# VFD or soft start to minimize power consumption on starting a motor?

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?

• If you don't get an response here, there are several regular contributors at https://electronics.stackexchange.com/ who have relevant experience. Feb 3, 2021 at 23:34
• Info on soft starter you have been looking at. Motor is 5kVA? What's driving three-phase generator? Feb 4, 2021 at 5:05
• This is an excellent question - I'm not an EE, a few years back when talking with one about start up current of motor they confirme that with a VFD/Frequency inverter start up current can be as low as we want it. So I'd say an FI could work for your problem. BUT you should check if the FI requires a stable power source and if your generator is stable enough.
– mart
Feb 26, 2021 at 6:42
• Talk to well pump people. How big a well pump? what is the lift? what type of pump drive system (shaft or immersion). What is the pump make and model? What is the purpose (irrigation, drainage, pressure water system supply)? How often will it cycle on and off? Diesel gensets designed for industrial motor starting don't need to be over rated to get most pumps moving, but a lot depends on the details. Feb 26, 2021 at 17:13
• Neither SRC current limiters or VFDs will let you use a smaller genset head, At least not under 50 kV·A or so. They both cause distortion to the wave form that lowers efficiency by more than having a slightly bigger gen head. What you want to do is engineer the plumbing system and select the proper pumps to maximize efficiency. Feb 26, 2021 at 17:38