Filter for power supply of the electric motor. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electric motors

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In everyday life, it often becomes necessary to power electrical appliances from a battery through a DC-to-AC converter. Most devices designed for a sinusoidal voltage also work quite normally from the rectangular pulses generated by such converters. Unfortunately, they do not include asynchronous motors, such as those driving circulation pumps in heating systems. A significant proportion of the harmonic components, which are rich in non-sinusoidal voltage, in such engines is uselessly converted into heat, the rest violate the uniformity of the rotation of the magnetic field. To suppress harmonics, a filter is needed, the scheme and calculation method of which is proposed by the author of the published article.

To power an asynchronous motor from a rectangular voltage source, the filter is best suited, the circuit of which is shown in fig. 1. It transmits the first harmonic to the load with little or no attenuation, attenuating the higher harmonics quite strongly.

The equivalent circuit of the filter loaded on the electric motor is shown in fig. 2. The motor is represented by a parallel connection of the active resistance Rd and its own inductance Ld. R1 is also taken into account - the active resistance of the inductor (choke) L1. Both oscillatory circuits - serial L1C1 and parallel LDS2 - are tuned to the input voltage pulse repetition frequency.

Let us calculate the elements of a filter designed for an asynchronous motor, on the nameplate of which the following parameters are given: voltage U - 220 V, frequency F - 50 Hz, power P - 75 W, cos<φ - 0,6. Further calculations will also require the value of the circular frequency Ω = 2πF = 6,28 50 = 314s-1 and the value of sinφ =√1-cos2φ = 0,8.

The active component of the current consumed by the motor lR = P / U = = 75/220 = 0,341 A, reactive - LL = IR (sinφ / cosφ) - 0,341 0,8 / 0,6 = 0,454 A, from where Rd = U / IR = 220 / 0,341 \u645d 220 ohms; Xl \u0,454d U / IL \u484d 484 / 314 \u1,51d 2 ohms; Ld \u50d XL / φ \u2d \u106d 2/106 \u3142d 1,51 Gn. In order for the resonant frequency of the circuit LdС6,58 to be XNUMX Hz, a capacitor with a capacitance СXNUMX = = XNUMX / (φXNUMXLd) = XNUMX / (XNUMX XNUMX) = XNUMX μF is required.

Assume that the filter has a choke from a lamp with fluorescent lamps with a power of 1 W as L80. The following data can be found on the nameplate of the throttle: supply voltage U - 220 V, frequency F-50 Hz, rated operating current IH - 0,84 A, cosφ - 0,5 (sinφ = √1-cos2φ= 0,866)

At resonance in the CS2 circuit, the reactive component of the motor current is compensated by the current of capacitor C2. The active component of the motor current (0,341 A) flowing through the inductor is much less than 0,84 A, so the temperature regime of the inductor does not cause concern

The power consumed by the lamp from the network is equal to РСв - UIн = 220 0,84 0,5 = 92,4 W, of which 80 falls on its lamp, and the remaining 12,4 is dissipated by R1 - the active resistance of the inductor. The active resistance of the entire lamp RCв \u220d U / IH cosφ \u0,84d \u0,5d 131 / (1 12,4) \u92,4d 131 Ohm is distributed between the lamp and the inductor in the same proportion as the power, therefore R0,134 \u17,60d RCB (XNUMX / XNUMX) \uXNUMXd XNUMX-XNUMX \uXNUMXd XNUMXm.

The inductive resistance of the lamp Хсв = U/In·sinφ = (220/0,84)∙0,866 = = 227 Ohm can be fully attributed to the choke, the inductance of which L1 = Хсвφ = 227/314 = 0,723 H. The oscillatory circuit L1C1 will be tuned to a frequency of 50 Hz if C1 \u106d 2 / ( φ1 L106) \u3142d \u0,723d 14 / (XNUMX - XNUMX) \uXNUMXd XNUMX microfarads.

Taking into account the equality of the reactances of the inductor L1 and capacitor C1 at resonance, we calculate the amplitude of the voltage across the capacitor during engine operation increases many times. In proportion to the current, the voltage also increases. Therefore, this capacitor should be selected with a permissible voltage that is ten times or more calculated above.

The coefficients of suppression by the filter of harmonics of the input impulse voltage can be calculated by the formula obtained from the one given in [1]:

dn = 1 - L1/Ld(1 - 1/n2)2, where n is the harmonic number.

With the values ​​of L1 and Ld found above, the third harmonic (frequency 150 Hz) will be suppressed at 3,4, the fifth (250 Hz) - at 11, the seventh (350 Hz) - at 22,5 and the ninth (450 Hz) - at 37,8, 3 times. Even-numbered harmonics in the input voltage waveform shown in fig. 1 (curve XNUMX) are absent, and it makes no sense to calculate their suppression coefficients.

The amplitude of the first harmonic of the input voltage (curve 2 in Fig. 3) is Um1 = 1,27Um, where Um is the amplitude of the pulses. The coefficient 1,27 for pulses of a different shape will be different, its values ​​can be found, for example, in [2]. The effective voltage of the first harmonic U1 = 0,707Um1 = 0,9Um, whence Um = 1.1U1 The voltage at the filter output is less by the drop on the active resistance of the inductor, therefore, for the motor to operate in the nominal mode, the converter must generate rectangular pulses ("meander") with amplitude Um \u1,1d 1 U1 Rd / (Rd + R1,1) \u220d 645 645 17,6 / (236 + XNUMX) \uXNUMXd XNUMX V.

The correctness of the calculations was verified by computer simulation of the developed filter using the Electronics Workbench program. The output voltage graph obtained on the model (see curve 3 in Fig. 3) differs from a sinusoid due to the presence of not completely suppressed higher harmonics in it and corresponds to what is actually observed on the oscilloscope screen when the motor is operating with a filter.

In the manufactured filter, as C1 and C2, groups of capacitors KBG-MN, MBGCH, MBGP, MBM connected in parallel were used to obtain the desired capacitance for a voltage of at least 1000 V (C1) and at least 400 V (C2). The inductor from a fluorescent lamp can be replaced by any other similar inductance that can withstand the current consumed by the motor without overheating. A homemade choke is wound on a steel magnetic circuit USh 16x30. Its winding is 870 turns of wire PEV-2 0,3.

Practice has shown that the filter requires tuning, and it is better to tune its serial and parallel branches independently. This will require a 220 V, 75 W incandescent lamp and an AC voltmeter. As a "signal source", observing all the necessary precautions, you can use the mains.

The serial circuit L1C1 is configured according to the circuit shown in fig. 4. The EL1 lamp serves as a load equivalent and at the same time as a tuning indicator. The initial capacitance of the capacitor C1 is taken slightly less than the calculated one. It is gradually increased by connecting additional capacitors of a smaller capacity in parallel with the main one. The goal is to achieve the highest brightness of the lamp or the minimum reading of the voltmeter.

The parallel circuit is configured according to the scheme shown in fig. 5, achieving the lowest brightness of the lamp or maximum voltmeter readings.

During tuning, the motor shaft must rotate without mechanical load.

Literature

1. Chucky F. et al. Power electronics. - M.: Energoizdat, 1982.
2. Bessonov L. Theoretical foundations of electrical engineering. - M.: Higher school, 1973.

Author: V.Volodin, Odessa, Ukraine

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