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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Meat grinder motor protection. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric motors The design features of the electric meat grinder make it advisable to introduce a control device into it, which combines a soft start with protection against overload and overheating. These functions are provided by the device described here. It can be used to control series-excited commutator motors in other household appliances. This protective device was developed for the screw electric meat grinder EMSh-35/130 "RATEP" with a 130 or 145 W commutator motor (DK76-60-15 or DK77-65-15R), but can be easily adapted to the drives of other household electrical appliances that work from a network of 220 V. The combination of soft start with current protection is important in such a control unit. The fact is that the meat grinder motors are made in one block with gearboxes, which contain plastic gears to reduce the speed of the output shaft. Overloading the gearbox in the absence of protective measures leads to breakage of the gear teeth, as the weakest link. The load during food processing changes relatively slowly, so the electronic current protection switches off the motor in an emergency in a timely manner. Another thing is the inclusion of an electric motor with a inhibited output shaft. At first, the motor armature rotates while the gaps in the gears are selected, and then it is instantly braked. The current protection for shock load increase does not have time to work, while the kinetic energy accumulated by the armature is already enough to break the gears. Soft start with slow armature acceleration provides a "softer" load increase [1], as a result of which the current protection switches off the motor in this mode as well. It can be objected that in order to avoid breakages, a replaceable sleeve is introduced between the meat receiver screw and the gearbox shaft, which breaks at a lower load than the gearbox allows. But this solution is not without drawbacks. The sleeve is a one-time fuse and may be scarce or absent in the drive. Its protective effect is weakened by a large multiplicity of operation according to the degree of overload (up to 3 ... 5 times) and a spread in characteristics. The speed of electronic protection is much higher, it is much more accurate in setting the threshold, and finally, it is more universal. Functionally, the protective device (see the diagram in Fig. 1) contains a soft start unit, current and temperature sensors, a fixation and status indication unit. The device does not provide for a self-start mode after the malfunction has been eliminated, since self-switching of an electrical appliance uncontrolled by a person can be dangerous for him.
A distinctive feature of the soft start node in comparison with [2] is the logical control of the combined inputs: the lower one according to the DD2.1 element circuit and the upper one - the DD2.2 element. In the presence of a high level voltage at the inputs, the generation of triac opening pulses is allowed, and a low level is prohibited. In addition, the duration of soft start (time constant of the C5R15 circuit) has been increased, since the inertia of the engine is higher than that of an incandescent lamp. The current sensor is formed by a resistor R18 and transistors VT1.4, VT1.5. It generates a high voltage at any polarity of the overload current, and the threshold value of the trip current is determined by the ratio of the opening voltage of the transistors to the resistance of the resistor. In the considered embodiment, the overload current is selected 1,8 times the rated current consumed by the motor and is 1,1 ... 1,2 A. Resistors R17, R19 limit the surge base currents of transistors, and resistor R20 allows you to refine the response threshold. The C6R16 integrating circuit eliminates the effect of high-frequency and impulse noise induced by a current or temperature sensor. Since the time constant of the circuit relative to the frequency of 50 Hz is insignificant, and the opening of the transistors occurs at the amplitude value of the sinusoidal load current, the motor is turned off by the protection already from the next half-cycle after the overload has been recorded. A capacitor C1 is introduced into the temperature sensor (R3-R1, RK1, HL1, C1.1, VT1) to reduce the effect of interference and interference on its operation, and the thermistor RK1 is placed on the engine. The threshold value of the sensor response temperature is 100°C. New in the device is a node for fixing and indicating states, which contains an RS-trigger DD1.1 and DD1.3, an inverter DD1.2, a two-color LED HL2. When connected to the network, the C2R4 circuit sets the trigger to a single state at the output of element DD1.3 and soft start begins. Note that the required time constant of the C2R4 circuit is determined not by the speed of the microcircuits, but by the processes of magnetization reversal of the magnetic circuit and the beginning of the movement of the armature in the electric motor, which create a short-term inrush of the consumed current, which is many times higher than the rated one, so the current protection must be blocked for this time. In the case of a cold engine, the resistance of the thermistor RK1 is increased and the transistor VT1.1 is open. The high-level voltage at both inputs of the DD1.1 element sets a low level at its output and at the top input of the DD1.3 element according to the circuit, so the trigger state does not change as the capacitor C2 charges. Smooth turning on ends with the transition of the triac to a permanently open state. Triac opening current pulses flow through the HL2 LED, which indicates the correct operation of the drive with green light. This mode remains until the sensors are triggered or until the network is turned off. Since now there is a high voltage at the lower input of the DD1.3 element according to the circuit, the operation of any of the sensors, leading to the appearance of a high level at the upper input of the DD1.3 element according to the circuit, puts the trigger into a low level state at the output DD1.3. As a result, from the next half-cycle, the triac will not turn on, and the HL2 indicator will indicate an overload with red light. Its glow is due to the current flowing through the LED and resistor R23 from the output of the DD2.4 element to the output of DD1.2 (at the output of the DD2.4 element, the voltage is high, and at the output of DD1.2 - low). This mode is also retained until the network is turned off. If the reasons for the protection operation are not eliminated when switching on again, the motor will be switched off again. A drawing of the printed circuit board of the device is shown in fig. 2.
