Three-phase electric motor: circuit breaker. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Three-phase electric motor: circuit breaker. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network

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Protecting an electric motor or other three-phase load from failure with low-quality power supply is a very urgent task, especially in rural areas. The low quality of the supplied electricity is manifested in the asymmetry of the effective voltage values in the phases of the three-phase network and even in the complete absence of voltage in one of the phases. This can lead to thermal overload of the motor and burnout of its stator windings. The proposed device will automatically disconnect the load from the network in the event of a dangerous situation.

Three-phase electric motor: circuit breaker. Schematic diagram of the device
Rice. 1 Schematic diagram of the device

The scheme of the machine is shown in the figure. The signal about the violation of the symmetry of the power supply is formed according to the principle described, for example, in the article by A. Sergeev "Signalizer" phase "skew" ("Radio", 2003, No. 11, pp. 42, 43). With the help of three identical reactances - in this case, capacitors C1-C3 - an "artificial neutral" is created. It can be shown that if the capacitance values of the capacitors are equal and the three-phase network is ideally symmetrical, the voltage between the artificial and real neutral is zero. At zero voltage in one of the phases (but in the absence of a break), the control voltage is approximately one third of the phase. At zero voltage in two phases, it reaches half, and with breaks in two phases, its full value. Thus, it is enough to set the machine to operate with a critical decrease in voltage in one of the phases, in other situations it will work even more confidently.

When you press the SB1 "Start" button, the phase voltage is supplied to the winding of the KM1 starter, and with its main contacts it connects the M1 motor or other load to a three-phase network. The auxiliary contacts of the starter block the SB1 button, which can now be released. The engine is turned off as a result of a break in the supply circuit of the KM1 starter winding when the SB2 "Stop" button is pressed or when the K1 relay is activated.

The winding of this relay receives a voltage proportional to the "phase imbalance" between the connection point of the capacitors C1-C3 and the neutral of the three-phase network N, rectified by the diode bridge VD1-VD4. The relay will operate if this voltage exceeds a certain threshold value, which can be adjusted by the variable resistor R1.

Capacitor C4 not only smooths out the ripple of the voltage supplied to the relay, but also provides the duration necessary to turn off the KM1 starter, keeping the contacts of the relay K1.1 in the open state. In addition, the capacitor prevents false trips of the machine, which can be caused by non-simultaneous closing of the KM 1.1 contacts when the starter is triggered. Zener diodes VD5-VD7 limit the voltage on the relay winding K1 and capacitor C4 at an acceptable level if the "skew" is too large.

As practice shows, it is critical for an electric motor to reduce the voltage in one of the phases to approximately 70% of the nominal, i.e., up to 150 ... 140 V in a 220/380 V network. In this situation, the effective voltage value between the artificial and real neutrals will reach 20 ... 25 V, and at the output of the rectifier bridge VD1 - VD4 - 28 ... 35 V (in fact, under the load created by the relay winding K1, the voltage will be slightly less).

In order to ensure the operation of the machine with such a "skew", the relay RP1 with a winding of 21 V DC and with a group of contacts for switching was selected as K24. The capacitance of capacitors C1-C3 is chosen based on the fact that their reactance should be much less than the resistance of the relay winding. Capacitors KBG-MN are used. It is possible to replace them with MBGO, MBGCH or imported ones for the appropriate voltage. Capacitor capacitance deviations from the nominal value should not exceed 5%.

Variable resistor R1 must be wire. Its power depends on the operating conditions of the machine. If large "distortions" in the network are not expected and it is necessary to protect the motor only from a sudden shutdown of one of the phases, the resistor can be 2 watts. If you have to work for a long time on the verge of triggering, its power will have to be increased to 10 W or more.

Starter KM1 - series PME-211 with a control winding for 220 V. Diodes 2D202R can be replaced with KD203G, KD203D or diode bridges KTs402A, KTs402Zh, KTs405A, KTs405Zh. Diodes with a lower allowable reverse voltage are not recommended. They can be damaged by voltage surges that occur when switching an inductive load.

Author: N. Kazakov, Volgograd; Publication: radioradar.net

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