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Helpful Resources - VFDs

Variable Frequency Drives

A Variable Frequency Drive (VFD)[1] controls the speed of an AC induction motor by controlling the power (voltage and frequency) that supplies the motor. By reducing the motor speed to match the needs of the application, VFDs have the potential to significantly save energy and reduce operating costs for a variety of motor driven applications[2]. Motor systems that should be evaluated for energy savings through the use of VFDs include:

  • variable load motor systems where output is throttled or damped below full rated speed
  • centrifugal fan, pump, or blower systems
  • systems that meet either criteria above that operate frequently (e.g., > 2000 hrs/yr)

The potential energy savings derived from reducing a motorís power to manage its speed are approximated by the Affinity Laws[3]. These engineering laws are used to express the relationship between flow, head, and consumed power in relation to shaft speed for pump and fan applications and can be summarized as follows:

  • change in flow is proportional to the change in shaft speed
  • change in head (pressure) is proportional to the square of the change in shaft speed
  • change in power consumed is proportional to the cube of the change in shaft speed

Energy Savings Example

To illustrate the relationship between motor speed and consumed power, assume a 50hp centrifugal pump (95% efficiency) operating 4,067 hours per year with a 75% load factor, and electric costs at $.07 / kWh, is using a throttling valve to regulate flow to 70% on average. Applying the third affinity law, the yearly electric cost of the motor running at full rated (100%) shaft speed (or flow) would be:

Annual Energy Cost (Throttling Valve) =

(hp/Emotor) * LF * 0.746 kW/hp * (% full rated shaft speed)3
* (annual operating hours) * (cost of electricity)

Or

50hp/0.93 * 0.75 * 0.746 kW/hp * (1.0)3 * 4,067 hrs/yr. * $.07/kWh = $8,564 per year

The same system is represented below, except an ASD (97% efficiency) replaces the throttling valve to achieve the same flow regulation by varying the motor's rotational speed.

Annual Energy Cost (ASD) =

(hp/Emotor) * LF * 0.746 kW/hp * (% full rated shaft speed)3
* (annual operating hours) * (cost of electricity) * (1/EASD)

Or

50hp/.93 * 0.75% * 0.746kW/hp * (0.7)3 * 4,067 hrs/yr. * $.07/kWh * 1/0.97 = $3,028 per year

Using the information from each scenario, potential savings are calculated: replacing the throttling valve with the ASD can achieve approximately 5,500 in annual energy cost savings. Nearly 65% energy cost savings is gained as a result of a 30% reduction in shaft speed (or flow). In other words, small reductions in speed and flow can lead to significant energy cost savings.

For additional summary information, please see CEE Motor Efficiency, Selection, and Management:  A Guidebook for Efficiency Programs.

 


[1] Variable Frequency Drives (VFD) are sometimes referred to as: Adjustable Speed Drives (ASD) and Variable Speed Drives (VSD).† ASD and VSD are more broad categorizations that include mechanical, electromechanical, hydraulic, and VFD controls.† VFDs uniquely control the frequency of the electrical power supplied to a motor.

[2] Actual savings depend on the application. When used in constant speed, high torque, sequencing, and soft start applications, energy costs may not change or actually be higher with a VFD.

[3] Affinity Laws express an ideal relationship and do not take into account energy losses in the motor system, see: http://www.engineeringtoolbox.com/affinity-laws-d_408.html