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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Simple booster devices. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Regulators of current, voltage, power The article provides descriptions of simple booster devices that allow you to increase the voltage in the electrical network by a certain amount or lower it by using conventional step-down transformers. In practice, it often becomes necessary, for example, to lower the increased voltage to the nominal level in order to extend the life of incandescent lamps or to increase the reduced voltage in order to increase the light output of the lamps. The simplest, most affordable and economical way to do this is with a conventional step-down two-winding transformer, turning it on according to the voltage boost scheme. Such an inclusion means that the secondary low-voltage winding of the transformer is connected in series with the load, and the primary, higher voltage winding is connected in parallel to the load or to the network terminals. Figure 1 shows the switching circuits of booster transformers and their vector diagrams.
In order to simplify, the vector diagrams are constructed without taking into account losses in the secondary windings of transformers. Figure 1, a shows the connection diagram of a booster transformer and its vector diagram with the consonant inclusion of its windings, in which the magnetic fluxes of the windings coincide in direction. Figure 1b shows the circuit when the windings are turned on in the opposite direction, which leads to the opposite direction of magnetic fluxes and, consequently, to a decrease in the resulting magnetic flux of the transformer. As can be seen from the figures presented, using a conventional step-down transformer, it is possible to increase or decrease the voltage at the load by ±∆U, depending on how its windings are connected - according to or counter. In other words, the required voltage boost is determined by the voltage of the secondary winding of a conventional step-down transformer. Consider an example. We have a single-phase step-down transformer of the type OSO-0,25 (lighting single-phase with a power of 250 VA) with a voltage of 220/36 V (commonly called a "boiler box"), wound on an L-shaped core. The secondary voltage of this transformer is 36 V and will be the value of the voltage boost U \u36d 220 V, which can be added to the mains voltage of 220 V or subtracted from it, depending on the consonant or opposite switching on of the windings: 36 + 256 \u220d 36 or 184−180 \u2d 180 (V ). Suppose that the voltage in the network is lowered and is 36 V, then using a conventional transformer according to the voltage boost scheme, with the windings turned on, it can be raised, brought closer to the nominal, since U216 = 1 + 256 = 2 (V). With an increased voltage in the network, for example, U1 = 256 V relative to the nominal load, it can be reduced by swapping the ends of any of the transformer windings. In this case, for our example U36=U220−∆U=XNUMX−XNUMX=XNUMX (B), i.e. at the load terminals we have the rated voltage. In cases where the required voltage boost does not correspond to the standard secondary voltages of transformers, the secondary winding is rewound to the required voltage, for example, 20 V. This does not exclude the possibility of winding or unwinding a certain number of turns of the transformer secondary winding in order to obtain the required voltage boost, so as the secondary winding is wound over the primary. The secondary winding of the transformer must be capable of withstanding the load current. The total power of the transformer through the secondary values S=U2I2, whence the current of the secondary winding I2=S/U2. For transformer OSO-0,25 220/36 V, this current will be I2=250/36=6,1 (A). Thus, a load current of up to 6,1 A can be passed through the secondary winding of this booster transformer. The power of a single-phase transformer, which is used for voltage boost, is several times less than the load power. It is determined by the formula: Sw=Snom⋅∆U/U=1000⋅22/220=100 (VA), where Svt is the power of a single-phase transformer used for voltage boost, VA; Snom - total load power, VA; ∆U - the value of the required voltage boost, V; U1 - voltage of the network to which the booster transformer is connected, V. For example, with the required value of the voltage boost ∆U=22 V, load power Snom=1000 VA and mains voltage U1=220 V, the power of the voltage boost transformer will be only Svt=100 VA, i.e. 10 times less load power. Consequently, the dimensions, weight and cost of such a booster device are relatively small. The efficiency of the booster device reaches values of 0,99 ... 0,995, the mass per unit of power is 2,5 ... 3 kg / kV⋅A. Losses of voltage and active power in such a transformer are small and, accordingly, equal to 0,5 ... 3, so they can be ignored. The circuits shown in Fig. 1 for switching on booster transformers allow you to increase or decrease the voltage at the load by a certain constant unregulated value, which is why they are called unregulated, or "silent", booster transformers. It should be taken into account that unregulated booster transformers create a voltage boost ∆U regardless of the network load mode. Due to this, it is necessary to choose the value of the allowance not according to the minimum (maximum) voltage mode, but according to the minimum load mode, when the voltage is higher. Therefore, an unregulated circuit for switching on a booster transformer is always acceptable where, regardless of the time of year and the load in all modes, it is required to increase, less often lower the voltage by ∆U. The booster device can be made three-phase. A schematic diagram of such a device is shown in Fig.2.
