The Case of the Over-Cranked Fan


An aircraft parts manufacturer had an aluminum heat treat oven that wasn’t meeting its surveys. The customer surmised that what the oven needed was more airflow, and they called ACES to make the adjustment.


The fan was running 500 rpm, and being driven by a 40 HP, 60 cycle AC motor. The fan was capable of another 100 rpm and the customer thought that would suffice to give them the airflow they needed to make their survey.

The CSI (Controls Systems Investigator) on the case ascertained that if he fed higher frequency into the motor — say, 70 cycles, he could run the fan faster and get the airflow they needed.

This would boost the horsepower on the fan, but would the motor overheat with the extra current?


A Variable Frequency Drive (VFD) gives the capability to overspeed the motor. Normally for a 40 HP motor you’d order a 40 HP VFD, but the CSI knew that it takes more horsepower to overspeed a fan than might appear at first glance, so he ordered a 40 HP heavy duty, which is equivalent to a normal 50 HP VFD.

The VFD could also be used to address the overheating concern: The CSI set the drive to run at the maximum current of the motor or up to 75 Hz, whichever was reached first. That way he could monitor the current coming out of the VFD and set a current limit on the motor while pushing it as hard as he could.

With the extra airflow the oven met its surveys, and the customer was able to fill their orders reliably and consistently.