Variable Speed Fan Efficiency

People do drive their kiln fans above rated capacity sometimes, but it's an inefficient use of energy. June 13, 2014

From Gene Wengert, forum technical advisor:
Does anyone over-drive their fans? That is, does anyone use more than 60 hertz and have the fans turn faster? I have seen this used a few times at the beginning of a run and wonder how often this is done.

Forum Responses
(Commercial Kiln Drying Forum)
From contributor W:
I have several customers who do that. The limit is about 20% higher RPM but because the power varies as the cube of the speed, most people cannot get nearly that far. A fan will use about 70% more power if the speed is increased by 20%.

From contributor P:
We run fans at faster than 60hz but only at higher temperature when drying softwood. Fans need to run faster at high temperatures to push the same volume of air.

From contributor W:
We too have done this on high temp kilns. It is easy to do. Just set the current limiting feature of the VF drive to maintain full load running current and the drive will speed up automatically as the air density decreases.

From contributor Y:
We use VFD for our heat treatment kiln blowers. It helps develop a heat bubble at the beginning of the process and then meet airflow requirements during treatment. It uses energy more efficiently that way. The problem is - you cannot get more CFM out of an electric motor that its horsepower rating. In other words, at 60Hz, or 100%, the horsepower required to create a specific CFM is at peak. Raising the speed of rotation does not increase the horsepower of the motor beyond its maximum.