Overload protection is to preventMain power supplyLine overload causes protectoroverheatedOverload protection equipment added due to damage.
Overload is a relatively broad term, which can refer to excessive load on electrical equipment or excessive force on objects.The protection for these behaviors beyond the "load" is collectively referred to as overload protection.
In the communication power supply system, the function of power supply equipment automatically disconnecting power supply due to excessive load is called overload protection.
It is allowed to pass through the electrical circuit continuously without overheating the wireelectric currentQuantity, called safe carrying capacity orSafety current。If the current flowing through the wire exceeds the safe current carrying capacity, it is called wire overload.Generally, the maximum allowable working temperature of conductor is 65 ° C.In case of overload, if the temperature exceeds this temperature, the insulation will age rapidly and even the line will burn.
In machinery, overload protection can prevent damage to equipment caused by overload when the shaft exceeds the load it can bear.
In microelectronics, the overload protection can automatically cut off the power supply or automatically switch the working mode when the current is too high somewhere in the circuit[1]。
Main reasons for overload
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The main reasons for overload are improper selection of conductor section, and the actual load has exceeded the safe current of the conductor;
There is also the phenomenon of "small horses pulling big cars", that is, too many high-power equipment are connected to the line, which exceeds the load capacity of the distribution line.
Overload protection shall be adopted for lighting lines in important material warehouses, residential areas and public buildings, power lines that may cause long-term overload of conductors or cables, and insulated conductors with ductile sheath laid on combustible or refractory building components.Automatic switch should be used for overload protection of lines.During operation, the setting current for long time delay action of automatic switch shall not be greater than the long-term allowable load current of the line.If usedFuseFor overload protection, the rated current of fuse melt shall not be greater than the long-term load current of the line.When the automatic switch is used as the protection device, its current release is grounded at the neutral pointThree phase four wire systemMedium, it should be installed on the phase line;In the three-phase four wire system with ungrounded neutral point, it is allowed to be installed on two phases;In the ungrounded two-phase two-wire system, it is allowed to be installed on one phase.
In the communication power supply system, the function of power supply equipment automatically disconnecting power supply due to excessive load is called overload protection.
Necessity of overload protection
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The amount of current in the electrical circuit that allows continuous passage without overheating the wire is called safe current carrying capacity or safe current.When the load is too large, the current flowing through the wire will exceed the safe carrying capacity, which is called wire overload.Overload of conductor will lead to temperature rise of conductor. Generally, the maximum allowable working temperature of conductor is 65 ° C.When the conductor is overloaded, if the conductor temperature exceeds the maximum allowable working temperature, the insulation layer of the conductor will age rapidly and even the line will burn, causing a fire.Therefore, in the actual power supply line, we usually have the function of overload protection equipment to prevent the overload from causing safety hazards[2]。
Realization of overload protection
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In the field of communication power supply, air switch and fuse are usually used to achieve overload protection function, and an air switch or fuse can be placed in the power supply line to achieve overload protection function.The two devices are implemented in different ways:
Air switch: when the current flowing through the air switch is greater than its rated current, the air switch will automatically trip.The air switch is a reusable device, which can be reused after being re closed after tripping.
Fuse: When the current flowing through the fuse is greater than its rated current, the fuse will open.The fuse is a disposable device, which cannot be used again after being fused.
Two speed motor and overload protection
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The speed change of the motor adopts the connection mode of changing the winding, that is, changing the number of magnetic pole pairs of the rotating magnetic field of the motor to change its speed.
The two speed motor (fan) has low speed at ordinary times, and sometimes the fan rotates at high speed, which is mainly realized by changing the winding connection mode of the motor coil by switching the following external control lines.
1. Two common windings with different pole pairs are embedded in the stator slot, and the connection method of the motor stator winding is changed by switching the external control circuit to change the pole pairs;
2. Two independent windings with different pole pairs are embedded in the stator slot;
3. Two independent windings with different pole pairs are embedded in the stator slot, and each winding can have different connections[3]。
