Motor protector working condition classification and wiring method
2, for the harsh working conditions, high reliability requirements, especially for motors involving automated production lines, should use high-end, full-featured intelligent protectors.
3. For explosion-proof motors, eccentricity due to bearing wear may cause high temperature friction at the explosion-proof gap, which may cause explosion hazard. The wear condition monitoring function should be selected. For special equipment such as large-capacity high-pressure submersible pumps, due to the difficulty in inspection and maintenance, the wear condition monitoring function should also be selected, and the temperature of the bearing should be monitored to avoid major economic losses caused by broom accidents.
4. Applicable to protectors with explosion-proof requirements. According to the specific requirements of the application site, the corresponding explosion-proof protector should be selected to avoid safety accidents.
Selection of wiring method for protector main current
The main current wiring method is divided into:
1. One-time core-through method (you can also use the secondary current transformer secondary circuit)
2, the terminal way (can also use the peripheral current transformer secondary circuit)
3, direct insertion method
One-time core-type wiring is convenient and safe, and avoids contact resistance heating caused by poor contact of the terminal. The rated current of the motor is above 5A, and it is generally possible to use one-time core wiring.
It is convenient to connect directly into the wiring mode, especially for those cases where the space is small and suitable for the installation position, the plug-in protector can be directly connected to the main contact of the contactor output.
Protector setting current range selection
In order to adapt to the selection of different power motors, the protector basically has a certain current adjustment range. When selecting the protector, select the value of the middle area of the set current range as much as possible according to the rated current value of the motor.
Working power selection
The working power supply is mainly for the internal circuit of the protector. Except for the working power type, the working power level is generally divided into:
AC380V, 220V, 110V, 36V. There is no special requirement for the working power supply, because it is an independent power supply unit, the user only needs to choose according to the motor control circuit voltage level.
The bimetal protector is actually a switch or relay made of bimetal, which is widely used due to its low price. The bimetal is formed by laminating two alloys having different coefficients of thermal expansion, wherein the larger expansion coefficient is called the active layer; A small expansion coefficient is called a passive layer. Due to the different expansion coefficients of the two layers of material, the bimetal will bend and deform when the temperature rises, and will return to its original state after the temperature is lowered. People use this phenomenon to make switches that can be closed or opened at a specified temperature.
For the compressor motor, when the winding temperature rises to a certain temperature (such as 110 ° C), it is necessary to disconnect the power supply in time to prevent burning; When the temperature drops to a certain temperature (such as 60 ° C), it can be automatically reset and the compressor resumes operation. This is how the bimetal protector works.
Bimetal protectors can be divided into two types: thermal protectors and overload protectors. The thermal protector itself does not generate heat, and heat comes from the heat of the protected part. There is an electric heater (heating wire or electric heating plate) in the overload protector. When the current is too large, the heating of the electric heater may cause deformation of the bimetal.
The thermal protector looks like a pencil head, often tied or pasted to a position where the stator winding temperature is relatively high, and the winding temperature is transmitted to the bimetal through the metal casing. When the winding temperature exceeds the set temperature, the thermal protector trips and the control circuit connected to the thermal protector is disconnected, thereby triggering the main circuit contactor to trip and the compressor to stop running. The thermal response time of the thermal protector is an important parameter and can generally be operated within a few seconds after reaching the set temperature. Be sure to ensure good thermal contact during installation, otherwise it will not be able to act in time and will not provide thermal protection.
Unlike the thermal protector, the overload protector has one or more small electric heaters (electric heating wires or electric heating sheets), and the electric heaters are connected in series in the single-phase or three-phase main circuit. When the motor is overloaded, the current increases, the temperature of the electric heater rises rapidly and the bimetal is deformed, the contacts connected to the main circuit are separated, and the compressor is stopped.
The overload protector can also transfer heat through the housing, so the overload protector itself is also a thermal protector. The overload protector is bulky and has a slow thermal response. In addition, the external overload protector cannot be used as a thermal protector.
In terms of installation, each has its own advantages and disadvantages. The thermal protector is independent of the main circuit, so there is almost no limit to the motor current, but it needs to be connected in series in the control loop, and the wiring is complicated. The overload protector is directly connected in series in the main circuit. It does not require additional wiring. It is simple and intuitive, but it is not suitable for electric appliances with large current to avoid contact arcing or soldering. The thermal protector can handle the motor overheating very well:
Such as voltage abnormality, phase imbalance or even phase loss caused by overheating; Overheating caused by insufficient motor cooling (such as refrigerant leakage and low return pressure); Overheating caused by high and low pressure series gas (broken disk damage, piston ring damage, pressure relief valve opening, etc.); Overheating caused by poor lubrication, holding shafts and even stalling.
Thermal protectors do not handle high current problems very well, so it is often necessary to have a main circuit overload protector or a current limiter. Overload protectors react quickly to large currents, and common phenomena that cause large currents include:
Power phase imbalance, phase loss, phase loss caused by contactor, excessive condensing pressure, misalignment caused by poor shaft, broken link or blockage caused by piston biting, damage to scroll or cross slip ring Stalled. Commercial compressors below 15HP generally use Klixon and other brand overload protectors.