I'll expand a little on what was already said. First, some background rules relating to what goes on inside of a VFD:
1) torque = current
2) current = heat
3) heat is mostly the result of switching losses in the transistors
4) the longer a transistor is in the circuit, the more switching losses it experiences
5) the lower the output frequency, the longer each transistor stays in the circuit.
So with that background, let's explore the nature of loads on a motor. Loads can mostly be divided into two main categories; Constant Torque (CT) and Variable Torque(VT). To make this easier, I'll only describe VT in detail, because everything else is CT. But briefly, because CT means torque remains the same as speed changes, current remains the same as speed changes (rules 1&2 above). That then means that with a CT load run at low speeds, the transistors will experience MORE heat than at full speed, because the losses are greater. Keep that in mind.
A VT load is different in that by definition, it comes about only in centrifugal machines (aka "quadratic power" to much of the rest of the world). These are almost always centrifugal pumps and centrifugal fans*. As a side note, this brings out the marketing folks, who use various euphemisms for this, because technical terms like centrifugal or quadratic mean little to non-technical people. Hence the term "HVAC drive" because 99% of drives used in HVAC will be for centrifugal pumps or fans. Others call them VT drives, pump drives, etc., and in the case of Rockwell, "Normal Duty" because statistically, over 70% of ALL AC induction motors are used on centrifugal loads, hence, "Normal".
What's different about the loads, and the reason the drives are different, is the nature of the load on the motor being "variable" with speed, following what's called the "Afinity Law of speed change" which dictates that as the speed changes on a centrifugal machine, the load expressed on the motor driving it changes at the cube of the speed change. So if a pump is run at 1/2 speed, the load required by the motor drops to 1/8th of what it was at full speed (.5 cubed = .125 = 1/8).
So now, apply this to the rules stated earlier. at 1/2 speed on a CT load, the transistors will get HOTTER, but at 1/2 speed on a VT load, the transistor will run COOLER because even though they are on longer, the current is 1/8th of what the transistor was sized for.
So now fold this into someone designing a drive. If its a CT drive, you have to design for the worst case scenario; full torque (current) at low frequency. You ALSO must design it to be capable of delivering the maximum torque from the motor, called "Break Down Torque"(BDT), because that is what the motor utilizes to accelerate the load, and more importantly, RE-accelerate a motor after a step change in load. BDT is actually higher than Locked Rotor Torque, it's about 200% of Full Load Torque, therefore the motor will want 200% current (rules 1&2 again). The motor can only take this briefly, but that's usually taken into account by the mechanical engineer, so the VFD only needs to deliver it briefly if necessary. In addition, most motors are capable of being run at lower levels of overload for slightly longer periods. But still, that means the VFD designer must account for all of that, and is called the "overload capacity" of the drive. It's based on the thermal charging capacity of the transistors at the rate they can dissipate into the heat sinks, assuming they were already at full capacity when this started, hence, larger transistors. So a CT rated drive will be capable of 150% overload for 60 seconds, 200+% for 2-3 seconds. That way it can basically do whatever the motor can do.
But if the VFD is going o be used on a VT load, there is no chance the load will EVER require BDT from the motor, it's something that can only happen if there is a problem, in which case you WANT to shut it down. It cannot even overload briefly (if selected for the pump or fan curve) because load = flow, unless, like in a pump, the pipe breaks and it goes "open channel flow", in which case again, you will WANT the drive to shut it down. So in a VT drive, the transistors can be a lot smaller, usually at least one size smaller, and still run the same motor size at full speed. Bottom line, the drive is cheaper. But you sacrifice the overload capacity of that drive. A VT drive will only be rated for 105-110% overload for 30 seconds (not 60), 150% for 2-3seconds. So it will NEVER be capable of delivering BDT if used on a CT load.
So using an HVAC rated drive on a conveyor may work if everything is perfect and never changes. But if anything ever does change, even though the motor could handle it, the drive will have to protect itself and shut down, whether you want it to or not. So weigh the cost of lost production against any perceived savings in hardware.
Class dismissed...
* Not all pumps or fans are centrifugal, always check.
