VFD Size

With a ruler....

Actually, most of them come with horsepower ratings at specific voltages and hertz. You just read the literature and pick one that equals or exceeds the motor characteristics, and has the features you need.

can i use 115% x motor kw (or Hp)?..anyways, this is an argument between practicioners in the company we worked for...the older ones would go into conclusion of having equal hp or kw rating of vfd w/r to motors..

The 115% reserve might be a good idea, especially in cases above the usually rated 50 Hz or 60 Hz, but sometimes it might kick you up to a larger VFD than you really need.

If a 10 hp motor has a 1.15 service factor, I don't know if a typical 10 hp VFD has an equal or higher service factor, so this is an interesting point. The next bump upward, if needed, would perhaps be a 15 hp VFD. Worth thinking about..., but I don't recall any cases of this being a problem (yet).

Modern VFDs are designed to be operated in overload conditions (above their rated kW) for short periods of time (seconds to minutes) so if you are only interested in momentary overloads then you shouldn't need to worry.

This overload information is included in the VFD manual and data sheets if you need to justify your position to others who may not be up to date with modern drive technology.

Actually, you should NEVER size a VFD by HP or kW alone, ALWAYS look at the motor nameplate FLC as a final check. Not all motors are created equal, the kW or HP rating just gives you a starting point.

There is a design philosphy of always over sizing everything "just in case", but this has a tendency to waste budget money. I look at the machine or process and evaluate it for any near term* future increases, then select only the amount of VFD power as is necessary to power the motor for that purpose. VFDs are designed to more than safely handle the same kind of starting duty as the standard motor it is connected to. The only time it's necessary to over size a VFD is when the starting duty is well above normal, such as a centrifuge or other large flywheel machine that has a long acceleration time and a special motor design.

There is also a "marketing" issue of "Heavy Duty" vs "Light/General Duty", or "Constant Torque" vs "Valiable Torque" ratings on VFDs. If you always choose CT and/or HD you will always be safe. The other choices are ONLY about saving money when the specific application is known to be non-stressful on the VFD overload capabilities. Such is often the case with centrifugal (quadratic) loads such as certain types of pumps and fans that will essentially NEVER be overloaded when the speed is low (the worst time for the VFD) because of the nature of the load's power demand curve. Remember that a CT or HD VFD can ALWAYS be used on a VT / LD application, but the opposite is not true.

* I personally feel there is no point in projecting out beyond 5-7 years for this, because in 5-7 years if the system expands and you need more capacity, there will likely be new VFD technology you may want to take advantage of by then.