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Is there any tips save energy with variable frequency inverters? 

Energy conservation is the talk of the day due to ever increasing energy costs. In a process plant, variable speed drives (VSDs), i.e., mechanical speed variators and hydraulic torque converters, have been used with induction motors to save energy. This is because induction motors, while the workhorses of industry, have a serious limitation; they are essentially constant-speed machines, and are inefficient when used with equipment that needs to operate at different speeds. 
In mechanical speed variators, an electric motor powers equipment via a belt. The motor pulley and the pulley of the driven device, mounted on splined shafts, have specially shaped and spring-loaded discs. Splined shafts are those having several shallow slots, instead of the customary single keyway. A lever moves the discs in or out along the axis of the shafts. This causes the belt to take different positions in the drive and driven pulleys, in effect, varying the diameter and, thus, the speed of the driven machine, though the motor operates at its rated speed. 
A hydraulic torque converter is a combination of a pump and a hydraulic turbine in single oil-filled casing. The motor drives the pump shaft, and the oil pressure delivered by the pump drives the hydraulic turbine shaft, with the equipment coupled to it. A set of guide vanes positioned by a lever emanating from the casing adjusts the angle of impingement of the oil onto the blades of the hydraulic turbine to vary the speed. Speed ratios range from 0.25 to 1. However, this device is suitable for large drives only (e.g., greater than 300 hp).
Both of these variation systems are complex, uneconomical and have limited options. Their need for high maintenance and limited applicability are key factors in making them uneconomical. 
Today, there is another option. Thanks to solid-state power electronics, the variable frequency inverter (VFI) permits stepless speed variations of 0.2 to 4 times the rated speed of induction motors. The system is energy efficient and requires practically no maintenance, eliminating the problems associated with the other two VSDs. 
VFIs accept single- or 3-phase 60-Hz AC, and convert it into 10-200 Hz AC. In addition, they change the output voltage to maintain the ratio of voltage to frequency (V/f) constant up to 60 Hz, and then keep the voltage at the line value - a requirement of induction motors to avoid overheating. A VFI renders the supply voltage into DC, then into variable-frequency AC to run a motor at different speeds. This is because the speed of an induction motor is proportional to the power frequency. 
VFIs are made up of a rectifier and inverter. The rectifier converts the AC into DC by using solid-state diodes. Most industrial motors use 3-phase AC and, so, a 3-- phase rectifier is necessary. The inverter changes DC back into AC, also via solid-state electronics. Such devices can produce outputs to even 400 Hz. To keep the ratio Vlf constant, the circuit varies the duration of time on vs. time off. Their energy savings potential can be realized rather quickly, usually with a payback of only a few months. 
To illustrate the benefits of VFIs, consider the following scenario for a fixed- and a variable-system resistance (VSR) in a pump or fan system. The pump delivers liquid into an absorber operating at 35 bar and has a fixed system resistance (FSR) of 35 bar; the frictional loss across the piping, heat exchangers and the rest of the system constitute the VSR. The VSR is proportional to the square of the flow rate. The sum of the two is the total system resistance. 
Centrifugal pumps. Conventional systems use a control valve at the discharge of the pump. The pump runs at full speed and the additional head generated is wastefully dissipated at the valve during lower flow rates. A motor powered through a VFI allows the pump to run at the correct speed, avoiding such dissipation. Let us calculate the energy savings of a pump that uses 100 kW at full throughput, but is run at only 70% for 12 h a day, i. e., 4,000 h/yr. Table 1 shows the energy savings for the cases, and a short payback for the second case. In this and the following table, P = power, N = speed, H = head at 100% flow, and Q = flowrate. 
The first case is at 70% of maximum flowrate with no VSR, and the second, also at 70% of flow, has a VSR = 50 m. The energy savings are substantial in both cases. In addition to the savings shown in Table 1, there will be reduced wear and maintenance of the motor owing to low-speed running, minimizing the purchase cost and recurring maintenance of control valves and other pieces of equipment. 
Positive-displacement pumps. Consider the top column pressure-control system of a 250,000 bbl/d crude distillation column that uses a screw compressor driven by a motor with the following specifications: 6.6 kV, 1 MW, 3,000 rpm, and 50 Hz power supply (the unit is not in the U.S.). A VFI powers the motor. The compressor runs at 1,800-2,100 rpm for most of the time to maintain the column pressure. Assume that the variable- and fixed-system resistances are 5 and 30 bar, respectively, at 100% flow. Again, the energy savings are substantial (Table 2). The cost of the inverter would be almost the same as that for the control system of a constant-speed motor. Thus, the plant enjoys a recurring annual savings via of lower power costs and reduced motor maintenance due to low-speed runs, with little or no additional capital expenditure.

Drives for large equipment. Drives for large rotary-- drum dryers, granulators and cement kilns usually have a motor driving a gear reducer. The output shaft drives a shaft-mounted pinion held between two bearings. The pinion, in turn, rotates the drum (at 
The drum of a modern cement kiln acts as the rotor, passing through stator of an 800-kW induction motor. The stator coils, wound for 120 poles and powered by an inverter of 10 Hz output, drive the rotor (kiln drum) at 10 rpm. All maintenance and high-cost items - the pinion, bull gear and gear reducer, and their recurring lubricating and repairing expenses - are eliminated. 
Retrofitting VFIs