Example calculation of energy cost reduction.

Using the Altivar 61 on fans and pumps can reduce your energy costs by thousands of dollars.

The basic principle is: lower motor speed = lower energy costs
Most HVAC systems are designed to keep the building cool on the hottest days and warm on the coldest days. Therefore, the HVAC system only needs to work at full capacity on the 10 or so hottest days and the 10 or so coldest days of the year. On the other 345 days, the HVAC system may operate at a reduced capacity. This is where a variable air volume system with variable frequency drives (VFDs) can be used to match air flow to actual heating and cooling demands. The VFD can reduce the motor speed when full flow is not required, thereby reducing the power required and the electrical energy used.

How Much Energy Can Be Saved? 

It's a general rule of thumb that a centrifugal pump or fan delivering 80 percent of its rated flow only requires 50 percent of the rated power. The graph demonstrates the relationship between flow and power requirements of a centrifugal pump or fan.
A= percent flow B= percent pressure C= percent power required

Consider the following example:

  • A 50 hp fan needs to supply air 10 hrs/day for 250 days 
  • The cost of running the fan at full speed would be:
    • 50 hp x 0.746 kW/hp x 2500 hrs x $0.08/kWhr = $7,460.00
  • Assuming the fan does not have to run at full speed all the time:
  • 25% of time at 100%; 50% of time at 80%; 25% of time at 60%
  • The cost of running the fan with a variable speed drive would be:
    •   50 hp x (1.0)3 x 0.746 x 625 x $0.08 = $1,865.00
    •   50 hp x (0.8) 3 x 0.746 x 1250 x $0.08 = $1,909.76
    •   50 hp x (0.6) 3 x 0.746 x 625 x $0.08 = $ 402.84
    •   Total = $4,177.60
  • Potential Annual Savings: ($7460.00 - $4177.60) = $3,282.40     
  • Payback Period of Variable Speed Drives

The payback period on a VFD averages 18 to 24 months, but can be less than 12 months. The period depends largely on the type and size of the system and how much time the motor is operating at full speed versus how much flow is actually required to heat or cool the building space. The life cycle of HVAC equipment in commercial buildings typically is between 20 to 50 years, so a one- or two-year payback period can generate a substantial return on investment.