交流電(dian)(dian)(dian)磁(ci)鐵勵磁(ci)線圈通入交流恒壓源時(shi)，線圈將(jiang)感應電(dian)(dian)(dian)勢(shi)(shi)與(yu)(yu)電(dian)(dian)(dian)源電(dian)(dian)(dian)壓平衡(heng)；感應電(dian)(dian)(dian)勢(shi)(shi)與(yu)(yu)磁(ci)通成(cheng)正比(bi)，略小(xiao)于(yu)電(dian)(dian)(dian)源電(dian)(dian)(dian)壓。∵電(dian)(dian)(dian)源電(dian)(dian)(dian)壓不變，磁(ci)通也(ye)近似不變（如若因(yin)某種原因(yin)使Φ減小(xiao)，E也(ye)將(jiang)隨之(zhi)減小(xiao)；從而使電(dian)(dian)(dian)流增大，以增大磁(ci)勢(shi)(shi)，讓Φ增加。反之(zhi)亦(yi)然）。∴說它為恒磁(ci)通型。

若(ruo)通電(dian)后銜鐵(tie)不能(neng)吸合，電(dian)流將(jiang)不能(neng)減(jian)小。這不但(dan)使(shi)設備(bei)不能(neng)工作，而且(qie)時間一(yi)長(chang)將(jiang)會(hui)使(shi)線圈因過熱(re)而燒(shao)毀。