Pump motor protection. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network, uninterruptible power supplies

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Electric motors of water pumps can fail and this is due to the fact that when the pump is running, the water level must be at a certain level. If the water level drops, the pump runs at idle, the motor overheats and the pump fails.

To protect the electric motor, a device is proposed, the electrical circuit of which is shown in the figure.

The device contains a sensor mounted on the pump housing, an electronic relay based on a VT1 transistor, a power source and a thyristor key based on thyristors VS1 and VS2. The device works as follows. As soon as the sensor contacts touch the water, the electronic relay is activated and the K1.1 contacts turn on the thyristor key and the pump motor. The indicator lamp HL1 signals that the pump is working normally. If the water level drops below the level of the sensor electrodes, the electronic relay switches off the thyristor switch.

Details. Transformer T1 - TPP230-50 with a power of 9 W, you can also use any transformer with a power of 10 W 220 V - 12 V. Relay RES-10 passport RS4.524.303. Capacitor C1 K50-35 470 uF x 40 V.

Design. The device is assembled in a suitable housing and installed in a convenient place for observation.

The device does not require adjustment.

Author: V.F. Yakovlev

Article notes

The first flaw in the motor protection circuit is as follows: the output of the transistor is connected to the wire and the electrode touching the water - the base, and the minus of the power supply through the resistor is connected to water.

Let us analyze the stability of the scheme. The base of a silicon transistor is pierced with a negative voltage of several volts. Induction from the network through the capacitance between the windings can be significant, especially if the installation is incorrect.

If the pump is damaged (vibration, moisture and other factors), a powerful signal appears on its body, which damages the wires, the power supply of the circuit through a resistor.

With the pump motor still running, the transistor easily breaks through, which entails the constant switching on of the circuit - no protection!

How should a diagram be built? A fragment of the improved circuit is shown in Fig.1.

It is necessary to connect the collector of the transistor to the "mass" of the structure (including an electrode long to the bottom), the potential of the common point of the automation circuit will follow the potential of the water (even with a pickup of 50 Hz), connect the short electrode to the base of the transistor through an RC filter . Protect the B-E junction of the transistor with a back-connected diode.

Block the relay winding with a back-to-back diode so that the self-induction emf that occurs at the moment of de-energizing the winding is not applied to the closed transistor and does not break through it. It should also be remembered that dropping the armature at the time of the passage of a small current also causes a surge in the self-induction EMF.

In such a circuit, the network pickup is grounded, the common output of the circuit is connected to the "ground". You can further reduce the pickup by phasing the power plug. The variable component of the signal is attenuated by the filter. A constant signal acts on the transistor with some delay, that is, short-term effects on the electrode do not pass to the output (the circuit must contain a couple of water levels).

The second flaw in the protection circuit: the use of thyristors for switching the motor, included in the gap of one wire! A variant is possible in which there will always be a phase on the motor winding, and sometimes (during its operation) a “ground” will appear on one of the winding terminals (through a thyristor). Of course, in standby mode, the neon light will not signal the presence of voltage.

It is easier to "amplify" the relay signal by means of an electromagnetic starter, which would switch both supply wires.

Remarks. It is undesirable to connect the control electrodes of thyristors without a limiting resistor. The rectifier bridge between the control electrodes does not affect the current flow in the thyristor electrodes, but only provides the current of one polarity to the relay contacts. According to experimental data, the supply of a contact pair with a rectified current is 3-7 times worse for contacts than an alternating current supply. In other words, a rectifier bridge in the thyristor circuit is not needed.

I would like the author not to introduce unnecessary nodes into the schemes. The thyristors connected in parallel and the neon light bulb motor and circuit connected in the same way look hardly recognizable.

The "unraveled" section of the circuit is shown in Fig.2. In such circuits, a resistor is needed in the thyristor control electrode circuits.

Addition. I did not want to change the scheme much, but I can give an example. In [1], the author turns on thyristors through a limiting resistor and, in addition, shunts the negative half-wave of voltage on the control electrodes using diodes. The man took into account the dangerous effects on the thyristor and prevented their consequences.

References:

1. Braichenko V.G. Automatic switch of radio equipment with protection against unauthorized inclusion//Electrician. - 2000. - No. 5.

Author: N.P. Goreiko

See other articles Section Protection of equipment from emergency operation of the network, uninterruptible power supplies.

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