Two speed single speed fan. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches

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As you know, the air in the bathroom always has high humidity. Water vapor is most often removed using an exhaust fan, which is installed on the opening of the ventilation duct. There are special moisture-resistant bathroom fans on the market. The higher the fan performance, the faster the moisture is removed, but also the higher its noise.

During the day, when outside noise is higher, the noise of the exhaust fan is inaudible. Then it is desirable to use it in full speed mode in order to quickly reduce the humidity of the air. In the evening, the operation of the fan is more noticeable. In this case, it can be switched to low speed mode.

These fans use an asynchronous motor. The speed of the induction motor is changed by changing the frequency of the supply voltage. There is an easy way to reduce the fan motor speed. Two factors contribute to this: the fan motor consumes relatively little power and its mechanical load is constant. The easiest way to reduce engine speed is by lowering the supply voltage, for example, by turning on a ballast reactive element - a capacitor.

To select a ballast capacitor, it is necessary to remove the dependence of the engine speed on the supply voltage.

As an example, in fig. Figure 1 shows the experimental dependence of engine speed on the supply voltage for a "Venis Turbo" fan with a power of 25 W and a capacity of 250 m³/hour. The dependence was removed before the speed was reduced by half. RPMs were measured with a digital laser non-contact tachometer.

Rice. 1. Experimental dependence of engine speed on the supply voltage

The table lists the experimental data on the dependence n = f(U_Pete). The dependence is approximated by a cubic polynomial using the least squares method.

The results of removing the dependence n = f(U_Pete)

Quadratic and linear approximating functions give large errors. This was established by comparing approximations. Thus, it was found that increasing the degree of the polynomial more than 3 does not provide any advantage.

The analytical approximation has the form:

n = 2,3524 10^-3 U³_Pete - 1,5116 U²_Pete + 328,22 U³_Pete - 21512 [rpm, V] (1)

The maximum deviation of the experimental points is 23 rpm.

The analytical approximation of the inverse function has the form:

n = 6,8928 10^-8 n³ - 3,5139 10^-4 · n² + 0,61694 n - 21,37 [V, rpm] (2)

The maximum deviation of the experimental points is 3,56 V.

Based on the obtained dependence, we determine the revolutions n_Mr. at U_pit1 = 220 V and the required supply voltage U_pit2 to get n_Mr./2. In a specific case, the results are as follows: n_Mr. = 2584 rpm (U_pit1= 220 V) and U_pit2 = 140 V for n_Mr./ 2 \u1292d \u790d XNUMX rpm. Experimentally, by selecting ballast capacitors, we determine the required capacitance value upon reaching the required voltage on the motor. In this particular case, it was XNUMX nF. This value is obtained by connecting several capacitors in parallel. Due to the spread of motor parameters, the value of the ballast capacitor may be slightly adjusted.

It is necessary to pay attention to one feature. The electric motor has a resistive-inductive impedance. The capacitor together with the motor windings forms a series oscillatory circuit.

At the resonance frequency of this circuit, increased voltages may appear on the electric motor and capacitor, depending on the quality factor of the circuit. In this case, a breakdown of the capacitor is possible.

On fig. 2 shows the theoretical dependence of the motor supply voltage on the capacitance of the capacitor. At high capacitance values, the supply voltage tends to the mains voltage. In this case, the curve is steeper in the region up to the resonant frequency. This dependence suggests that the selection of the capacitance of the capacitor should be started from small values ​​by slowly increasing them, and not vice versa!

Rice. 2. Dependence of the supply voltage on the fan on the capacitance of the ballast capacitor

On fig. 3 shows a schematic diagram of the fan control. The ballast capacitor group C1 *, the discharge resistor R1 and the power supply circuit for the operation indication on the elements C2, R2, R3, VD1-VD5 are located inside the plastic fan housing. The red neon has been replaced with a blue LED, better associated with airflow. Circuit R2 R3 C2 is current limiting. Bridge rectifiers (VD1-VD4, VD6-VD9 and VD10-VD13) practically operate in short circuit mode, so the reverse voltage of non-conductive diodes is practically absent. You can use diodes with U_OBR up to 50 V.

Rice. 3. Fan control and alarm circuit (click to enlarge)

A 3-wire supply line is connected to the fan. It is controlled by two switches SA1, SA2 on the same panel, with independent mechanical movement of each half, switch SA1 only turns on/off the fan, and switch SA2 determines the rotation speed - 50% or 100%. The fan operation mode is indicated by the blue LEDs VD15 and VD16. The VD14 zener diode protects the VD10...VD13 diodes from breakdown in the event of a possible break in the wires connecting the VD15, VD16 diodes.

In the 50% RPM mode, the motor is powered through the ballast capacitor C1. The optocoupler LED is powered through resistor R5 and bridge VD6-VD9. The phototransistor of the optocoupler shunts the VD16 LED, so only the VD15 LED is lit.

In the 100% rpm mode, the engine is powered directly by the optocoupler LED, no current flows, the phototransistor is closed, and both VD15 and VD16 LEDs are lit.

All LEDs can also be powered by half-wave rectifiers, but then flickering appears and the intensity of the glow is lower.

For greater safety, a phase conductor is connected to the SA1 switch.

Author: Georgi Dimitrov

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