Variable autotransformers

Power delivered by the tetrodes to the output transformer, and thence to the corona reactor load, is controlled by the oscillator and the variable autotransformer in the tetrode plate power supply. The autotransformer control keeps the total power consumed by the equipment during an experiment as low as possible, to minimize heat developed in the cabinet and to extend tube life. 

For alternating supplies, the conventional method is to use a motor-driven autotransformer. In this case it is necessary to ensure that the steps caused by the turns of the autotransformer are less than 2% of the result. Since most variable autotransformers arc wound with 100 turns, it is necessary that the result be greater than 50% of the maximum output, necessitating a further stae of adjustable transformer ratio if a wide range of materials is to be tested. 

For measurements of heat transfer, the NBS 0.1-ohmstandardresistor was replaced by a manganin shunt ( 0.001 ohm) which had been calibrated in terms of the 0.1-ohm standard, and the battery was replaced with a three-phase bridge rectifier driven from a variable autotransformer. Filtering reduced ripple to 0.7% peak to peak. 

Equipment plugged into an electrical receptacle should include a fuse or other overload protection device to disconnect the circuit if the apparatus fails or is overloaded. This ova-load protection is particularly useful for equipment likely to be left on and unattended for a long time, such as variable autotransformers (e.g., Variacs and powerstats), vacuum pumps, drying ovens, stirring motors, and electronic instraments. Equipment that does not contain its own built-in overload protection should be modified to provide such protection or replaced with equipment that provides it. Overload protection does not protect the worker from electrocution, but it does reduce the risk of fire. 

Do not use variable autotransformers to control the speed of an induction motor because such operation will cause the motor to overheat and perhaps start a fire. 

Schematic diagram of properly wired variable autotransformers.

The external cases of all variable autotransformers have perforations for cooling by ventilation, and some sparking may occur whenever the voltage adjustment knob is turned. Therefore, these devices should be located where water and other chemicals cannot be spilled onto them and where their movable contacts will not be exposed to flammable liquids or vapors. Variable autotransformers should be mounted on walls or vertical panels and outside of hoods they should not simply be placed on laboratory benchtops. 

Pressure-containing systems designed for use at elevated temperatures should have a positive temperature controller. Manual control using a simple variable autotransformer, such as a Variac, is not good practice. The use of both a back-up temperature controller capable of recording temperatures and shutting down an unattended system is strongly recommended. 

Autotransformers are frequently wound on a cyh-ndrical core, and the output taken from a movable contact as shown in Fig. 2.40. Such a variable autotransformer could provide a voltage from 0 V to about 10% above the Hne voltage. 

Yield of product and tin content was studied as a function of stirring rate. The blender was plugged into a Powerstat Variable Autotransformer (Type 16B Superior Electric Company, Briston, Connecticut), where stirring rate was varied as a function of voltage output. 

XI.6.4 After setting the hydrogen flow, connect the heater to a variable autotransformer. The setting should be approximately 55. Allow the catalyst to condition for 24 h at 300°C. Norm operating temperature for the methanator should be 325 25 C. 

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