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Variable Frequency Drives: Opening Door for Energy Savings in Pump Application

Variable Frequency Drives (VFDs) is an electronic device that converts Alternating current (AC) of one frequency to another frequency which gives an opportunity to run the AC motors in variable speed and provide an opportunity for saving energy on many industrial applications. Several control methods such as V/F control method, Vector control and VVC+ have been used to achieve the desired speed of AC motors such that it meets industrial process requirements. AC motors have been extensively used for pumps, fans and air compressor applications. According to the International Energy Agency, electric motors consume 46% of the electricity generated in the world. They account for nearly 70% of the total industrial electricity consumption in industries. As per European Commission’s report the pumping system accounts for nearly 22% of the energy supplied. VFDs have a tremendous potential to save energy, especially in pump applications.

VFD: Saving Energy through Motor Speed Regulation 

In many water pumps and flow applications, a mechanical throttling device such as a mechanical valve or electro mechanical valve is used extensively to control flow of water where a fixed AC supply is used to run the motor. Although this method is an effective means of control, it leads to wastage of energy. Figure 1 below represents a pumping system connected to mechanical throttling valve which has a manual or automatic control. In this case, the AC motor is run with fixed AC supply and therefore it runs the motor at full speed and flow control via throttling device leading to energy wastage. To overcome the energy loss, VFD based water flow control is shown below in Figure 2 where AC motor is powered via VFD to control the speed of the motor which gives an opportunity to save energy depending on the speed at which the motor is operated. 

Fig: 1 Constant Speed Pumps controlled Via Throttling Devices

Fig: 2 Variable Speed Pumps controlled Via VFD

 

A VFD operating at 70% speed can result in 65.7% cost saving 

Significant energy savings and hence cost savings opportunities are available if we use VFDs for pump applications. It’s also interesting to note that VFDs offer plenty of advantages such as elimination of mechanical components, reduced power line disturbances, smooth starting and stopping and hence longer motor life.

Cost savings accrued due to VFDs can be better understood by comparing a 2.2 kW motor being operated on fixed AC supply (i.e flow control via throttling devices) vs. variable speed operation through VFD. 

Constant Speed Pumps: Inefficient in Energy Saving 

Case 1: Assuming a pump operates at regular intervals at full speed for 263 days annually, 12 hours each day at fixed AC supply (at rated grid voltage and frequency). The operating cost for the pump can be calculated with the following formula:

Cost=Power (kW)*Running Time*Cost/kWh

 

Energy Cost in INR/Year =2.2 (KW) X12 (Hours) x 5 (INR/kWh) X263 (Days) =34716.0 (INR)

So, the Energy Cost for operating this pump is Rs 34716 per year if the unit cost of electricity is Rs 5. This pump uses a conventional method of flow control via throttling devices which leads to wastage of power and hence energy losses.  

VFD: Key to Energy Efficiency and Reduced Energy Bills

Case 2: To overcome this energy losses in case-1, the pump can be operated through VFDs wherein we can get variable speed of the motor and hence desired flow of water. This means we run the motors at desired speed to get desired flow of water. Potential energy savings can be understood with the help of affinity laws. The affinity laws for pumps are used to express the relationship between variables involved in pumps (such as head, volumetric flow rate, shaft speed) and power.

Flow is proportional to shaft speed is given by 

Q1/Q2=N1/N2

Pressure or Head is proportional to the square of shaft speed

H1/H2=(N1/N2)^2

And Power is proportional to the cube of shaft speed

 P1/P2=(N1/N2)^3

The energy savings calculation for 2.2 kW at 70% motor speed is as follows: 

Power(@70%)= Power(@100%)* (N(@70%)/N (@100%) )^3=2.2 kW* ((70%)/(100%))^3=0.754 kW

 

Energy Cost in INR/Year@ 70% Speed =0.754 (KW) X12(Hours) x 5(INR/kWh) X263(Days) =Rs 11907.59 which gives about 65.7% on cost. Once again an energy savings opportunity depends on speed at which pumps to be operated. 

Having seen the energy savings and cost, VFDs can be treated as an energy saver and extended to various other pump applications such as boiler feed Pumps, circulator pumps, domestic pumps, dewatering pumps, fire umps, flooded suction pumps, industrial pumps, irrigation pumps, marine pumps, pond pumps, pool pumps, process pumps, sewage pumps, sprinkler pumps open and bore well pumps. 


Authored by:

Dr. S. Paramasivam, Mr. M. Thiruvenkadam, Danfoss Industries Pvt. Ltd, India

Author: sustainabilityoutlook