrial and commercial sectors use belt drives—most of them standard V-belts. While they are popular for their low acquisition costs, wide availability and quiet performance, V-belts simply are not as energy-efficient as synchronous belt drives.

Efficiency of any power transmission system is a measure of the power loss associated with the motor, the bearings and the belt drive. It is defined by these formulas:

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As these equations show, energy losses in belt drives are separated into two categories: torque loss and speed loss. These vary in V-belt and synchronous drives as a result of the belts’ inherently different physical characteristics.

Although properly maintained V-belt drives can run as high as 95-98% efficient at the time of installation, such efficiencies deteriorate by as much as 5% during operation. Poorly maintained V-belt drives may be up to 10% less efficient. Synchronous belt drives, on the other hand, remain at an energy efficiency of 98-99% over the life of the belt. A proven, viable alternative to V-belt drives and roller-chain drives, they are generating savings across a variety of industrial applications.

Here, we look at how to calculate energy costs, energy savings and payback period—all of which are important factors in realizing the value of belt-drive conversion.

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The calculations
Plant maintenance managers leverage improved energy efficiency by converting V-belt drives to synchronous belt drives in one of two ways: 1) maintaining current capacity while using less power; or 2) increasing capacity slightly using the same power. For example, if the current airflow in an HVAC application is satisfactory, a synchronous drive would use less energy to do the job. If the current airflow is insufficient, a synchronous drive could increase airflow without increasing use of energy with proper attention to design.

To determine the kilowatt-hours saved when using synchronous drives instead of V-belt drives, the following formula is used:

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In this equation, .746 is the conversion factor from HP to KW, and .05 is the estimated 5% energy savings gained by converting.

Short payback

Estimating potential energy savings and the payback period for a synchronous belt drive is simple with these formulas:

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Consider this example: If energy costs are $0.10 per KWh, the annual energy cost for a 40 hp motor running at 89% efficiency, 8736 hours per year, totals $29,290.14. The annual energy savings is $1464.51. If a new synchronous belt drive costs $342.83, the payback period is .23 years—less than three months.

When the annual dollar savings amount is multiplied by the number of similar motors in a plant, and added to the savings from motors of all other types, a facility’s overall energy-savings impact is clear.

Maintenance savings: an added benefit
It’s important to note that energy savings are not the only positive impact of conversions to synchronous belt drives. Many manufacturers recommend that a newly installed V-belt be retensioned 24 hours after installation. Employees must lock out the power, remove the belt guard, retension, secure the belt guard and resume power. The run-in process is time-consuming but necessary. Companies that are too busy for this 30-minute task are later burdened by premature belt failure. That can lead to costly belt replacement.

For optimum performance, V-belts should also be retensioned regularly as part of a scheduled preventive maintenance program. Like run-in, each procedure takes approximately 30 minutes—during which the drive must be shut down and even more productivity is lost.

Synchronous drives, which require no run-in procedures or retensioning, help eliminate maintenance costs and downtime. Continuing with the example of our 40 hp motor, we can compare the maintenance costs incurred in a one-year period for a synchronous belt drive and a V-belt drive. If each is replaced once a year, installation time doesn’t differ: It’s $160 ($40 per hour x 2 technicians x 2 hours). The recommended run-in procedure for the V-belt drive uses the same two technicians for another hour, so the additional cost is $80. Assuming the drive is well maintained, retensioning occurs four times for $320. Thus, the V-belt’s annual maintenance cost is $560 (versus, as previously noted, only $160 for the synchronous drive).

It all adds up
Inefficient drives waste energy. Many companies are converting to synchronous belt solutions that drive down energy usage and have the added benefits of lower maintenance costs and less downtime. Payback from converting to synchronous belt drives is typically much less than one year. MT

Brent Oman is manager, Product Application Engineering with Gates Corporation.

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