There are two issues here, first being the motor and second being the load.
The motor: Torque (specifically flux) in an induction motor is proportional to the ratio of "volts to hertz". As long as this ratio is kept constant, the motor will produce constant torque (within limits...read on). If you reduce this ratio the flux will decrease and the torque will decrease as well. However, increasing this ratio beyond the nameplate ratio will not produce a linear increase in torque (due to core saturation) and will result in core and winding heating that will reduce the life of the motor significantly.
Back to your question, it may be by coincidence or may be by design, I honestly don't know. Take the IEC rating 380V/50hz=7.6 . Now consider the NEMA rating 460V/60hz=7.6 . In sum, the motor will produce the same torque (and flux) at the new voltage and frequency as the old and will experience a resulting increase in horsepower due to RPM increase offset by an increase in windage and friction losses, also due to RPM increase. Using the equation HP=(T*RPM)/5250 and disregarding windage and friction losses, when T is constant and RPM increases by 20%, HP will increase by 20% too. The best part of the whole thing is that current is constant. That sounds too good to be true but, W=V*I*sqrt3. V has increased by 20%, so W can increase 20% with constant I. With constant I and constant flux, the result is that heating would be the same in the core and winding, meaning no reduction in motor rating or life. In addition, the motor's centrifugal fan will produce 20% more flow (at a cost in efficiency)resulting in more cooling effect. The gains in power are not ideal, as the increased friction and windage losses take a toll.
Now, the down side.
The load: You must consider the load requirements. A constant torque load, ie. conveyor, will require the same torque at any speed, where a variable torque load will require increasing torque with increasing speed depending on the type of load. I am not sure what type of blower/pump your application uses and I am certainly not an expert on such things by any means, but I would feel safe assuming variable torque. This is an issue. For example, in the case of a centrifugal fan or pump, HP1/HP2=(RPM1)cubed/(RPM2)cubed. In other words, a 20% increase in speed and flow requires a 73% increase in power. This is far greater than the motor power increase of 20%. In addition, your vac pump/blower unit may not function properly at the new speed. Again assuming centrifugal: 20% more flow, 44% more pressure, 73% more torque (major consideration with respect to the pump).
The fact that you are concerned about D-flange means that you have a direct coupled unit. As such, the flange size is the limiting factor here since running your unit at 60hz (ie. approximately 3650 and 1750 rpm respectively) will require significantly more power and a subsequently larger motor frame than that currently used and will be difficult to match with the current flange. In the past, facilities with large amounts of euro equipment had to use 60/50hz MG sets to address this, many are in service today (been there, seen that). These days however, the cheapest and easiest option would be to use a variable frequency drive. VF drives operate on the volts/hertz principle for constant torque operation, so a 460V VF drive operating at 50hz will output 380V (voila!). As well, like computers, they are cheaper every day. Since your motors are used and may not be inverter duty, I would recommend buying new inverter duty motors if reliability is critical. IEC motors (and of course nema drives)are available in stock in the states these days, so that is not an problem. As well, I would not buy a top of the line drive, but would look for the simplest/cheapest one which will provide the ability to select a preset speed upon a single input (ie. hit start and ramp quickly immediately 50 hz). The conversion to VF drives and new motors will cost a few thousand dollars in the size you are looking at, so you will want to price the conversion against whatever the cost of a new unit is or against the cost of whatever other suggestions the forum somes up with.