It can be created from improvised means that almost every enterprise has, namely: from three single-phase boiler rooms (OSO-0,25, OSM-0,4U3) or welding transformers. The secondary windings of these transformers with a voltage of 12 ... 36 and 40 ... 60 V are designed for high currents and can be used for inclusion in the line cut as serial ones. These windings create an additional voltage ∆U. The primary windings of these transformers act as exciting ones and can be connected directly to a three-phase network in a star or delta circuit. Such transformers can be used in extended industrial and agricultural networks. For domestic purposes, suitable transformers from radio and television equipment can be used as booster transformers, as, for example, shown in [1]. Booster transformers are most often used to increase voltage, although they can be made reversible. A diagram of such a booster device is shown in Fig. 3.
It differs from the circuits shown in Fig. 1 by the presence of a two-pole switch SA1 with three positions with a neutral position of the moving contacts in the middle position. An example of such a switch is a toggle switch of the VT3 type for a switching current of 3 A (up to 660 W) and a switching voltage of ~ 220 V with fixation of the control knob in the middle and extreme positions. When contacts 1-2 and 3-4 of switch SA1 are closed, the windings W1 and W2 of the VT transformer are connected to the network, and the voltage at the output of the device is increased by +∆U relative to the mains voltage. If the contacts 2-5 and 4-6 of the switch are closed, then the ends of the secondary winding W2 of the transformer are reversed. Consequently, the magnetic fluxes of the windings are directed oppositely, and the voltage at the output of the device will be reduced by -∆U. In the middle position of the switch knob SA1, the winding W2 is disconnected from the network and does not flow around, the load and the primary winding W1 of the VT transformer do not flow. When operating a booster device, it should be borne in mind that opening the primary winding W1 of the VT transformer during operation of the device is unacceptable, based on safety conditions and the rules for the technical operation of electrical installations. The fact is that when the primary winding W1 is opened, the current in the secondary winding W2 will remain the same and equal to the load current. In fact, this mode of operation of the transformer is an idling mode, but with an idling current of the transformer equal to the load current, which is many times greater than the normal idling current of the transformer, and this current is completely magnetizing. This leads to a significant increase in the magnetic flux of the transformer. Losses in the magnetic circuit of the transformer are proportional to the square of the magnetic flux. As a result, a strong heating of the transformer core occurs, which is dangerous for its insulation, and even spontaneous combustion of the transformer is possible. In addition, the EMF of the primary winding W1 increases in proportion to the magnetic flux and can reach large values that are dangerous both for the transformer itself and for life. surrounding. The research carried out by the author of the OSO-0,25 type transformer in the booster mode with the primary winding open and even not fully loaded led to the appearance of EMF at the terminals of the primary winding of 500 V, and with increasing load, the EMF value increased. With high load currents or the need for remote control of a booster transformer, magnetic starters or high-current relays can be used as a switching device. A schematic diagram of such a booster device is shown in Fig.4.
It works in the following way. In the initial pre-start state, the coils K1 and K2 of the magnetic starters are de-energized, and their power contacts K1.1, K1.2 and K2.1, K2.2 in the circuit of the secondary winding W2 of the VT transformer are open. As a result, the VT transformer and the load are de-energized. To increase the voltage at the load by ∆U, press the "More" button. As a result, the coil K1 of the first magnetic starter is energized, the starter is triggered and the power contacts K1.1 and K1.2 connect the windings of the transformer VT to the network, at the same time the contact K1.4 blocks the "More" button, and the contacts K1.3 of the electrical blocking open. If it is necessary to reduce the voltage at the load, press the "Stop" button, in this case the circuit returns to its original state (all power contacts are open), and then press the "Less" button. The circuit works in a similar way, but at the same time, the second magnetic starter is activated, which closes its power contacts K2.1 and K2.2 in the secondary winding circuit W2 of the VT transformer, as a result, the current phase in it changes to the opposite, and the voltage at the output of the booster device decreases by the value of ∆U. In addition to two conventional magnetic starters, one reversible one can be used for this circuit, for example, of the PME-11-3 type for a current of 10 A and a voltage of 380 V with a voltage of the closing coils of 220 V, which is equipped with a mechanical blocking device from the simultaneous switching on of all power contacts of the starter . References:
Author: K.V. Kolomoitsev
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Comments on the article: Victor On fig. 2, 3, 4 you have the input and output mixed up. Kolya Fig 4. The scheme does not work
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