Regulators may be supplied for use with 230 mm, 300 mm, 380 mm, 450 and 600 mm single phase fans. They have four speeds, are metalclad and of auto - transformer type for 450 mm 1400 r.p.m. and 600 920 r.p.m. fans and of choke type for others. Electronic Stepless Regulators can also be supplied with all single phase fans.