A building engineer, or HVAC engineer, designs the ventilation system. In the specifications section, he specifies how much air a motor/fan assembly has to move. This is specified in Cubic Feet per Minute, or CFM. The variable frequency drive (VFD) must power up the motor to deliver the requested CFM. For example, the University of Michigan has specifications for the VFD. One specification is that a separate VFD is to be used for each motor. Another specification by the University of Michigan and the University of Washington is the VFD is to be located as close to the motor as possible.
A VFD has to be overloaded momentarily, and the engineer takes this into account. When a motor first starts up, it draws a little bit more power than when running. This is called the inrush current. Inrush current is the primary reason for designing in over-current condition in a VFD. The engineer sets the overload specifications. For example, at the University of Pennsylvania, the overload is specified to be no more than 115 percent of the supply voltage for 1 minute for motors with adjustable torque. Adjustable torque means the horsepower output can vary.
Because a VFD controls motor speed by varying frequency of the input voltage, specifications are developed for minimum and maximum frequencies. At Indiana University, VFD controllers must have a frequency range of 15 to 66 Hertz for the maximum frequencies output. A Hertz is a unit of measurement of frequency. It is stated as cycles per second. A 60-Hertz frequency cycles on and off 60 times in 1 second.