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