Power electrical equipment in your home

The power electrical equipment of the house includes pumps, fans, compressors, mechanisms for opening gates and other mechanisms equipped with electric motors.

If the house is powered by a three-phase circuit, then it is advisable to use three-phase power (and thermal) equipment.

To drive such mechanisms with three-phase power, it is most often used three-phase asynchronous motor.

Information about the engine is indicated in its passport (in the documentation and on a metal plate attached to the body). Nominal values ​​are given here, i.e. those for which the engine is designed during its normal operation at the highest permissible load.

For example, the nameplate indicates: P = 1.1 kW; U = 380/220 V; I = 2.5 / 4.3 A; f = 50 Hz; n = 2810 rpm; Efficiency = 77.5%; cosp = 0.87.

This means: the rated net power on the motor shaft is 1.1 kW, or 1100 W; the connection of the windings with a star corresponds to the linear voltage of the network 380 V, in this case the linear current (in the wires supplying the motor; 2.5 A; the connection of the motor windings with a triangle corresponds to the linear voltage of the network 220 V and in this case the linear current is 4.3 A; mains frequency should be equal to 50 Hz; rated rotation speed, i.e. engine speed at rated load is 2810 rpm nominal efficiency (the ratio of the useful power on the shaft to the consumed power of the electricity received from the network paid by the meter ky) is equal to 77.5%, the power factor (also called "power factor") is 0.87.

Power factor is the ratio of the active power of electricity, i.e. one that can be converted into another form, in this case, into a mechanical one, to the full power of the electric power.

The power formula connecting these parameters for a three-phase induction motor is as follows:

P = l, 73Ulηcosphy

where: U, I - line voltage and current, η - Efficiency, cosphy - power factor.

Fractional passport values ​​of voltage and current mean that if the linear (ie between the linear wires) voltage of the three-phase line is 380 and the phase voltage is -220 V, then the stator windings of this motor must be connected by a star.

For star connection, the ends of all three windings, displayed on the shield of the engine input box and marked C4, C5, C6, must be connected to one point, called neutral, and the line wires of the network are connected to the starting points of the windings, which are marked C1, C2, Sz.

If the line voltage of the network is 220 and the phase voltage is 127 (the latter is currently rare), then the stator windings of the motor should be connected by a triangle. For this, the end of the first winding (C4) is connected to the beginning of the second (C2), the end of the second winding (C5) is connected to the beginning of the third (C3), and the end of the third (C6) is connected to the beginning of the first (C1), and the resulting three terminals are connected linear wires.

In both cases, the phase voltage on each of the windings will be 220 V, and the motor power will remain unchanged, but because of the difference in the magnitude of the current, the cross section of the supply wires in the second case will have to be increased.

If the engine drives the mechanism, the moment of resistance on its shaft slows down the rotation of the rotor. With increasing load, the engine speed decreases, which leads to an increase in the engine torque, and it overcomes the resistance of the mechanism. This is possible even with some (one and a half to two times) short-term excess of the rated load, but up to a certain limit, called the critical moment of the engine, increasing the load above which will stop the engine.

At rated engine load, its efficiency and power factor are maximum. When the engine is idling, its efficiency is zero, and the power factor is very low. Therefore, avoid prolonged underloading of the engine, or idling it.

When starting an induction motor, a very large, albeit short-term, starting current occurs, which is 5-7 times higher than the rated value. Inrush current can sometimes lead to a significant reduction in line voltage. In order to reduce inrush currents, you can use soft starters.

To reverse (change the direction of rotation) of an induction motor, it is enough to swap any two wires when connected to the motor terminals or, if you need to do this often, use reversing starters.

Three-phase nutrition of individual houses is still very rare at present. If the power is supplied by a single-phase circuit, then the motors must comply with this. In this case, the following special types of engines are used.

Commutator motor. Its feature is the presence of a collector and brushes, which is usually not the case with an induction motor (and this is one of its advantages). But there are advantages to the collector motor: the ability to work from single-phase alternating current circuits, the ability to obtain high rotation speeds at the usual frequency of 50 Hz, smooth speed control when powered by an autotransformer, and an increased power factor.

Capacitor induction motor. Such an engine can operate from a single-phase network with the inclusion of capacitors. An additional capacitance turns a pulsating magnetic field of a single-phase current into a rotating one.

These motors develop slightly less (approximately 30%) torque compared to a three-phase motor of the same size and have somewhat worse performance. The optimal capacity for such schemes depends on the design features of the engine and its electrical parameters.

For an engine with the passport data listed above, the formula for the circuit should be k = 2800, phase voltage 220 V, phase current 2.5 A, regardless of whether the motor windings are connected by a star or triangle. The desired capacitance is 32 μF.

The calculation formula is approximate and therefore it is necessary to find the optimum value of the capacitance at the site by disconnecting or connecting additional small-capacity capacitors in order to find the optimal variant with the highest engine torque by successive approximation (the increase and decrease in engine torque can be felt by its operation under load) . The developed power in this case is the rated power of the capacitor motor.

As a rule, to start the engine requires additional capacity, included in parallel with the working only during start-up. During start-up, especially under load, the switch should turn on an additional capacity, the value of which is selected so that the full starting capacity, including the working capacity, exceeds the working capacity by 2–3 times. Capacitors can be installed directly near the engine or in a special power supply. There are capacitor motors with built-in capacity.

See more about this here: Single-phase connection of a three-phase motor andTypical schemes for connecting a three-phase motor to a single-phase network

When working with capacitor motors, additional safety rules must be observed. Condenser batteries should be enclosed in a fireproof box and secured against shock and vibration. Fuses must be replaced when the circuit breaker is disconnected. After the engine is turned off, the disconnected container must be closed with a switch.

It must be remembered that with alternating current, electrolytic capacitors cannot be used (their terminals are marked with + and -), intended only for direct current. Otherwise, a capacitor explosion may occur.

It should also be remembered that the capacitor retains a charge for a relatively long time after disconnection, which is dangerous for humans when touching the terminals of the capacitor. The charge is higher, the larger the capacitance and the higher the voltage of the capacitor. The discharge of the capacitor should be removed after each shutdown of the motor by shorting to a piece of insulated wire.

Turning on and off stationary, i.e. non-portable electric motors are most conveniently produced using magnetic starters, which consist of an electromagnet with contacts fixed on its movable part, closing and opening when the electromagnet coil is turned on.

Turning on and off the coil itself is done with buttons installed here or taken to the right place, maybe even at a fairly large distance. Instead of a button, you can use photo relay, float or other relays that automatically turn on the current in the coil when changing certain parameters.

Thus, the magnetic starter has at least two obvious advantages: the ability to control the mechanism (or lighting system) at a distance and the ability to automatically control without human intervention. The metal housings of the magnetic starters and control buttons must be zeroed (see article "Protective grounding").

An example of automatic control of a pump supplying water to a tank located at a certain height is a magnetic starter, which is switched on by a float switch placed in the tank.

When the liquid level in the tank reaches a lower critical position, the float provided with contacts includes contactor coilwhich attracts the moving part of the contactor during current flow and, with its contacts, turns on the electric motor. In the upper position, the float turns off the coil, and it turns off the engine.

One of the simple and reliable pump control schemes that can be assembled independently is given in the article. "Automation of pump control in the country".

Of great importance is the control of grounding and insulation resistance. An external examination in this sense is recommended to be done before each working cycle of the appliance, and once a year to take measurements of the insulation resistance and the presence of grounding using appropriate devices.

Vladimir Reprintsev