普通小型电力变压器上都有一个无载分接调压开关。当用户设备端电压在因电网电压波动、供电线路过长、线路损耗大或因负载量变化等引起输出电压低于或高于额定电压时，就可通过该分接开关在-5%、+5%范围内进行调节。
There is a no-load tap-changer on the ordinary small power transformer. When the terminal voltage of user equipment is lower or higher than rated voltage due to fluctuation of grid voltage, excessive length of power supply line, large line loss or variation of load, the tap-changer can be adjusted in the range of -5% and + 5%.
调节时必须断开所有负载，使变压器处于空载条件下进行调节操作。调压原理是通过改变开关位置时，开关内部的触点将相应地改变变压器原边绕组的匝数，从而改变输出电压值。
When adjusting, all loads must be disconnected to make the transformer operate under no-load condition. The principle of voltage regulation is to change the output voltage by changing the position of the switch and the number of turns in the primary winding of the transformer.