On the supply of heating elements with direct current. Scheme, description

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On the supply of heating elements with direct current. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electrician's Handbook

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Heating elements (NE) are not cheap, especially for powerful NE. Therefore, naturally, there is a desire to increase the service life of the NE.

It has long been known a way to extend the service life of NE by using several NE instead of one. Four identical NEs can be turned on so that they are equivalent to one in resistance, and hence in power. The reliability of such a battery increases several times. But in an electric stove, there will be little benefit from such an inclusion. Yes, and the cost increases four times.

The easiest way is to install a non-standard NE in the electrical appliance, i.e. more powerful, and reduce the voltage supplied to this NO. The NO will operate in an understressed mode, which ensures the electrical and thermal safety margin.

Electric oven repair. Replacing the wire NE with an identical one took a lot of time. Unfortunately, six months later, the coil burned out again. Further, a more powerful NE was installed in the electric oven, and more than one year has already passed, as they forgot about the repairs. It is possible to bring the power of the new NE to the level of the previous one by using a thyristor or triac power controller.

It is even easier to "knock down" part of the power by supplying the NE with direct current, for example, through a diode bridge rectifier. In this case, the voltage on the NO is equal to 190-200 V instead of 220 V. Observations have shown that when operating the NO with a diode rectifier, the service life increases by 1,3-1,5 times. We are talking about such appliances as electric stoves, electric ovens, boilers, irons. For the experiment, the same type of instruments were used under the same conditions, so we can talk about the objectivity of the observations. Let me give you the best example. My neighbor and I have the same electric stoves and work in the same conditions.

For 10 years, my stove, working through a rectifier, has never failed. Neighbor's NE changed at least 3 times.

For NEs operating on alternating current, there is also an unpleasant phenomenon associated with leakage to the case. There is a threat to human health when an electrical appliance begins to "beat" with current. The most annoying thing is that the electrical appliance is otherwise perfectly serviceable. This is why electrical appliances must be grounded (many of them are equipped with an appropriate terminal).

But which one of us does it? So, the power supply of the device with direct current allows you to do without grounding.

The problem of electric boiler and electrical safety. This very convenient device in everyday life does not have the ability to connect a ground wire (zero protection class). Before installing the diode bridge, the boiler was checked with a phase indicator and an ampervoltmeter, which showed the presence of alternating voltage on the body of the boiler and even in ... water.

According to the safety regulations, it is strictly forbidden to use electrical appliances in such a situation. But due to high prices for electrical goods and low salaries, our people are forced to use dubious electrical appliances. In civilized countries, no one rewinds soldering irons, restores heater coils, they are simply thrown away.

In principle, if the device "beats" with current, then sometimes the situation can be corrected simply by changing the position of the plug of the electrical appliance. If the place of breakdown of the spiral on the body is in the middle of the spiral, then a voltage divider is formed. The ratio of the shoulders of this divider determines how strong the blow you will receive. The strength of the blow also depends on how wet your hands are. So, if you install a rectifier in such an electrical appliance, the strength of the electric shock decreases sharply.

You can completely get rid of electric shock by using a standard three-pole plug with a grounding terminal. If this is not possible, find the "0" network and connect it to the body of the appliance. But you should make sure that the plugs of the electrical panel do not break exactly the zero bus (I have also met such cases). The fact that "miracle electricians" manage to connect a zero bus through a switch to a chandelier, I think, is known to many.

About fuses. In front of the diode bridge (in the break of the network wire) it is necessary to turn on the fuse. Usually, manufacturers of electrical goods do not bother to use a fuse. And the absence of a fuse leads to the fact that a short circuit inside the appliance can cause a fire in the room. At the same time, they rely on fuses (plugs) in electrical panels. And if there are "bugs"? We can only advise you to install fuses in your electrical appliances. If the NO is powered through the diode bridge, then the fuse, among other things, "saves life" for the diodes in emergency situations. This is possible due to the fact that diodes are able to withstand large current overloads in a short period of time. For NE with a power of 1 kW, the diodes are placed on three heat sinks. On one of them with an area of 200 cm2, two diodes are placed, and on the other two, with an area of 100 cm2, one diode each. Diodes of the D246 type (others for a voltage of more than 400 V and a current of at least 5 A are also suitable). It is better to have double the current margin. As a material for heat sinks, you can use any sheet (for example, tin plate with a thickness of 0,5-0,8 mm). For a load of 10 A, the diameter of the copper wire of the fuse is 0,25 mm, for a load of 20 A, 0,41 mm.

About bimetallic contact power regulators. These are often used in household electrical appliances. Over time, such regulators lose their reliability and stability of parameters, and then the operation of the devices is possible only when the contacts of the regulators are short-circuited. And the very principle of operation of bimetallic regulators contributes to the accelerated destruction of the materials from which the NE is made. After all, repeated switching on and off of NEs cannot in any way positively affect their service life.

About power regulators on thyristors. Conventional phase-controlled regulators give a high level of interference on the network. Therefore, in the literature, thyristor regulators were advertised with the passage of individual periods of the mains voltage ("noise-free"). For lighting, of course, such regulators are unsuitable because of the flickering of the bulbs, but they are suitable for NE.

In a stable urban power grid, this is true. But in rural areas and summer cottages, the operation of such a regulator at a load of several kilowatts causes a sharp jump in the mains voltage over long sections of the power line. The whole street can experience the work of such a regulator in the form of a kind of "light music" that appears on incandescent lamps.

In conventional thyristor power regulators, interference can be well suppressed by installing a low-pass filter between the regulator and the power cord. For this purpose, regular PFP television filters are used, the boards of which are now cheap. But a load of 1 kW for such filters is too tough. Therefore, the standard DF110PTs choke should be replaced with a home-made one. It is performed on a M600NN ferrite rod with a diameter of 8-10 mm and a length of more than 60 mm, on which 200-300 turns of wire with a diameter of 0,8-1,0 mm are wound.

Author: A.G. Zyzyuk

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Comments on the article:

Alexander
I wanted to draw the author's attention to the fact that direct current is more dangerous than alternating current. In the event of an insulation breakdown, the chances of survival will be much less.

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