I want to use the smallest generator possible to run a standard three-phase AC well pump. According to this website and others, it is advisable to use a generator capable of 3x the running kVA if the pump is started directly, and 2x the running kVA if the pump is stepped up using a star-delta starter. So for all but the first few seconds of use, the generator will be running at between 33% and 50% of its rated load. This seems wasteful:
- Bigger generators costlier, bigger, and heavier
- Diesel generators, which are typically the only economical choice over 10 kVA, decline steadily in fuel efficiency as the load falls below about 85%, and below about 60% there is a risk of cylinder glazing
- The application of dummy loads, as the linked website above suggests, will indeed prevent glazing, but at the cost of significant useful efficiency.
So I am thinking of installing either a soft starter (which gradually increases voltage) or a variable frequency drive (which gradually increases frequency, and hence power demand) in front of the well pump.
Soft Starters
I guess you can think of a star-delta as a coarse, step function-like soft starter. In the initial Y-connected phase, voltage is comparatively lower, then when the motor is switched to Δ-connected, voltage increases by a factor of $ \sqrt{3} \approx 1.73 $. The website linked above says that with a star-delta starter, the generator should be rated for at least 2x the running kVA. The soft start I've been looking at, however, can ramp up from 30% of running voltage to 100% in as long as 30 seconds. My thinking is that $\frac{30\%}{1/\sqrt{3}} = 0.5196\ldots\approx\frac{1}{2}$, so the soft starter should need about half the kVA of the star-delta. So I should be able to start a pump that wants 5 running kVA with a 5 kVA generator, right?
Variable Frequency Drives
My understanding is that VFDs can be used to soft start a motor by gradually increasing frequency (and possibly voltage with it). I understand that VFDs provide many advantages over soft starts. But do they use any less peak power on startup?