Variable-speed drives gain favor as a way to minimize energy costs.
Senior Drive Application Engineer
Edited by Ken Korane
Manufacturers once assumed that energy costs were, for the most part, beyond their control. But such attitudes became outdated with the advent of power monitoring and control devices such as energy-efficient motors, sophisticated controllers and software, and variable-speed drives (VSD). They make sure that key processes use less energy, thus turning energy into a manageable expense.
The potential savings are mind-boggling. Domestic manufacturers spend more than $33 billion on electricity every year, with motor systems consuming approximately 63% of the total. More than half of the motors run either fans or pumps key areas with significant potential for energy savings.
As a result, manufacturers are looking for ways to reduce power consumption for these applications. In general, any application in which pump or fan speed varies can potentially benefit from a variable-speed drive. Here's how to determine if a VSD is worth the price.
Most fan applications do not require maximum air movement all of the time. So, to vary airflow, managers use methods such as:
- Cycling, typically used in residential settings and generally not applicable in industrial applications.
- Outlet dampers with control louvers to restrict outlet airflow.
- Variable inlet vanes that modify the physical characteristics of the air inlet, which changes airflow.
- Variable-speed drives, which change the fan speed.
In industrial settings, variable-speed drives are generally the most effective way to control energy use because they regulate motor speed. Inlet vanes and outlet dampers only control airflow, not the motor.
As a rule, energy consumption in fans and pumps varies by the cube of motor speed, also called centrifugal load. For example, if the fan motor speed can be cut in half, energy consumption = ( 1 /2) 3 = 1 /8, meaning the energy used to power the motor actually decreases by 7 /8. The same rule applies to pumps.
To illustrate, let's compare outlet dampers and VSDs. Changing airflow or fan speed can affect the system's natural operating point, as well as a fan's efficiency and power requirements.
Outlet dampers affect a system by increasing resistance to airflow stated mathematically as:
P = KQ 2
where P = pressure required to produce a given flow, K = resistance to airflow, and Q = airflow. Outlet dampers affect K. Power requirements for this type of system gradually decrease as flow decreases, as shown in the Outlet damper graphic.
Variable-frequency drives take advantage of changes in fan behavior as speed changes. This is quantified by a set of equations called affinity laws. These laws, for centrifugal pumps and fans, relate how changes in speed or geometry affect capacity or power consumption.
Q2 /Q1 = N2 /N1,
P2 /P1 = (N2 /N1) 2 , and
H2 /H1 = (N2 /N1) 3 ,
where N = fan speed, rpm; Q = flow, cfm; P = pressure, static in. H2O; and H = horsepower.
Reducing fan speed significantly reduces power requirements, as shown in the Variable speed graphic.
Comparing energy consumption when controlling airflow using outlet dampers to that used with variable-speed drives requires a load profile (based on the application) and a fan curve (supplied by the fan manufacturer). Based on the accompanying fan curve, let's assume the fan runs at 300 rpm and 100% flow = 100,000 cfm. Also assume the following load profile:
100% flow for 10% of the time; 80% flow for 40% of the duty cycle; 60% flow for 40% of the cycle; and 40% flow for the remaining 10%. For outlet dampers, the horsepower required for each operating point can be read from the fan curve. Multiplying the horsepower at each point by the percent of time the fan operates at this point produces the weighted horsepower. Weighted horsepower provides a glimpse into the amount of energy the motor uses to control airflow at each point in the duty cycle. Summing the calculations produces the total weighted horsepower, which represents the fan's average energy consumption over a complete cycle. The chart shows the complete results.
For variable-speed operation, the fan curve does not contain enough information to read horsepower values for all operating points. Therefore, affinity laws are incorporated into the calculations.
Substituting Q for N in the horsepower equation gives the following results:
Q2 = 80%, H2 = 18 hp; Q2 = 60%, H2 = 7.56 hp; Q2 = 40%, H2 = 2.24 hp.
From this data, one can calculate total weighted horsepower for variable-speed operations.
Results show the outlet damper has a weighted horsepower of 32.6, significantly higher than the variable-speed drive's 13.9 weighted horsepower. If the fan runs 730 hr/month and electricity averages $0.07/kW-hr, energy costs for the dampered fan would exceed $1,242. Meanwhile, the drive-controlled fan would use approximately $531 in electricity, representing a significant savings. Although the example is quite basic and does not consider motor and drive efficiency, it does illustrate the potential savings variable-speed operations offer.
Variable-speed drives offer similar energy savings for pumps. Pumps typically control flow with throttling valves or by changing pump speed. Just as calculations show the energy savings possible for fans, similar calculations estimate energy savings for pumps. Generally, reducing pump speed by 20% reduces motor horsepower by nearly 50%.
In addition to energy savings, variable-speed drives relieve stress on mechanical systems by starting gradually and ramping up to speed. The correct drive can also tighten process control.