推荐回答（1个）
因为热继电器的触点动作时间与被保护的电动机过载程度有关，所以在分析热继电器工
作原理之前，
首先要明确电动机在不超过允许温升的条件下，
电动机的过载电流与电动机通
电时间的关系。这种关系称为电动机的过载特性。
当电动机运行中出现过载电流时，
必将引起绕组发热。
根据热平衡关系，
不难得出在允许温
升条件下，
电动机通电时间与其过载电流的平方成反比的结论。
根据这个结论，
可以得出电
动机的过载特性，具有反时限特性
了适应电动机的过载特性而又起到过载保护作用，
要求热继电器也应具有如同电动机过载
特性那样的反时限特性。
为此，
在热继电器中必须具有电阻发热元件，
利用过载电流通过电
阻发热元件产生的热效应使感测元件动作，
从而带动触点动作来完成保护作用。
热继电器中
通过的过载电流与热继电器触点的动作时间关系，称为热继电器的保护特性
热继电器中产生热效应的发热元件，应串接于电动机电路中，这样，热继电器便能直接
反映电动机的过载电流。
热继电器的感测元件，一般采用双金属片。所谓双金属片，就是将
两种线膨胀系数不同的金属片以机械辗压方式使之形成一体。
膨胀系数大的称为主动层，
膨
胀系数小的称为被动层。
双金属片受热后产生线膨胀，
由于两层金属的线膨胀系数不同，
且
两层金属又紧密地贴合在一起，
因此，
使得双金属片向被动层一侧弯曲，
由双金属片弯曲产
生的机械力便带动触点动作。
双金属片的受热方式有
4
种，即直接受热式、间接受热式、复合受热式和电流互感器受
热式。
直接受热式是将双金属片当做发热元件，
让电流直接通过它；
间接受热式的发热元件
由电阻丝或带制成，
绕在双金属片上且与双金属片绝缘；
复合受热式介于上述两种方式之间；
电流互感器受热式的发热元件不直接串接于电动机电路，
而是接于电流互感器的二次侧，
这
种方式多用于电动机电流比较大的场合，以减少通过发热元件的电流
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