Speed control

In addition to the speed control system, a second overspeed system was developed to prevent operation of a rotor above its maximum rated 

---

A powerful stirrer, driven by a flexible driving shaft between the motor (I h.p.) and the stirrer, is depicted in Fig. II, 7, 3. The motor may be placed at a distance from the stirrer head and reaction vessel, thus enabling the assembly to be used for inflammable, corrosive or fuming liquids without damage to the motor. Furthermore, any laboratory retort stand and clamp may be used since the stirrer head weighs only about 250 grams. A variable speed control (500-2000 r.p.m.) is provided. 

The centrifugal pump directly driven by a variable-speed electric motor is the most commonly used hardware comoination for adjustable speed pumping. The motor is operated by an electronic-motor speed controller whose function is to generate the voltage or current waveform required by the motor to make the speed of the motor track the input command input signal from the process controller. 

The most popiilar form of motor speed control for adjustable-speed pumping is the voltage-controlled pulse-width-modulated (PWM) frequency synthesizer and AC squirrel-cage induction motor combination. The flexibility of apphcation of the PWM motor drive and its 90 percent- - electric efficiency along with the proven ruggedness of the traditional AC induction motor makes this combination popular. 

Motor-driven compressors usually operate at constant speed, and other methods of controlhng the capacity are necessary. On reciprocating compressors discharging into receivers, up to about 75 kW (100 np), two types of control are usually available. These are auto-matic-start-and-stop control and constant-speed control. 

Constant-speed control should be used when gas demand is fairly constant. With this type of control, the compressor runs continuously but compresses only when gas is needed. Tiiree methods of unloading the compressor with this type of control are in common use (1) closed suction unloaders, (2) open inlet-valve unloaders, and (3) clearance unloaders. The closed suc tion unloader consists of a pressure-actuated valve which shuts off the compressor intake. Open inlet-valve unloaders (see Fig. 10-89) operate to nold the compressor inlet valves open and thereby prevent compression. Clearance unloaders (see Fig. 10-90) consist of pockets or small reservoirs which are opened when unloading is desired. The gas is compressed into... 

Motor-driven reciprocating compressors above about 75 kW (100 hp) in size are usually equipped with a step control. This is in reality a variation of constant-speed control in which unloading is accomplished in a series of steps, varying from full load down to no load. Three-step eontrol (full load, one-half load, and no load) is usually accomplished with inlet-valve unloaders. Five-step eontrol (fuU load, three-fourths load, one-half load, one-fourth load, and no load) is accomphshed by means of clearance pockets (see Fig. 10-91). On some machines, inlet-valve and clearance-control unloading are used in combination. 

The first method is used most frequently. The next preference is for the last method, mostly used in small compressors due to problems with speed control of electrical motors. Other means of capacity control are very seldom utilized due to thermodynamic inefficiencies and design difficulties. Energy losses in a compressor, when capacity regulation is provided by lifting the suc tion valves, are due to fric tion of gas flowing in and out the unloaded cylinder. This is shown in Fig. 11-84 where the comparison is made for ideal partial load operation, reciprocating, and screw compressors. 

The two principal elements of evaporator control are evaporation rate a.ndproduct concentration. Evaporation rate in single- and multiple-effect evaporators is usually achieved by steam-flow control. Conventional-control instrumentation is used (see Sec. 22), with the added precaution that pressure drop across meter and control valve, which reduces temperature difference available for heat transfer, not be excessive when maximum capacity is desired. Capacity control of thermocompression evaporators depends on the type of compressor positive-displacement compressors can utilize speed control or variations in operating pressure level. Centrifugal machines normally utihze adjustable inlet-guide vanes. Steam jets may have an adjustable spindle in the high-pressure orifice or be arranged as multiple jets that can individually be cut out of the system. 

Secondary Control of Wound-Rotor Motors Wound-rotor motors may be effectively reduced-voltage-started or have their speed controlled by using external secondaiy resistance. The addition of resistance into the secondaiy circuit of a wound-rotor motor reduces the starting current and affects the speed under load conditions. 

Speed-Control Systems The most common sensing element is mechanical some systems are hydraulic or electronic. For valve positioner they all have a hydrauhc servo as first choice, with an occasional choice of pneumatic for lighter loads. 

The speed control operates the governing valve to maintain steam flow commensurate with load demand while holding speed essentially constant. For sudden load changes there will be a short-time overshoot, and a special case is the instantaneous loss of load, load dump at full load. The usual specification states that the overshoot on load dump must not exceed 9 to 10 percent of rated speed. The settled speed rise will of course be equal to the regulation, 4 or 6 percent for a NEMA Class C or B governor and less than I percent for Class D. 

The trip valve is provided as a second line of defense in case of overspeed The trip valve is frequently equipped with a trip-actuating solenoid which can be operated by push button, by low od pressure, or by some other process upset. When the speed control functions as described above, the trip will not be actuated by load dump. 

Control system, speed control, pressure control, and process control, so that consideration can be given to remote control, speed or pressure variation that can be tolerated, and system response speed... 

Automatic speed control of slip-ring motors 5/95... 

But where accurate speed control is the process requirement, static controllers, termed slip recovery systems (Section 6.16.3) are recommended, which in addition to exercising extremely accurate speed control, also conserve slip losses. Static drives are discussed in Chapter 6. Below we will describe a procedure to determine the value of resistance, its steps and switching and control schemes for these steps for a rotor resistance starter. 

An electrolyte starter is almost a standard product like a motor and the manufacturer, depending upon the number of starts and the speed control requirement, can adjust the quantity of electrolyte, depth of electrodes etc. 

The resistance units, when used only for starting purpose, are in the circuit for only a short time and are thus short-time rated. However, when they are employed for speed control, braking or plugging operations, in addition to the starting duty, they may be rated for continuous duty. The resistance units are thus classified according to their duty demand, i.e. number of operations per hour (c/h). See Chapter 3 on the duty cycle. 

Moreover, high-temperature variation may result in large variations in the resistance of the grid and may vary the performance of a variable-speed drive if care is not exercised in selecting a proper alloy of the resistance. An alloy such as aluminium-chrome steel should be preferred when it is required to perform a speed control or a speed varitition. as discussed above. 

By makitig the electrodes move through ti geared motor, it is possible to tiehieve even automatic speed control of slip-ring motors through such starters. 

Speed control through. solid-state technology 6/99... 

Speed control in slip-ring motors has been discussed in the previous chapter. Squirrel cage motors have limitations ill their speed control in view of their fixed rotor parameters. Speed variation, in fixed steps, however, is possible in such motors if the stator is wound for multipolcs and such motors arc known as pole changing motors. Up to four different speeds can be achieved in such motors economically, in combinations of 2/4, 4/6. 4/8, 6/8, 6/12, 2/4/6. 4/6/8. 2/4/6/12 and 4/6/8/12 poles etc, or any other similar combination. For limitation in the motor size and tlux distribution, winding sets of more than two are not recommended. I he two windings can be arranged for two. three oi (maximum) four different speeds. 

---