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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.
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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