Ceramic capacitors are selected from among small-sized K10-17 or KM-6. Capacitor C5 can be K53-1, K53-4, etc. with a leakage current of not more than 0,5 μA or K10-17, KM-6. Capacitor C11 - K73-17 (K73-16) for a rated voltage of 630 V. Thermistor RK1 - MMT-1. Resistor R18 - C5-16V (C5-16MV). Fuse FU1 - a jumper from one core of the MGTF wire with a cross section of 0,07 ... 0,12 mm2, laid in an insulating tube removed from such a wire. When placed outside the board, the fuse and fuse holder can be of any type. The triac is equipped with a heat sink made of a copper (or aluminum) plate with dimensions of 55x15x1 mm and assembled with it through a gasket is attached to the board with a screw. The thermistor is attached to the stator winding of the electric motor and therefore must have high-quality heat-resistant heat-conducting insulation. To do this, it is necessary to put fluoroplastic tubes on its conclusions with extension conductors from the MGTF wire, and direct the conclusions themselves in one direction. Then, on the body of the thermistor with one of the leads pressed against it, tightly put another fluoroplastic tube of a larger diameter. To the stator winding, press the thermistor in the tube, tie it up or glue it with heat-resistant glue to ensure both thermal contact and strong fastening. Adjustment of the device consists in its adaptation to the protected motor, if it differs from the above types. Initial checks and adjustments are best carried out using an electric lamp of suitable power instead of an engine. The resistance of the resistor R18 is determined by the amplitude value of the overload current, which can be taken as 1,5 ... 2 rated motor currents. The power dissipation of the resistor and the dimensions of the heat sink of the triac are determined by the values of the overload current and the voltage drop across them. The rated current of the fuse should be approximately twice the overload current. Turning on the device and increasing the load current with the help of additional resistors or a rheostat, measure the current protection threshold. Within small limits, it can be changed by selecting a resistor R20. The permissible heating temperature of the motor winding wire can be in the range of 90...130°C. To set the threshold for overheating protection, you can heat the used thermistor in boiling water and determine the desired resistance of the resistor R1 for a temperature of 100 ° C. Install a resistor of the next lower value in comparison with the measured value in the device. The inertial properties of the motors are different, so the duration of the soft start must be clarified by changing the parameters of the C5R15 circuit. With an increase in the ratings of the elements, the duration of the start-up increases, and vice versa. To determine the optimal time constant of the C2R4 circuit, you can do the following. Starting with a capacitor capacitance of 0,1 uF and increasing it; through 0,1 μF, determine the moment when, when the motor is connected to the network, the current protection does not work. A capacitor with a capacity of 1,5 ... 2 times more is installed in the device. When choosing ceramic capacitors of groups H50, H70, H90, it should be borne in mind that the actual capacitance may differ significantly from that indicated. The HL2 LED can be moved outside the board to indicate the drive status in a place that is more convenient for observation during operation. During the manufacture, adjustment and operation of the protective device, it should be remembered that all its elements are under mains voltage. Therefore, the device must be placed in a housing made of insulating material, and the connecting wires must be reliably insulated. Literature
Author: V.Zhgulev
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