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High Br/Bs and High 脤m Hanocrystalline Cores
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| (1).Comparision of nanocrystalline cores with other soft magnetic cores. | ||||||||||||||||||||||||||||
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| (2).Typical characteristic curves | |
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(3)Nanocrystalline cores used in summation current transformers for voltage independent RCCBs. ①Principles of operation.Fig.1 illustrates the functions of a voltage independent RCCBs.  Under normal conditions for the summation current transformer,the magnetising effects of current carrying conductors,in accordance with kirchhoff's low,cancel each other out.There is no residual magnetic field remaining,which could induce a voltage in the secondary winding.However if a defect in insulation causes a fault current,the balance becomes disturbed,and a residual magnetic field remains in the transformer core.this produces a voltage in the secondary winding,which via the release and the contact latching mechanism disconnects the circuit with the insulation defect. ②The properties demand of summation current transformer cores for voltage independent RCCBs. a. High permeability μm.The fault current given by the summation transformer will directly feed to the release,without any amplifying of signal through an electronic unit.Consequently it is important for higher permeability μm to obtain a higher voltage value in the secondary winding with low fault current. b. Lower coercivity Hc.There are two respects for Hc to influence the operating properties of the summation current transformer.First,it is necessary for low Hc to obtain high μm,and most importantly,Coercivity Hc has a great influence on the impulse current withstand capability for voltage independent RCCBs.after a surge current occurs to the summation current transformer,the cores will be quickly magnetized to saturation.When the surge current is eliminated,theoretically the saturate induction Bs should equal to zero,but practically the remance Br exists.when the undemagnetized cores are magnetized again,the permeability will decrease ,the secondary winding voltage value for summation current transformer will also decrease.This will increase the rated fault current of the voltage independent RCCBs whose value is dependent on the coercivity of the croes,e.g. the higher the value of Hc,the more reduction of permeability and the secondary winding voltage value.For a small reduction of permeability of the cores under surge current,it is necessary to reduce the coercivity of the cores. c. Sufficient ascending value of △E2.In addition to low coercivity and high permeability,higher ascending value of △E2 for the summation current transformer cores is also needed to guarantee the operating stability for voltage independent RCCBs.For the cores whose rated fault current are 42mA,when the exciting current value equals to 36-48mA, △E2≥1.2mV;for the cores whose rated fault current is 70mA,when the exciting current value equals to 60-80mA, △E2≥1.8mV. d. Excellent stress stability.Viberation will occur during delivery and assembly for the voltage independent RCCBs,which will result in the worse of magnetic properties for soft magnetic cores.In particularly the permeability will seriously reduce while Hc increase when the nanocrystalline cores are broken.consequently good stress stability is necessary for nanocrystalline cores to guarantee the excellent magnetic stability. e. Excellent temperature and time stability.When the soft magnetic cores are under long-term service at high temperature and rated operating temperature range,the magnetic properties will decrease slowly.The value of reduction can not influence the operating properties of the voltage independent RCCBs. ③Summation current transformer nanocrystalline cores for AC fault current voltage independent RCCBs. Fig.2 shows the hysteresis loop of nanocrystalline cores for AC fault current voltage independent RCCBs  Fig.3 shows the V-A properties for 30-100mA and 300mA nanocrystalline cores  ④Summation current transformer nanocrystalline cores for pulsating DC fault current voltage independent RCCBs. In various types of electrical equipment in industry such as frequency converters,medical equipment,control systems and computers,when fault produces,the fault current is pulsating DC type in stead of AC type current.These cores used in this kind of voltage independent RCCBs is very different from that of AC type cores..A single-phase pulsating square wave voltage is connected to the cores,the cores work on the first quadrant of the hysteresis loop,the induction changes between Bs and Br.It is necessary to reduce Br to obtain high △B=Bs-Br. So that high secondry winding voltage E2 can reach. Fig.4 illustrates the hysteresis loop of nanocrystalline cores for pulsating DC fault current voltage independent RCCBs 
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