本文件描述了交流电机的甚低频（VLF）测试。它包括安装后在工厂或现场对新机器进行验收测试。还包括在用机器的例行维护测试。为了便于调查人员之间的沟通和比较，本文件建议使用的VLF为0.1 Hz+/-10%。对于交流电机，3.3kV至30kV之间的电压被视为高压电机。

This document describes very low frequency (VLF) testing of ac electric machines. It covers acceptance testing of new machines in the factory or on-site after erection. Also covered is the routine maintenance testing of machines that have been in service. In order to facilitate communication and comparison among investigators, this document recommends that the VLF used be 0.1 Hz ±10%. For ac electric machines, voltages between 3.3 kV to 30 kV are considered to be high voltage machines.