We turn on household appliances at low mains voltage. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network, uninterruptible power supplies
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Unfortunately, often in rural areas, and often in cities, the mains voltage in houses is less than the nominal voltage, and in the evenings it drops so much that refrigerators, pumps, washing machines, etc. refuse to work. And if the problem of TV power supply can be solved by purchasing AC voltage stabilizer, then with asynchronous electric motors used in household appliances, everything is much more complicated: their launch is accompanied by significant current overloads, and buying a powerful stabilizer is unreasonably expensive.
On the other hand, for electric motors, the increased voltage is not so “painful” as its disadvantage: with a reduced voltage, they lose the ability to start, and as a result, the stator winding burns out. In other words, one can simply increase the voltage without much fear of further fluctuations in the network.
However, the step-up transformer is bulky and expensive. Therefore, to solve the problem, I propose a non-standard use of a conventional small-sized step-down transformer. You just need to add the necessary "volts" by summing the mains and voltage from the secondary winding of the transformer to the required output value. The principle of operation is easily explained by the diagram shown in the figure.
The figure shows that the current in the secondary winding and in the load is the same, respectively, the criteria for choosing a transformer are the required voltage and current in the secondary winding. There is no need to achieve an exact value of 220 V, already at 210 V everything will work.
You will need a transformer with a primary winding of 220 V, a secondary winding for the required "missing" voltage, and the maximum current of the secondary winding is sufficient even for low-power step-down transformers! It is easy to pick it up from the standard range for industrial transformers.
An approximate calculation (and this is more than enough for the task at hand) can be done using the following formulas.
First, we calculate the current of the secondary winding of the transformer - the load current:
where In - rated load current, A; Рн - nominal (passport) load power, W; UH - rated load supply voltage (220...230), V.
Then, knowing what voltage needs to be added, we determine the required power of the transformer
where P is the power of the transformer, W; I - rated current of the secondary winding, A; U2 - voltage of the secondary winding, V.
We select a ready-made transformer with suitable parameters for nameplate power and output voltage.
Now you can calculate the final result - the new voltage on the load:
where Ktr - transformation ratio; U - rated voltage of the primary winding (220), V.
From the last formula it can be seen that the voltage on the load can be both increased and decreased! To correctly phase a transformer, it is enough to swap the conclusions of one of its windings. The desired result can be achieved even by assembling a "garland" of several transformers: their primary windings should be connected in parallel, and the secondary windings in series and do not forget about phasing.
Let me give you a few practical examples.
- To increase the voltage by 30 ... 35 V for a pump with a power of 0,5 kW, it is enough to use a transformer with a voltage on the secondary winding of 42 V and a power of only 100 W.
- Lighting lamps in the entrance will last much longer if the voltage on them is slightly reduced - a 220/24 V transformer with a power of 160 W will easily "serve" the entire entrance of a multi-storey building with a total lighting power of 1,5 kW.
- A 1/6 V transformer with a power of 220 W will cope with the start of the electric motor of the refrigerator compressor (IN = 42, ZA, at startup - up to 63 A) with a voltage reduced by 45 V in the network.
Similarly, you can very effectively increase or decrease the voltage in a three-phase network - this will require, respectively, three transformers.
Author: Yu.Arkhipov, St. Petersburg
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