Adjustable Frequency Drive

The three basic types of drives are alternating current (ac), direct current (dc), and hydraulic. While a number of drives have been used in extruders, the most common are dc silicon control rectified (SCR) and ac adjustable frequency drives. A dc SCR drive is a sohd-state dc rectifier connected to a dc motor. The base speed is about 1 percent, but reduces to 0.1 percent when a tachometer is added to the drive. These drives are very reliable, can handle high starting torques, can maintain a constant torque through a speed range of 20 1, and are relatively easy to maintain (that is, replace brushes). However, since the drives have brushes, they are limited to noncorrosive polymers. 

The three basic types of drives are alternating current (AC), direct current (DC), and hydrauhc. Although a number of drives have been used in extruders, the most common are DC silicon control rectified (SCR) and AC adjustable frequency drives. 

Enei y aver, Adjustable frequency drives, GE Fuji Drives USA, Inc. 

The two AC drive systems used on extruders are the adjustable transmission ratio drive and the adjustable frequency drive. The adjustable transmission ratio drive can be either a mechanical adjustable speed drive or an electric friction clutch drive. 

The ratio of voltage to frequency must be held constant to maintain a constant torque capability as the motor speed is varied. Almost any speed/torque characteristic can be obtained by varying the voltage to frequency ratio. Because of limitations of the SCR cells, the maximum rating of adjustable frequency drives is presently around 300 hp. As better SCR cells are developed, this maximum rating is likely to increase. 

The line-to-shaft efficiency of DC motors is around 0.85 and about 0.80 for variable frequency AC motors. Considering that a typical two-stage gearbox has an efficiency of about 0.95, the overall efficiency for a DC drive is about 0.80 and about 0.75 for a variable frequency AC drive. The overall efficiency of a well-designed hydrostatic drive is around 0.70. Thus, the hydrostatic drive compares reasonably well with the AC and DC drives with regards to overall efficiency [31]. The drive efficiency of the DC motor drive and the adjustable frequency drive increases with rated speed, whereas the drive efficiency of the hydraulic drive is relatively independent of rated speed. The efficiency of the DC motor is better than other motors in the range of 20 to 100% rated speed below 20% rated speed the hydraulic drive is more efficient. 

The merits of a variable-speed motor would appear to be obvious, as many compressors in the past have benefited from the variable speed available in a steam turbine. A compressor may be adjusted as required to meet the process needs. The advent of the variable-frequency drive returns some of the benefits to the process operator that were lost when the more favor able electric energy caused motors to replace steam turbines. 

Inverter-AC Motor Drives. An adjustable-frequency control of AC motors provide efficient operation with the use of brushless, high-performance induction, and synchronous motors. A typical system is shown in Figure. 3-14. Such a system consists of a rectifier (which provides DC power from the AC line) and an inverter (which converts the DC power to acljustable-frequency AC power for the motor). Inverter cost per kilowatt is about twice that of controller rectifiers thus the power convertor for an AC drive can approach three times the cost of a DC drive. 

Figure 3-14. Typical invester AC motor drive consisting of rectifier-DC link, adjustable-frequency inverter, and induction of synchronous motor [10].

Advantages Good long-term frequency stability, moderate distortion, easily adjusted, moderate drive capability Disadvantages High parts count... 

Variable Frequency Drives. An important energy by-product of solid-state electronics is the relatively low cost variable speed drive. These electronic devices adjust the frequency of current to control motor speed such that a pump can be controlled directly to deliver the right flow without the need for a control valve and its inherent pressure drop. Figure 11 shows that at rated load the variable speed drive uses only about 70% as much power as a standard throtde control valve system, and at half load, it uses only about 25% as much power. 

Adjustable speed drives (ASDs) produce noise signals that are very often troublesome. The noise frequency generated by the ASDs is typically higher than the harmonic frequencies of the fundamental voltage. Because of this, the noise could find its way into sensitive data and signal circuits unless such circuits are sufficiently isolated from the ASD power lines. 

Harmonic number (h) refers to the individual frequency elements that comprise a composite waveform. For example, h = 5 refers to the fifth harmonic component with a frequency equal to five times the fundamental frequency. If the fundamental frequency is 60 Hz, then the fifth harmonic frequency is 5 x 60, or 300 Hz. The harmonic number 6 is a component with a frequency of 360 Hz. Dealing with harmonic numbers and not with harmonic frequencies is done for two reasons. The fundamental frequency varies among individual countries and applications. The fundamental frequency in the U.S. is 60 Hz, whereas in Europe and many Asian countries it is 50 Hz. Also, some applications use frequencies other than 50 or 60 Hz for example, 400 Hz is a common frequency in the aerospace industry, while some AC systems for electric traction use 25 Hz as the frequency. The inverter part of an AC adjustable speed drive can operate at any frequency between zero and its full rated maximum frequency, and the fundamental frequency then becomes the frequency at which the motor is operating. The use of harmonic numbers allows us to simplify how we express harmonics. The second reason for using harmonic numbers is the simplification realized in performing mathematical operations involving harmonics. 

Variable frequency drives (VFDs) are sometime used to adjust the operation of typical (US) standard of 480VAC, 3PH, 60hz operation of the motor. The functionality of a VFD is to convert frequency measured in Hertz (Hz) to motor speed. One Hz equals 1 cycle per second. When voltage is being received (input to the VFD), it is in the sinusoidal waveform. The sine wave is converted to a digital square wave that now controls the revolutions per minute (RPM) of the motor. 

A variable frequency drive (VFD) adjusts the speed of a motor to alter the discharge pressure (see Chapter 6.2). This is useful when... 

The use of variable-speed drives in pilot plant and manufacturing plant vessels is recommended for development and scale-up of crystallization processes. This capability provides the opportunity for critical experimentation at the pilot plant scale to determine the effect of impeller speed on PSD and other variables. On the manufacturing scale, the ability to change impeller speed is the most readily adjustable parameter for manipulation on scale-up. Modem variable-frequency drives provide an excellent means to vary speed over a wide range. The added cost of variable-speed capability is minimal compared to all other methods of changing mixing... 

Adjustable Speed Drive - An electronic device that controls the rotational speed of motor-driven equipment such as fans, pumps, and compressors. Speed control is achieved by adjusting the frequency of the voltage applied to the motor. 

DC motor drives have always offered high torque at all speeds and exact control of motion speed. AC induction motors have reliably converted electricity into rotary power for many years, and recently adjustable-frequency controls add variable-speed capability. While AC motors were originally relegated to relatively simple tasks, such as varying the flow rates of fans or pumps, advances in both motor and control technologies have allowed their use in higher performance operations. They are reliable sources of fixed-speed and variable-speed rotating power. Electric drives with appropriate closed-loop control operate only when required. However, to avoid unsuccessful apphca-tions, it is important to properly match the load, motor, and controller. 

Use of Variable Frequency Drives (VFD) for the air blower motor to adjust the blower speed as per the production rate desired. 

The inlet pressure to the membrane array is adjusted manually by a globe valve (when the RO pump speed is controlled by a variable frequency drive (VFD) the membrane inlet pressure control valve does not require adjustment). The reject water flow rate is controlled manually by adjusting the reject flow control globe valve. Both valves are adjusted to achieve the desired PWR (%). The pressure required to achieve the permeate and reject flow rates is set during the initial run. As the membrane performance deteriorates with time, these valves require adjustments. 

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