Frequency-controlled motor

The core element is a frequency-controlled motor and a holder for the rotating substrates (e.g., disks). Depending on the application, liquid, optical, and possibly electrical interfacing between rotating onboard components and stationary lab-frame components such as dispensers or detection units have to be incorporated. A process control software has to synchronize events such as liquid handling or optical readout to the periodic motion of the substrate. Figure 14 shows device concept of the LabCD platform. 

Roots Vacuum Pumps with Bypass Valve, Respectively with Frequency Controlled Motor... 

The starting torque ciurent is substantially less for an adjustable-frequency controller/motor combination than the locked rotor current for an induction motor started across the line. This results in a soft start for the controller/motor... 

V ri ble Frecjuency Drives. An important energy by-product of soHd-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 direcdy to deUver the right flow without the need for a control valve and its inherent pressure drop. Eigure 11 shows that at rated load the variable speed drive uses only about 70% as much power as a standard throttle control valve system, and at half load, it uses only about 25% as much power. 

This is an important and useful motor drive arrangement for many process and nonprocess applications. For a variable-frequency controller, see reference 94. 

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. 

Options for connecting the motor drive to the shaft depend on the shaft orientation. A vertical-shaft cantilever design would prefer a belt drive to reduce the cost of manufacture of the support structure and to facilitate maintenance. A horizontal shaft has the additional option of direct coupling. Variable speed can be accomplished through a gearbox or preferably through variable frequency control on the motor. In addition to the power requirements discussed previously, the startup power to overcome the torque of the rotor must be considered. This startup power is related to the time required to reach the desired rotor speed. 

The rotary atomizer machines are high-speed machines traditionally built with a step-up gear to increase the speed from the 3000 or 3600 rpm of the standard two-pole electric motors to 10,000 to 20,000 rpm normally required to achieve sufficiently fine atomization. Newer designs feature high-speed electric motors with frequency control of the atomizer speed. Table 12-41 gives the main operational parame-... 

The major advantages of variable frequency controllers for submersible motors are -... 

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. 

Since about 1990 the paddle or infeed rolls can also be driven separately, each from a slip-on type geared motor. Frequency control gives infinite variabihty of the paddle shaft speeds, so that these can be optimally adapted to the auger shaft speed (Fig. 38). 

Occasionally d.c. motors were used in the 1980s in order to provide an infinitely-variable control of the auger shaft speeds, but since then frequency-controlled squirrel-cage motors have become the standard. 

Control of input, flow, or pressure is generally more gradual and more precise with variable frequency drives (VFD see glossary) on pumps, blowers, and fans than with control motors and valves, or (worse yet) with dampers. If many zones are supplied from one blower, VFD is not practical therefore, careful linearization of both actuators and valves is necessary. 

Provide variable-frequency drive motors for pumps and blowers wherever possible instead of throtthng the delivery valves for flow control. 

Special consideration is required for heavy loads such as crushers, high capacity electric furnaces, big pumps, or blowers which may cause flucmations in the incoming supply grid and disturb other nearby industries or power consumers. Hence, arrangements must be provided in the equipments to start on a minimum load and then slowly increase the load. Some of the methods for this could be provision of variable-frequency drive motors, throttling inlel/exit valves, controlled feed rates of solid lumps to crushing machines, etc. 

Figure 15.9. The operating temperature in the dryer is set at the desired level by adjusting the condenser capacity, and the required air humidity is maintained by varying the compressor capacity via frequency control of the motor speed. This set-up permits drying at temperatures from -20°C to 60°C and air humidities from 20% to 90%. The flow rate of the drying agent (air) permitting a superficial velocity from 0.3 to 5.0 m/s is adjusted by controlling the fan rpm.

Extruder motors are usually electric, but some systems utilize hydraulic motors. For example, injection molding machines use hydraulics to develop clamp tonnage. Electric motors may be of the direct current (DC) or alternating current (AC) types. Traditionally, DC motors, which regulate speed through voltage control, have been more popular because they could provide the necessary power at a lower cost. However, recent advances in frequency control - the technique used to regulate speed in AC motors - have caused this type of motor to become more widely used. 

A more suitable and modern approach in coimection with chemistry applications is that of smoothly cutting-in a Roots vacuum pump in to a process without relying on a bypass valve but instead by using a frequency and thus speed controlled motor. Now the Roots vacuum pump wUl no longer start to operate at a certain operating pressure at full speed. Under certain circumstances it may be started even at atmospheric pressure, but under speed, respectively frequency control, and at a lower rotational speed and thus a low theoretical volumetric speed. The characteristic is the same as for a Roots vacuum pump equipped with a bypass valve, except that now the valve can no longer fail because it is missing. 

Induced draught fans shall be used to maintain a negative pressure in the total system. Radial fans are the preferred type of off-gas fan. Control of the off-gas fans may be ensured by throttle dampers or by frequency control of the electric motor. The casing and frame of the off-gas fans shall be made of steel or special alloys if necessary. The fan casing should be provided with a condensate drain at its lowest point. Vibration monitors should be provided on the induced draught fans. 

Limits to practical operation of motors controlled by variable frequency usually involve heat dissipation and noise. As a TEFC motor is slowed, the attached fan becomes less able to remove heat by blowing air across the outside of the motor. At low speeds (low frequency) the motor can also be very noisy. Another limit to motor design is usually torque, so as the speed is reduced, so is the output power. A variable speed motor at one-fourth speed can produce no more than... 

It is possible to rewire a three-phase motor into a single-phase motor, and thus reduce its speed from 3600 to 1800 rpm. Then its power consumption would drop by a factor of 8. I ve done this once at an East Coast refinery, which had a hugely oversized sulfur plant incinerator air blower. For most projects, a more typical option would be to retrofit the motor for variable-speed operation, using frequency control. However, not all motors are amenable to frequency control, which is a relatively expensive option. Feedback from my clients that have retrofitted pump drivers with variable-speed AC motors is often unfavorable. 

Fig. 22.24 Illustration of the spray drying tower. The feed is pumped from the vessel (2) through the pipe (5) to the LamRot (4), which is coupled with the motor (5) by the shaft. The fans (6, 7) and heater (8,9) prepare the air for the drying process. The main product is collected in a vessel (10) under the conical part of the tower. The drying air still containing small product particles leaves the tower through the pipe (11). The exhaust air than passes a cyclone (12) and a pocked filter (13) subsequently. The suction fan (14) controls the pressure within the spray tower. (7) and (14) are frequency controlled...

The concept of variable friction drives is described and a comparison is made between frequency controlled AC and hydraulic drives. Some of the features and benefits of using hydraulic drives on a roll mill and mixer are ontlined and a functional description is given of a radial-piston type motor with a rotating cylinder block/hoflow shaft and a stationary motor case/housing (Hagglunds Marathon motor). 

Speed control by way of adjusting power frequency is becoming more and more important for economical throughput or presstme capacity variation of modern process machinery. Several key parameters that must be considered when applying induction motors to adjustable frequency controllers include the load torque requirements, current requirements of the motor and the controller ciurent rating, the effect of the controller wave-shape on the motor temperature rise, and the required speed range for the application. 

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