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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Motor overload protection
Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network The problem of reliable protection against overloads of electric motors, and, accordingly, the mechanisms in which they are installed, is still very relevant. Especially in production, where violations of the established rules for the operation of mechanisms often occur, leading to overloads, and accidents of worn-out equipment sometimes occur (the gearbox is jammed, the bearing “crumbled”, a short circuit in the cable or a break (of different wires). In all these cases, the protective devices under consideration work reliably when the engine is turned off. The first of the devices considered in the article replaces two blocks of an electromagnetic starter, which, in the event of a malfunction, are quite difficult to restore - protection blocks for maximum current (PMZ) and operating current (TZP). It significantly surpasses them in terms of reliability and accuracy of setting the thresholds. In addition, the limits of threshold regulation are much wider.
On fig. 1 shows a diagram of this device. When you press the SB1 "Start" button, K1 is activated - an intermediate relay of the electromagnetic motor starter, and with it the starter itself, the auxiliary contact groups of which KM 1.1 and KM 1.2 are closed. The first of them blocks the SB1 button, which can now be released, and the second turns on the rectifier on the VD5-VD8 diode bridge. The voltage of 12 V from the output of the stabilizer on the zener diode VD9 and the transistor VT1 is supplied to the power supply circuit of the device. The 1 V voltage required to power relay K36 is available in the starter. Usually it is possible to find an alternating voltage of 12 ... 18 V for the rectifier there. Immediately after the power is turned on, the capacitor C6 is charged through the resistor R10, on which a pulse is formed that sets the triggers DD1.1 and DD3.2 to their initial state with a low output level. Usually, in electromagnetic starters, two current transformers are used to control the current consumed by a three-phase motor. In the blocks PMZ and TZP, the output current of the transformers is compared with the exemplary one. Comparison nodes are built on MLT-2 resistors, which, when the permissible current values are exceeded, get very hot, sometimes even fail. Overheating results in ring cracks at the soldering points of these resistors. In the device under consideration, the voltage comparators at the op-amp DA1 and DA2 monitor the voltage amplitude across the load resistors of the current transformers T1 and T2 (R1 and R2, respectively), which is proportional to the controlled current. It is possible that the voltages taken from these resistors will turn out to be too small compared to the thresholds of the comparators. In this case, they can be amplified with the help of op amps connected according to the standard non-inverting amplifier circuit. As DA1 and DA2, it was not by chance that the K140UD11 OU was chosen, which have protection against exceeding the permissible input voltage and against shorting the output. When replacing them with microcircuits of a different type, the non-inverting inputs of the amplifiers should be protected by connecting D814D zener diodes between them and the common wire (anodes to the common wire). To protect a single-phase motor, when the current is controlled in only one circuit, a T2 current transformer is not needed. It is excluded from the device along with the resistor R2 and the diode VD2, and the upper (according to the diagram) output of the tuning resistor R4 is connected to the same output of the resistor R3. With the start of the motor, the non-inverting input of the op-amp DA2 receives positive half-cycles of voltage from the engine of the tuning resistor R4. Their amplitude is much higher than the exemplary voltage at pin 2 of the op amp, since the starting current of the electric motor is usually 5 ... 7 times greater than the working one. As a result, at the output of the op amp DA2 there are logic level pulses. The front of the first of them starts single vibrators on triggers DD1.2 and DD3.1. The first generates a pulse with a duration of 5 s, the second - 3 s. The elements of the DD2 chip connected in series create a delay due to which, when the single vibrators are simultaneously started, the high level at the input D of the DD3.2 trigger is set later than at the input C, so the trigger remains in its original state, and the short circuit relay winding is de-energized. If the motor current has not decreased to the operating value in 3 s and the pulses at the output of the op-amp DA2 have not stopped, the single vibrator will be restarted on the trigger DD3.1. Since the high level previously set at input D of trigger DD3.2 remains the same, this trigger will switch, the short circuit relay will work, its contacts K3.1 will open the winding circuit of relay K1. The engine will be turned off. Similar processes will occur with an increase in current in excess of the permissible operating current as a result of a mechanical overload of the motor. If its duration is less than 3 s, the engine will continue to run, and if it is longer, it will be turned off. It should be borne in mind that in the case when the contacts of the SB1 button or the relay of the remote control unit (RCD) that performs its functions remain closed for more than 3 s, after an emergency shutdown of the engine, it will turn on again for another 3 s. To prevent this, you can, for example, replace a conventional short-circuit relay with a bistable one (remote switch) and use its second winding to return the protection device to operating mode after the cause of the accident has been eliminated. The second channel of the device, assembled on the current transformer T1, op-amp DA1, trigger DD1.1, transistors VT2, VT3 and relay K2, immediately turns off the engine when the permissible value of the starting current is exceeded. The overload pulses that appeared in this case at the output of the op-amp put the trigger into a state with a high level at the output, which leads to the operation of relay K2, which opens the power circuit K1, an intermediate relay of the starter. To eliminate the consequences of too long pressing the SB1 button, it is recommended to replace the remote switch and relay K2.
The printed circuit board of the considered device is shown in fig. 2. Its establishment comes down to checking the duration of single vibrator pulses on triggers DD1.2 and DD3.1 and setting the protection thresholds with trimming resistors R3 and R4.
This problem is successfully solved by the device assembled according to the scheme shown in Fig. 3. The control circuit of the intermediate relay of the contactor is not shown here, but the positional designations of the relay and their contacts coincide with those in fig. 1. Like the previous one, the protection device turns on when the auxiliary contact group of the KM1.2 starter is closed, and the contacts of the K2 relay, when the protection is triggered, open the winding circuit of the intermediate starter relay. With the advent of a stabilized voltage of 1 V at the emitter of transistor VT12, capacitor C3 is charged through resistor R4. A positive voltage drop across this resistor starts a single vibrator on a DD1.1 trigger, which generates a high logic level pulse with a duration of 5 s. During this time, the trigger DD1.2 is held in a low-level state at the output and is insensitive to changes in the level at input C. Relay K2 is de-energized, the motor, having accelerated, switches to the operating mode during the pulse. After 5 s, the level at the input R of the trigger DD1.2 becomes low, after which the first overload pulse received at the input C of the trigger from the output of the op-amp DA1 will switch the trigger to the opposite state. Transistors VT2 and VT3 will be open, relay K2 will work, turning off the engine.
The printed circuit board of this version of the motor overload protection device is shown in fig. four. Relay K2 and short circuit in the first and K2 in the second protection device - RES22 with passports RF4.500.122, RF4.500.129 or RF4.500.233. In the absence of a factory-made current transformer, it can be made from an electromagnetic relay with an anchor fixed in the attracted position. The wire, the current in which must be controlled, is passed through the window of the resulting closed magnetic circuit. The relay coil serves as the secondary winding of the transformer. It must be shunted with a resistor, as shown in the diagrams in Fig. 1 and fig. 3. Author: A. Mankovsky, pos. Shevchenko, Donetsk region, Ukraine; Publication: radioradar.net
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