VFD Power

V/f ratio is different in different cases

Let take V/f ratio from reverse computation

1. V = (1163*1000)/1.732*1427.22*.87 = 540 V i-e v/f = 13.2776 @ 40.67 Hz

2. v= (1184*1000)/1.732*1452.4*.87 =541 V so v/f= 13.27 @ 40.67 Hz

3. v =(861*1000)/ 1.732*1166*0.87 = 490.04 V v/f= 14.13 @ 34.66 Hz

4. V= (350*1000)/1.732*731.05*0.87 = 317.726 V v/f= 9.174 @ 34.63 Hz

I think the problem is with the computation of v/f. but how many v/f we have

You are calculating power based on the motor nameplate PF, which is based on its effect on a fixed sine wave power supply. The VFD changes all of that (among other things); it can alter the output waveform based on the PF to maximize shaft power vs input power. The algorithm for calculating output kW in a VFD driven motor is very complex, something the microprocessor in the VFD is doing for you.

ACS800 drive controls the motor speed by producing just the required torque. It will not follow the standard constant V/F ratio. It will adjust the V/F ratio continuouly to match the varying load requirement.

For example let us consider the case when your fan runs at 692.2 rpm. You are seeing two different torque readings at the same speed because the load requirement of the driven fan are different. So the drive adjusted the V/F ratio and produced just the required torque to maintain the fan at 692.2 rpm.

In this VFD, they don't follow the predefined switching frequency for the inverters. The IGBT's are switched on only when required ie the drive finds out what is the optimum switching frequency and the IGBTs are switched on accordingly. They say that the number of switching is less by about 30% compared with the traditional PWM inverters.