AC drives operate by converting the incoming AC power to a DC signal and then re-transmitting the power signal to the motor at varying frequencies and voltages. AC drives can operate rotating equipment at speeds ranging from nearly 0RPM to as high as 150percent of the rated speed for the motor. The use of an AC drive requires installing high-efficiency Class F insulated motors that can withstand the variations in voltage and current flux.
Some good applications include 1) replacing outlet dampers or variable inlet vanes (VIVs) in supply fan systems controlling variable air volume (VAV) boxes; 2) controlling air supply to multiple zones by adding an AC drive and isolation dampers in zones with different occupancy and operating schedules; and 3) controlling pump speed by maintaining a pressure setpoint. Other advantages of AC drives include less wear on the motor due to reduced speed and torque, gentler starting through gradual acceleration, and fewer moving parts (i.e., no damper or inlet vanes to wear out). Following Figure shows a typical fan application requiring two AC drives with pressure control feedback.
Energy Information
The real energy savings of pumps with AC drives come from the basic laws of fan and pump operation. Theoretically, fan power input will dro according to the cube of the flow rate (load). This is illustrated in the equation below:
W2=W1×(Q2/Q1)3
As an example, if the Power (W1) is at 100percent flow (Q1) and the flow is reduced to 80percent, the new Power (W2) will be
W1 × (0.8)3 = 0.51 or 51 percent.
When a fan is throttled back by an inlet vane, the power drops along with the flow rate to a certain value. However, the poor inlet or outlet conditions cause fan performance to suffer, motor efficiency drops as the load is removed, and power savings are reduced. In the case of an AC drive, the motor and fan speed are reduced and the losses are much smaller, approaching that of the ideal case.
Comparison
Below figure illustrates the difference in power consumption of a typical AC drive compared to three common retrofit applications, including VIV control, outlet damper control and constant speed. To estimate the energy savings from an AC drive installation, the duty cycle of the equipment must be established. For example, if a fan system was off for eight hours per day, and operated for eight hours at 50percent flow, six hours at 80percent flow and two hours at 100percent flow, the power saving from AC drive control would be 40percent over an inlet vane and 55percent over outlet damper control.
