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Variable Frequency Drives

How Much Will I Save?

The potential energy savings from installing a VFD is illustrated in the following example. Here, a 40 hp motor is used in an HVAC system with a flow-control damper. The system operates 365 days a year with the load/time profile shown in Table 1. The damper is removed and a VFD installed. The estimated annual energy savings realized from the use a VFD is shown in Table 1.

VARIABLE FREQUENCY DRIVES Energy Efficiency Reference Guide

Table 1. Potential Energy Savings
From Replacing a Damper With a VFD

Airflow Volume
(percent of maximum)
Daily Operating Time (hours) Energy Consumed Using a Damper(kWh/year) Energy Consumed Using a VFD(kWh/year) Difference in Energy Consumption (kWh/year)
50% 2 18 500 4 800 13 700
60% 3 29 300 9 800 19 500
70% 6 61 700 26 800 34 900
80% 6 63 300 35 900 27 400
90% 4 44 200 32 600 11 600
100% 3 34 200 35 200 –1 000
Total 24 251 200 145 100 106 100

The above example shows a possible electrical energy saving of 106 100 kWh per year, resulting from replacement of the existing damper-control system with a VFD. Savings would be less if the existing flow-control system used variable inlet vanes. At energy rates of $0.085/kWh, annual savings are $9,018.

At an estimated cost of $4,200 for the VFD and $750 for line and load reactors, the simple payback period is approximately seven months. When installation costs are added, the payback increases to one year.

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