so, if i were making a motor, why not make one that is multi-use? 380v/50Hz/15hp, same motor can run at 460v/60Hz/20hp, same motor that can wear different nameplates (multi-market use, etc). you make it to work at the higher hp, stepping it down by a tad wont hurt it. thats what i meant by same motor different nameplates, etc.
That's essentially the way things are, but as Besoeker said, there are different design philosophies between IEC and NEMA. In a nutshell, NEMA design philosophy calls for designing off-the-shelf motors to be useful in the vast majority of applications by looking at the near worst case scenario. IEC design philosophy is to make motors that are more tailored to the exact task necessary and only use what needs to be used. There are all kinds of valid reasons for each approach, some rooted in history and geography. But nonetheless, if you understand the differences, most 380-400 and 440-480V motors can be interchanged across borders, and this is done on a regular basis. Many of the motors sold in both areas
are different only on the nameplates. However if you sell a NEMA designed motor in IEC world, it has no problem delivering full torque at the reduced speed, but it will be a larger frame because of being designed for a Service Factor, which means nothing in IEC world. Then if you use an IEC motor in NEMA land, it too delivers rated torque at the higher speed, but will NOT have a Service Factor as we are used to. Those facts limit the interchangeability to some extent.
Another of those differences is in the units of power measurement on the nameplate and that's where people get confused. In North America we still use HP (the UK used to, but has officially changed), in the rest of the world, they use kW. But some of the confusion lies in converting. Yes, there are 746W/HP, but when speaking of motor nameplates, BOTH the HP and kW values expressed are MECHANICAL output, not electrical input. As I said earlier, HP (or mechanical kW) is just a shorthand expression of a given amount of torque at a given speed. So if the torque is the same but the speed is different, the HP or kW is different. But for the majority of machines (that are NOT centrifugal pumps and fans*), what we need is TORQUE, with speed as a consequence that is dealt with in gear/sheave ratios. So if you have a conveyor, milling machine, rock crusher etc., it's fairly easy to deal with the speed change either in the machine design or with something like a VFD, because the beauty of a VFD is that it allows the motor to provide the SAME torque at any speed.
Where you can get hung up in the conversion aspect is if you take the IEC kW rating as ELECTRICAL kW, not mechanical. They use two different terms for "power" in order to differentiate; "absorbed" power and "rated" power. The "rated" kW is the mechanical output, what we often refer to as the shaft power, but what we call the HP of a motor. The electrical power that the motor consumes is called the "absorbed" power, and just like here, is NOT shown on the nameplate. You can infer it by looking at the maximum rated power and efficiency rating, but remember, that is always the MAXIMUM, not what the machine actually uses. That holds true for NEMA motors as well, we just have to convert the mechanical HP to mechanical kW first, then apply the efficiency.
Where the biggest problem lies in import/export machinery powered by AC induction motors is in the market for "230V" class machines. Here in the US, we have more 208V 3 phase systems than 230V. In IEC world, 230V is largely used for single phase, but there are many areas that use 230V 3 phase as well, but never 208V (let's leave Japan out of this for now). So an IEC motor designed for 230V is 230V 50Hz, so it wants a V/Hz ratio of 4.6:1. Here in NA, our 230V 60Hz is going to be at 3.83:1, 17% lower. That means the running torque will be at 83.3%, but more importantly the
peak torque output of that motor will vary by the square of the difference, so .833^2 is .69, meaning the peak torque output of that motor is only 69% of what it was designed for. Peak torque (Break Down Torque) is what the motor uses to accelerate, and more importantly RE-accelerate, the motor under load. So even if you downgrade the motor mechanical power by 17%, as some bottom feeder exporters do, the loss of accelerating torque is likely going to result in the motor running hotter and failing sooner than you might expect. Most of the time they will outlast the warranty, which is all the bottom feeders care about, but the end user ends up paying the long term cost. Then if you use that IEC motor on 208V 60Hz, it gets even worse; the difference is 25% instead of 17% and peak torque drops to just 56% of normal, risking a stall. Conversely, if you send a NEMA 230V 60Hz motor to IEC land, the motor will get OVER excited by 20% and over excitation means that the extra voltage will produce MORE torque, which may damage the mechanical components, but will also result in saturating the windings which means more heat for the same amount of work done. They too then don't last as long as they should unless the load is significantly smaller than the motor, something not done in IEC world. Bottom line, despite claims otherwise, its never a good idea to assume a 230V machine will import/export without changing the motor. Whatever someone claims as a working solution will be a compromise (unless a VFD is involved).
* Centrifugal machines like pumps and fans present a different challenge with frequency change because of the Affinity Laws, one of which dictates that the power REQUIRED by the centrifugal machine varies at the CUBE of the speed difference. So if I have a centrifugal pump designed around a 50Hz AC motor speed, and I hook it up to a 60Hz supply, the motor will spin at 120% speed. That means the pump attached to it will
demand mechanical power from the motor that is 1.2 x 1.2 x 1.2 (173%) the amount it required at 50Hz. The motor output power will increase to 120%, but it will still fall way short of what the pump requires.
{Besoeker, you are right, I missed the frequency change issue in stating that current follows torque 1:1...}
