Frequency Increase Through VFD

A VFD is technically capable of putting out any frequency, but most manufacturers limit it to practical specs, i.e. 400Hz maximum. That does not mean however that any motor can be operated at that speed.

Once you increase the frequency above the design frequency of the motor at the rated voltage, the motor can only operate in Constant HP mode. This is because you need voltage to mach the frequency applied at the same ratio as the design levels. So for example, a 460V 60Hz motor is designed to accept a V/Hz ratio of 7.7:1 (460/60) so as long as you can maintain that ratio, the torque remains constant. But if you are doing that, and the frequency goes above 60Hz, how are you going to keep the ratio the same? You will run out of voltage at 460V. So what happens is the ratio diminishes, and with it the output torque, at the square of the voltage difference. So at 120Hz, the motor is getting 1/2 of the voltage required to maintain torque at that speed, so the output torque will have dropped to .5 x .5 or 25% of the original rated torque of the motor.

One trick you can do however is to use a motor rated for less than your supply voltage. So for instance, if you have 460V, use a motor rated for 230V. Then program the VFD to provide 60Hz at 230V, the design rating of the motor, and if you need to run at 120Hz, you will be giving it 460V at 120Hz. This will effectively double the HP capacity of the motor, but of course, that will be at the higher speed. The motor cooling becomes a problem however, because not only will the motor losses increase proportionately, but often times the cooling fans no longer function effectively at that speed. Hence the earlier advice that you really must check wit the motor mfr. before doing this. There is also the issue of the speed rating of the bearings in the motor. most are not rated for use beyond about 150% of design speed.

There are motors designed to be run at high frequencies and speeds however, they are usually referred to as "spindle motors" and are significantly more expensive that standard off-the-shelf AC motors.

As you say, most VFDs can operate with a V/f ratio covering rated voltage at 25 up to 400Hz. Most common motors are specified at rated supply voltage at 50 or 60Hz.

The rated frequency of the motor is the Base frequency which is coupled with the rated voltage at this Base Hz to give the ratio characteristic for VFD operation.

Above the Base Hz you correctly advise theoretical constant HP, so at double the rated speed, this would be 1/2 torque, not 1/4 as you stated. Of course, motor losses increase with friction and windage making a standard motor audibly noisier. So, in practice, maybe a little less should be used compared to theoretical. The performance of the motor at higher Hz (supersynchronous) is also a function of maximum pullout torque for this motor as a ratio of the square of the voltage applied vs theoretical multiplied by peak pullout e.g. if at 460V / 120Hz on 460V / 60Hz motor then voltage is 1/2 of required constant torque voltage (920V), square this gives 1/4 and if max motor pullout is 240% then max will be 1/4 x 240% = 60%. If you wanted constant power so your torque demand is 50% then this is OK. If demand goes to 60% the motor stalls very quickly. If max was 200%, you'd be at absolute limit at 50% torque so don't do!!

Another consideration is the speed holding or slip characteristic which is different when operating supersynchronous - the slip curve will 'flatten' giving more slip for the same load changes at different Hz (as you are increasingly undervolting the motor at high Hz.

Going back to the original query, the main reason to do this is to:

1. get a wider speed range of operation as the low speed performance will determine minimum Hz, probably

2. have a higher starting torque capability

3. reduced motor torque at lower Hz means you can have a benefit running at lower speeds without overheating on constant torque applications

The mechanical motor design speed capability should be checked with the supplier but most motors up to 7.5kW have common designs for different pole numbers, so if you use a 4 pole, 120Hz is only 2 pole speed which it's designed for normally anyway. Vibration will, of course, increase with speed. I've seen standard 50Hz motors operate up to 180Hz on the 50Hz connection in some applications

Normally, operating at 230 or 400/460V should have no impact on insulation since this should be selected to be more than 2 x line to line whatever on any decent motor, if in doubt, check with the supplier again.