seanleed said:
At 50Hz, we were using 1.2x the energy that would be used DOL.
VFDs are generally about 95-97% efficient. So yes, at full speed there are losses, but 20%? I find that difficult to believe. There has to be more to that story...
Still, it's a valid point in that if you are going to run a motor at full speed most of the time, a VFD may cost you in energy losses. The energy saving value is in being a more efficient way of VARYING the flow in a centrifugal machine. If the flow is not varying, don't use a VFD (unless there is no other way to accelerate it, which happens sometimes).
waross said:
This is the first time that I can remember disagreeing with you Jeff.
Transformers saturate from over-voltage not overload.
As the impedance of the load drops, the current increases and the terminal voltage drops until the load impedance reaches zero Ohms and the current reaches the available Short Circuit Current as determined by the the transformer impedance.
Apart from that I am in agreement with you.
Bill, to be quite honest, I never really thought that through too much, I simply observed it (the first time on the motors for the drawbridge at Ford Island, going to the USS Arizona memorial at Pearl Harbor**) and made the leap to comparing it to a reactor start. I will, in the future, stop using that explanation in favor of yours. Makes a lot more sense.
**I built Soft Starters for the twin 75HP drawbridge motors for that back in the late 90s. I got called out there to troubleshoot them after installation, because they "weren't working". Turned out they put 75KA transformers feeding the 75HP motors, thinking that the soft starters would "magically" fix this problem. They of course didn't, because by the end of the ramp time, there was still no voltage to give to the motors and the OLs would trip out. So we bypassed the OLs for 30 seconds and it allowed the motors to come up to speed. I basically turned the ramp time to near zero, because the Soft Starter was not doing anything useful anyway, but I never told them that... Since then I have seen similar situations with under sized transformers resulting in a quasi-soft starter arrangement, but only if the motor is the only load affected.
" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden 
