Stock air conditioner capacitor vs SoftStart

Watch my High Rate Data Logger (HRDL) reveal how soft start technology works

Dear Mike,

It would be informative to also show a plot of a “hard start “capacitor with both the standard capacitor and the SoftStartRV. That should dispel some rumors that keep popping up of how a $30 hard start capacitor does the same thing as the soft starter. —Jim

Dear Jim,

I’m getting ready to run a HRDL (the High Rate Data Logger I designed) graph using a Supco^®Hard Start Capacitor on my Dometic Penguin II 15kBTU air conditioner. That should help explain the differences between a hard start capacitor, stock starting capacitor, and soft start controller technologies.

“If wishes were horses, beggars would ride…”

Sadly, no matter how much we all wish it were true, a hard start capacitor won’t help start an air conditioner compressor using a small generator.

I know that these aftermarket soft start controllers cost a lot of money, especially compared to a $10 Supco hard-start capacitor. But they aren’t the same thing at all. Hard start capacitors are really only useful on a capacitor start induction motor connected to low-voltage utility power.

So if you have an air compressor out in the barn on a long electrical run from the house, then a hard start capacitor could work. But generators don’t have a voltage sag issue – they have electronic controls to limit the peak current so they don’t destroy themselves.

Why hard-start capacitors don’t help generators start an air conditioner

While even a standard home electrical service panel can muster up a few hundred amperes of peak current to kick an induction motor up to speed, a inverter generator might be able to supply only 2 or 3 times its rated amperage (at most) for a fraction of a second before shutting down. This suggests that a 2,000-watt inverter generator that can output 16 amps of current for a while might be able to provide 30 or 40 amperes of peak current long enough to kick start a stock 13.5kBTU air conditioner compressor, but maybe only on a good day with nothing else turned on in the RV.

Here’s a detailed graph of what’s going on inside of a soft start controller compared to the stock starting capacitor from the factory.

If you zoom in to take an even closer look, you can see HRDL data showing the actual sine wave of the 60 Hz waveform. Note that in the SoftStartRV plot on the lower half of the graph it appears that the waveform is sort of chopped up.

And that’s exactly what these soft start technologies do. By rapidly switching the starting current on and off hundreds of times a second, they slowly ramp up the amperage, and keep the generator (or heavily loaded pedestal circuit breaker) from tripping off.

Yes, the RV manufacturers could include this sort of technology in higher-priced air conditioners, and there are a few inverter-powered air conditioners on the market. But once again, it’s all about the money. Since inexpensive air conditioners work just fine on unmetered shore power, there’s not a lot of incentive to up the price of an RV to include a more efficient air conditioner that will run on a smaller generator.

For more fun, here’s my latest video which uses variable speed playback of HRDL data to show how soft start technology works compared to a stock capacitor.

Click the link below to watch the video showing how it works in real time and slow-motion. I think it’s a really descriptive way to show how a complex event occurs. Watch my latest video HERE.

What about a hard-start capacitor graph?

As you can see, a hard-start capacitor still pulls the maximum LRA (Locked Rotor Amperage) as the stock capacitor, but for a longer time of 1.5 seconds compared to 0.150 seconds of the stock starting capacitor. There’s really no way that a hard-start capacitor can help a smaller generator start the compressor. But more on that later….

Please stand by for another HRDL graph article showing how a Supco Hard Start Capacitor works by in detail, and why it’s not a good choice for starting your RV air conditioner from a portable generator.

Let’s play safe out there…. Mike