Control stepless

Moderate to accurate, depending upon the accuracy of controls. Stepless up to 20% of V, at constant h.p. and up to 33% of N, at constant torque is possible. Pumps, ID fans etc., that call for speed variation during a process need may not necessarily be too accurate. Or variation in flow of fluid, gas or temperature etc. not calling for very accurate controls, that such drives find their extensive use. It may be made more accurate, but at higher cost of controls... 

The inverter may be a current source inverter, rather than a voltage source inverter (.Section 6.9.4) since it will be the rotor current that is required to be vtiried (equation (1.7)) to control the speed of a wound rotor motor, and this can be independently varied through the control of the rotor current. The speed and torque of the motor can be smoothly and steplessly controlled by this method, without any power loss. Figures 6.47 and 6.48 illustrate a typical slip recovery system and its control scheme, respectively. 

They are suitable for stepless speed variation and can be controlled through speed, torque, temperature or flow of a process. They can also be programmed for any sequence of operation. 

A fast VAr control is achieved through thyristor switching, which by itself is capable of a stepless variation. But switching of capacitors, which are switched in banks, is not steples.s. The SVCs may be of the following types. 

The use of hydraulic systems for the setting, operation and control of machine tools has increased significantly. Hydraulic mechanisms being inter-linked with electronic controls and/or feedback control systems. In machine tools, hydraulic systems have the advantage of providing stepless and vibration-less transfer of power. They are particularly suitable for the linear movement of tables and slideways, to which a hydraulic piston may be directly coupled. 

Hot plates. The electrically heated hot plate, preferably provided with three controls — Low , Medium and High — is of great value in the analytical laboratory. The heating elements and the internal wiring should be totally enclosed this protects them from fumes or spilled liquids. Electric hot plates with stepless controls are also marketed these permit a much greater selection of surface temperatures to be made. A combined electric hot plate and magnetic stirrer is also available. For some purposes a steam bath may be used. 

In the proportional control mode, the final control element is throttled to various positions that are dependent on the process system conditions. For example, a proportional controller provides a linear stepless output that can position a valve at intermediate positions, as well as "full open" or "full shut." The controller operates within a band that is between the 0% output point and the 100% output point and where the output of the controller is proportional to the input signal. 

Component Electric Hoist Motors and Controls. Both hoists have enclosed, nonventilated, 220/440-V, three-phase, 60-Hz, 1200-rpm (rev/min) motors. The control used on hoist motions is referred to as the static stepless magnetorque control on the manufacturer s drawings. With this control, drive motor torque is controlled by means of fixed resistors and saturable reactors in the... 

A hydraulic drive generally consists of a constant speed AC motor driving a hydraulic pump, which, in turn, drives a hydraulic motor and, of course, the associated controls. The entire package is often referred to as a hydrostatic drive. Some of the advantages of a hydrostatic drive are stepless adjustment of speed, torque, and power smooth and controllable acceleration ability to be stalled without damage and easy controllability. 

True proportional control is obtained with solid-state switching of loads through SCRs, triacs, and similar solid-state devices. This allows a reduction of cycle time to the millisecond level. If the cycle time is reduced to one-half the power line period (8.3 ms for 60 Hz), then the proportioning action is referred to as stepless control or phase-fired control. A 50% phase-fired output is shown in Fig. 4.33. 

The BOR-60 pumps are driven by standard motors with stepless speed control. For the BN-350 pumps special squirrel-cage induction motors with two speeds (250 and 1000 rpm) are used. For the BN-600 pumps controllable synchronous motors are used coupled to synchronous-rectifier drivers with phase rotors. 

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