The authors describe the successful development of a variable-frequency test generator suitable for testing rotating electrical plant with voltages up to 31 kV (peak) (22 kV RMS) over a frequency range DC to 1 kHz subject to the current limit of 100 mA (peak). The design, which is based on a high-voltage class A/B amplifier in a totem-pole configuration with nonlinear control, uses a fibre optic system to provide electrical isolation and is capable of supplying a range of waveforms subject only to rating limitation. This makes the generator a valuable research tool permitting many types of investigation including the effect of distortion on measurements obtained from the standard 0.1 Hz generators.