Motor protection device. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Motor protection device. 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, uninterruptible power supplies

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As typical elements of protection of electric motors, electrothermal relays are most often used. Designers are forced to overestimate the rated current of these relays so that there are no trips during start-up. The reliability of such protection is low, and a large percentage of engines fail during operation.

(click to enlarge)

The scheme of the motor protection device (see figure) from open-phase modes and overload is characterized by increased reliability. Transistors VT1, VT2, together with the elements attached to them, form an analogue of a dinistor, the turn-on voltage of which (Uon) depends on the ratio R6 / R7. With the ratings indicated on the diagram Uon \u30d 36 ... 15 V in the temperature range t \u20d -XNUMX ... XNUMX ° C.

Resistors R1...R3 form a vector adder, at the output of which the voltage is 0 if the motor is powered by full-phase. Transformer T1 is a current sensor for one phase of the motor. The outputs of the current sensor and vector adder are connected to a rectifier made on diodes VD1...VD3. In normal mode, the voltage at the output of the rectifier is determined by the current in the primary winding T1 and the ratio of turns w1/w2. With the help of resistor R4, this voltage is set below U on VT1 and VT2.

If a phase failure or motor overload occurs, the voltage at the rectifier output will exceed Von, transistors VT1, VT2 will open, and the KV1 relay will be energized. Contacts KV1.1 will break the magnetic starter interlock circuit and the engine will turn off. Contacts KV1.2 block transistors VT1 and VT2, preparing the circuit for the next cycle of operation.

The speed of protection operation depends on the capacitance of capacitor C3.

The duration of the switching pulse of the relay KV1 depends on the resistance of the winding KV1 and the capacitance of the capacitor C3. Within certain limits, the protection speed can be changed by selecting the values of C1, C2, R1 ... R3.

All parts, except R1 ... R3 and T1, are mounted on a printed circuit board. After assembling the board, you need to connect a voltmeter to C3 and, by applying an alternating voltage of 1-2 V to one of the rectifier inputs (C20 or C24), check the turn-on voltage of VT1 and VT2. If necessary, you need to choose the resistance of the resistor R6. Relay KV1 will turn on periodically. The circuit is finally adjusted on the operating engine.

First you need to select the number of turns w1 within 1-10 turns (depending on the engine power), so that in normal mode 4-5 V is supplied to the R22, R24 chain. Resistor R4 sets the threshold for overload protection.

If a trigger occurs at start-up, then it is necessary to increase the capacitance of the capacitor C3 without changing the position of the R4 slider. After that, check the operation of the circuit, simulating the open-phase mode.

As VD1 ... VD4, you can use any rectifier diodes with Uobr\u100e 30 V and a current of more than 1 mA. Transistors VT2, VT361 can be replaced with KT315 or KTXNUMX.

Relay KV1 for 24 V with contacts suitable for switching in 220 V AC circuits. In the T1 transformer, it is best to use an easily collapsible tape core with a cross section of 24 cm2. The secondary winding can be wound with a wire of 0,15-0,2 mm 10001500 turns, and the primary winding with a mounting wire of a suitable section. Resistors R1...R3 must be installed directly on the engine, filling them with bitumen to protect against moisture.

References:

Kolomoytsev K., Romanyuk Yu. Protection of the electric motor from operation in two phases // Radio amateur.-1999.-№1.-P.29.

Author: A.R. Zherdev

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

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