Constant Power mode

Most systems are equipped with power supplies that can be operated in constant voltage, constant current, and constant power modes. Common limits are 30 kV and 300 liA for constant voltage and constant current, respectively. Software-controlled polarity switching and programmable gradients are conveniences. 

An advantage of the constant power mode is that the voltage applied to the IEF cell automatically increases as the current drops. However, constant... 

Figure 2. The method ofcontrolled pulse heating ofa thin wire probe characteristic heating curves in the constant power mode P(t)-const (2a) and the temperature plateau one Tpi = T(t > tg) -const (2b). Here tg is the time period required for transition to the regime. Here ami further, arrows show the moment of spontaneous boiling-up (t = t ) for the liquids.

FIGURE 2.1 Particle size distribution of fresh and used Pt/C electrocatalysts, tested in constant-power mode, 0.12 W cm" at 333 K over 529 h. (Reproduced from Guilminot, E. et al. 2007b. /. Electrochem. Soc. 154 B96-B105. With permission from The Electro Chemical Society.)... 

Variable microwave power means that the microwave energy is emitted in a pulsed mode by turning on-off the microwave power during the drying process, or that the microwave emission is declining continually with time. Compared with the constant power mode, the variable microwave power mode is able to stabilize the temperature of the sample at a value that depends on the microwave power used, the duration of the power-on and power-off periods, and the other operating conditions, which indicates that thermal runaway can be inhibited in the variable microwave power mode. 

In the constant-power mode, the current is lowest at the beginning of the discharge and increases as the battery voltage drops in order to maintain a constant-power output at the level required by the equipment. The average current is lowest under this mode of discharge, and hence, the longest service time is obtained. 

FIGURE 14.53 Discharge characteristics of cylindrical (spirally wound electrode) Li/MnO cells (CR123A-size) under constant-power mode at 20°C. 

The discharge characteristics of the nickel-cadmium battery under the constant-power mode, at several different power levels, are shown in Fig. 28.18. These are similar to the data presented in Fig. 28.7 for constant-current discharges, except that the performance is presented in hours of service instead of percent discharge capacity. The power levels are shown based on the -rate. The -rate is calculated in a manner similar to calculating the C-rate, but based on the rated watthour capacity. For example, for the E/5 power level, the power for a battery rated at 780 mWh is 156 mW. 

The discharge characteristics of the nickel-metal hydride battery under the constant-power mode, at several different power levels, are shown in Fig. 29.12. These are similar to the data presented in Fig. 29.3a for constant-current discharges, except that the performance is... 

By way of comparison, Jonsson et al. [6] used Kanthal AE and Kanthal AF as reference materials in their recent paper on Cyclic oxidation testing by resistance heating . The wire samples were U-shaped, 224 mm long and 0.7 mm dia. and therefore similar to the larger diameter wires used in this study. The samples were tested in constant power mode using a 2 min on/2 min off cycle at a power of 155 W (a peak surface load of 31.5 W/cm ). The temperature achieved at the hottest part of the sample was 1265°C. Such results agree well with the trends observed in this study, confirming our belief that this new rapid cycle, environmental test facility can test to the ASTM standard. 

Current mode control has an inherent overcurrent protection. The highspeed current comparator provides pulse-to-pulse current limiting. This form of protection is a constant power form of overload protection (see Section 3.11). This form of protection folds back the current and voltage to maintain a constant power into the load. This may not be optimum for all products, especially where the typical failures slowly increase the failure current. Another form of overload protection can also be placed in the circuit. 

Mossbauer spectra are usually recorded in transmission geometry, whereby the sample, representing the absorber, contains the stable Mossbauer isotope, i.e., it is not radioactive. A scheme of a typical spectrometer setup is depicted in Fig. 3.1. The radioactive Mossbauer source is attached to the electro-mechanical velocity transducer, or Mossbauer drive, which is moved in a controlled manner for the modulation of the emitted y-radiation by the Doppler effect. The Mossbauer drive is powered by the electronic drive control unit according to a reference voltage (Fr), provided by the digital function generator. Most Mossbauer spectrometers are operated in constant-acceleration mode, in which the drive velocity is linearly swept up and down, either in a saw-tooth or in a triangular mode. In either case. 

Regulated direct current (DC) power supplies designed for electrophoresis allow control of every electrophoretic mode. Constant voltage, constant current, or constant power conditions can be selected. Many power supplies have timers and some have integrators allowing runs to be automatically terminated after a set time or number of volt-hours (important in IEF). All modes of operation can produce satisfactory results, but for best results and good reproducibility some form of electrical control is important. The choice of which electrical parameter to control is almost a matter of preference. The major limitation is the ability of the chamber to dissipate the heat generated by the electrical current. 

P 8] The resistance between the electrodes was measured in situ. The power supply was operated in constant-voltage mode [96], A low-volume tracer was injected with a syringe and then applied a voltage of 4 V across the channel. The current was of the order of pA and was below the resolution level of the power supply (10 mA). 

Not surprisingly, the final temperature of the solvent relies on the volume used, especially if experiments are performed at constant power. In such experiments, a decrease of the final temperature was observed with increased volume. Obviously, it is not possible to directly compare single-mode experiments with multimode experiments at an identical output power. Due to the significantly higher power density, the heat transfer of single-mode reactors is substantially higher. 

A Heath/Schlumberger dc power supply, Model SP-2711, 30 V, 3 A, operating in its constant current mode, was used. 

The Mossbauer transmission spectra were recorded in the constant acceleration mode with an Elscint Mossbauer drive unit and a model MFG 3A Elscint function generator, an MVT-3 linear velocity transducer and an MD-3 transducer driving unit, y-ray detection was done with a Reuter-Stokes Kv-CH4 proportional counter driven by an Ortec 401A/456 high voltage power supply. Voltage pulses were introduced into an Ortec 142 PC preamplifier and an Ortec 571 spectroscopy amplifier. Data were collected on a Tracor-Northern NS-701A multichannel analyzer. The data were later analyzed on an IBM 360/370 computer. 

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