Repair and modernization of the electric fan SMF-3RDEA. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electric motors

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When disassembling an old faulty microwave oven for parts, an SMF-3RDEA electric fan was removed from it among potentially useful components. When examining the electric motor, traces of a short circuit in the form of spattered copper balls were found on its winding. The fan worked, but the motor winding became very hot a few seconds after being turned on. Since the fan turned out to be very quiet and created good airflow, it was decided to repair it.

I did it like this. Using a vise, a hammer and a metal screwdriver, the coil with the winding and the magnetic circuit was carefully knocked out of the stator of the electric motor, after which the magnetic circuit was squeezed out of the plastic frame of the coil and cut off its entire winding. The new winding - 4800 turns with a tap from the 4000th - was wound in heaps with PEV-2 wire or a similar diameter of 0,11 mm, evenly distributing the turns over the coil frame. If the wire broke, its ends were tied in a knot, welded in the flame of a match or a gas lighter, and insulated with a thin film. The finished winding was impregnated several times with transformer varnish. The winding resistance at room temperature is approximately 580 ohms.

Since the fan motor had to be dismantled for repair, it was decided at the same time to improve it, giving it a second function - a step-down transformer. To do this, tightly wrapping the winding (in the transformer it will be primary) with several layers of varnished cloth or office paper impregnated with zaponlak, I wound the secondary winding over it (turn to turn) - 430 turns of PEV-2 wire 0,27 mm. I also impregnated this winding with varnish and protected it with two layers of PVC electrical tape. The view of the coil assembled with the magnetic circuit is shown in fig. 1.

Fig. 1

I assembled the motor in the following order. Clamping the stator in a vise, thickly smeared the magnetic circuit with BF-2 glue and inserted it into the coil. Then I smeared the joints of the magnetic circuit and the stator with the same glue and carefully drove the magnetic circuit into the stator as quickly as possible. The view of the fan assembly is shown in fig. 2.

Fig. 2

Due to the tap from the main winding, the fan after repair can operate in two modes - normal and forced, when not the entire winding is used, but its part of 4000 turns.

The fan speed is noticeably higher in this mode.

An alternating voltage of 17 ... 19 V can be removed from the additionally wound secondary winding in idle mode. It can be used for various purposes, for example, to power LED backlights or automatically control fan operation modes. However, it should be taken into account that if the fan winding turns out to be de-energized, the devices powered by the fan will also be de-energized.

Fig. 3

An example of using a modernized electric fan as a stabilized power supply with an output voltage of 12 V is shown in fig. 3. Mains voltage is supplied to the primary winding of the fan motor through a protective resistor R1 and closed contacts of the switch SB1. In its position shown in the diagram, the fan operates normally. Varistor RU1 protects the primary winding from power surges. From the secondary winding of the fan, an alternating voltage of 17 ... 19 V through a self-restoring fuse FU1 is supplied to the bridge rectifier VD1-VD4. Capacitor C5 smooths out the ripple of the rectified voltage.

The voltage regulator is made on the 78R12L-TF4-T (DA1) microcircuit, which is a linear voltage regulator of positive polarity with an output voltage of 12 V. The maximum input voltage is 35 V. The microcircuits of this series differ from the usual ones (series 78xx, 78Mxx, KR142) by the presence of an input control and reduced voltage drop between input and output (for 78R12L-TF4-T it does not exceed 0,5 V at a load current of 1 A).

Microcircuits of the 78Rxxx series are available in different packages for several fixed output voltages (3,3; 5, 9, 12 V) and with adjustable output voltage. On fig. 3 shows the pinout of the 78R12L-TF4-T chip in a four-pin TO-220F-4 package. If additional cooling is needed, the chip is mounted on a heat sink.

Diodes 1N4001 can be replaced by any similar ones, for example, series 1N400x, KD208, KD243, KD247. Capacitors C5, C8 are oxide imported, the rest are small-sized film or ceramic capacitors with a rated voltage not lower than that indicated in the diagram. Resettable fuse FU1 - any type with an operating voltage of 30.60 V and a trip current of 0,15.0,2 A. It is advisable to install a non-flammable R1-1 resistor or an imported breakable resistor R7. Varistor 07K275RJ (RU1) can be replaced by any of the series FNR-14K431, FNR-20K431, FNR-07K471. Pushbutton switch SВ1 - any type with contacts designed for switching alternating voltage not lower than 250 V.

By changing the number of turns of the secondary winding of the electric motor, you can make a power source with a different output voltage, but the power consumed by the load, in any case, should not exceed 3 watts. In the absence of a load on the secondary winding, the fan consumes 90 mA from the mains in normal mode, and 110 mA in forced mode. At room temperature, the heating of the winding and stator of the electric motor does not exceed 45 °C. An increase in the current of the load connected to the fan leads to a decrease in the rotational speed. Reducing it by increasing the load on the motor shaft, for example, by braking the impeller by hand, does not lead to a change in the voltage at the terminals of the secondary winding.

Other similar electric motors can be modified in a similar way. By the way, the so-called electric motor-transformers were used in the 70-80s of the last century in some domestic tape recorders, where an electric motor with an additional winding, powered by a 220 V network, simultaneously served as a step-down transformer for the power supply of electronic components.

Author: A. Butov

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