three phase asynchronous motors protection devices have permeated all fields such as power generation, power supply and power consumption systems. They are an important component of power generation, power supply and power consumption systems and are related to various aspects such as industry, agriculture, national defense construction and people's lives. The damage of three phase asynchronous motors is mainly caused by overheating of the windings or reduced insulation performance. Therefore, the most effective way to protect three phase asynchronous motors is to directly monitor the actual operating temperature of the windings or bearings. Implement protective control over it. To monitor the actual working temperature, corresponding temperature-sensing elements need to be installed inside the motor. Temperature-sensing elements are generally divided into two major categories: fixed value control protection and non-fixed value control protection.

Fixed value control protection

The fixed value control protection is that when the temperature at the internal monitoring point of the three phase asynchronous motors exceeds the specified value, the temperature measurement element operates and the three phase asynchronous motors immediately cuts off the power. There are usually two types of fixed-value sensing elements: bimetallic strip switch (i.e., thermal switch) sensing and PTC thermistor sensing.

1.1 Thermal switch

The thermal switch is a switching type bimetallic strip thermal element. Generally, the K1, K2, and K3 thermal switches are respectively pre-embedded in the three-phase coils of the three phase asynchronous motor A, B, and C. When the temperature of any phase in the three-phase coils of the three phase asynchronous motors reaches the temperature control point, the switch of this phase is disconnected, the electromagnetic switch K is de-energized, and at the same time, the three phase asynchronous motors is de-energized. So as to prevent three phase asynchronous motors from burning out. Of course, the thermal switch can also be placed on the bearing cover of the three phase asynchronous motors and connected in series with the thermal switch of the coil of the three phase asynchronous motors. When the temperature of a certain bearing rises to the specified value (such as 90℃), the thermal switch at that point is disconnected. K lost power, three phase asynchronous motors lost power.

The rated switch temperature of the thermal switch is within the range of 60℃ to 200℃ (in series every 5℃), the rated voltage is AC 220V 50Hz, and the rated current is available in specifications such as 3A and 17A. When selecting the type, the model should be determined in combination with the expected temperature control point. For example, for YX3-280M-4, MK1-145-DK-1000 40 40 1000(sensitive element, switch type 1, F-class insulation Tk, 145℃, three-branch structure) should be selected. For single-phase motors, a single-branch structure thermal switch can be used to directly cut off the motor power supply to protect the motor.

1.2 PTC Thermistor

PTC thermistors are small in size, have a low heat capacity and a fast response speed. They are suitable for protecting motors from faults such as undervoltage, overvoltage, overload and phase loss. However, they must be equipped with dedicated motor protectors (such as GRB motor protectors) when in use. Generally, R1, R2, and R3 thermistors are respectively pre-embedded in the A, B, and C three-phase coils of the motor. When the temperature of any phase in the motor's three-phase coils reaches the temperature control point, the value of the thermistor in that phase rapidly rises above 3000Ω (the resistance value at normal temperature is less than 100Ω), and the arc-free relay contacts inside the GRB protector act. Drive the main control circuit to cut off the power supply. PTC thermistors can also be used to protect the bearings. The temperature of the thermistor is within the range of 80℃ to 180℃ (in series every 5℃), the maximum working voltage is DC 30V, and the working voltage of the motor protector is AC 220V 50Hz. Motors of different insulation grades should be equipped with thermistors of different Tk. For instance, for the YX3-280M-4 motor, MZ6-150-DK-1000 40 40 1000 is selected (sensitive element, switch temperature control type, F-class insulation Tk 150℃, three-branch structure).

2. Non-fixed value control protection

Non-fixed value control protection is to display the actual working temperature of the monitoring points inside the motor in real time, and then control it according to the operating conditions of the motor. There are usually two types of non-fixed sensing elements, namely thermal resistance sensing and thermocouple sensing.

2.1 Thermal resistance sensor

The non-fixed value control protection of thermal resistance is based on the linear change principle of thermal resistance, that is, for every 1℃ increase in temperature, its resistance value also increases by the same amount (for example, PT100 is 0.385Ω/℃). This signal is sent to the KLB instrument, and the instrument converts the change in resistance value into a temperature display. In this way, KLB constantly monitors the temperature changes at A certain point of the motor (usually one each for the front and rear bearings, and one each for phases A, B, and C of the coils). When the temperature of the thermal resistance sensing element at a certain point exceeds the set value, an alarm will be triggered or the motor will be powered off, thereby preventing the motor from burning out. In motors, three-core PT100 wires are generally used. Two wires of the same color are considered the same wire and can be connected to the same point. Any one of them can be a compensating wire. For instance, the commonly used WZP-3×16-T(WZP stands for PT100, 3×16 represents the diameter × length of the sensor, and T indicates ceramic packaging), WZPM is a platinum resistance thermometer for bearings. When in use, drill a through hole in the motor bearing chamber and tap the corresponding thread, then install the platinum resistance thermometer firmly so that the sensor end face contacts the outer ring of the bearing.

2.2 Thermocouple sensor

Thermocouple non-fixed value control protection is based on the principle that thermocouples generate different electromotive forces at different temperatures. This signal is sent to the KLB instrument, which converts the magnitude of the electromotive force into a temperature value for display. In this way, KLB constantly monitors the temperature changes at A certain point of the motor (usually one each for the front and rear bearings, and one each for phases A, B, and C of the coils). When the thermocouple sensing element at a certain point exceeds the set value, an alarm will be triggered or the motor will be powered off, thereby preventing the motor from burning out.

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