Rotary kilns have always had variable speed drives. From the earliest times there was always, at least, an option of "full speed" and "half speed". This allows the operator to vary the rate at which the feed advances down the kiln, and in particular, allows the kiln to be warmed back up again if for some reason the burning zone has become too cold to sinter the clinker. On early drives, speed change was brought about by use of "fast and loose" pulleys of various sizes. However, with DC motors it was also possible to vary the speed of the motor itself, and this ability was one of the main reasons for the early adoption of electric power. However, variable speed was not viable for more powerful motors, so this placed a limit on the size of kiln that could be turned with a single motor, the maximum being around 250 kW.