Soft start vs Hard start for inverters

Fabian

Can someone explain the pros and cons for soft start and hard start on a inverter.
I am about to buy a inverter but i have to choose between soft or hard start. I am kinda confused. Which one is the better choice?

BB.

What appliance/devices are you looking to power?

Soft Start ard hard start (as I understand) are more or less just different aspects or ways of looking at a problem--The starting of an Induction motor--Typically driving a load that has "high torque" when starting (like starting a refrigerator compressor, air compressor, water pump against back pressure, etc.).

Soft Start is the attempt (through capacitors, switches, and/or electronics) to start induction motors with less surge current. For single phase induction motors, the "natural" magnetic field inside the motor is just alternating North/South magnetic field alertnating between two electromagnets (the poles). There is no "rotational" element to the field, and an induction motor would just sit there buzzing vs trying to rotate.

The use of a starting/run capacitor is to create a separate current/voltage source that is ~90% with respect to the other current. Put in a second set of coils that are offset by 90 degrees, and put a capacitor in series, you now have a "rotating" magnetic field, that tries to drag the rotor of the motor around. Giving you a starting torque.

Problem is that it takes a lot of current to create this rotational torque. For something like an electric fan, the motor just needs to slowly start turning until it gets up to speed--Not much torque/current required to turn. For "Hard Start" devices, like a refrigeration compressor, the motor needs a lot of torque to start rotating against the pistons/backpressure.

So, a Hard Start kit for a compressor, can be just a different size of capacitor provide more starting torque (current) to get the motor quickly turning. Or a smaller capacitor to reduce starting current draw, but hopefully provide enough current to still start the motor.

Or, through the use of electronics, to provide higher starting current/torque more efficiently to the motor.

When buying an inverter to run a difficult to start load (typically induction motors on refrigerators/similar), The surge capability of the inverter needs to be high enough to provide sufficient starting current without shutting down. For typical induction motors, the starting surge is something like 5x the running current (i.e., a 120 Watt refrigerator motor needs >600 VA to start--Yes, I did change Watts to VA--Another long discussion on power factor). It is generally assumed that a "low frequency" conversion inverter is better able to start inductive loads (like induction motors). High frequency AC inverters (which tend to weigh less and be cheaper to build) can have difficulties starting inductive loads.

For example, we typically recommend a minimum of a 1,200 to 1,500 Watt AC inverter to reliably start a refrigerator with a 120 Watt pump.

There are other ways to design/make "hard to start" appliances such that they do not take a lot of Watts/VA to start. Nominally, these are usually called "inverter" type refrigerators and air conditioners. They use electronics to create variable frequency rotating fields (the "inverter") and can reduce the starting loads to be no more than the running loads. Also, they may use more efficient "permanent magnet" motors (PM) vs the standard Induction Motor.

-Bill

Fabian

Looking.to power fridge and washing machine. Which start would be better.  Soft or hard?
In regards to LF and HF. Which start woRKS. BEST on each of them?

BB.

Soft Start devices should have lower starting current requirements.

Hard Start devices have higher starting current requirements and usually require a larger inverter that is able to supply these higher starting currents.

What is a little confusing is that "hard start" usually refers to "kits" that are retrofitted to existing devices (air conditioners, refrigerators, etc.) to make them easier to start. I have not seen an appliance labeled as "hard start" (sort of anti-marketing).

Hard Start "kits" (new/possibly larger capacitor) are generally to "fix" a hard starting (or failing to start) A/C or similar compressor. And, may times are just "patches" until the unit finally fails and gets replaced:

https://www.hunker.com/12615547/what-is-an-air-conditioner-hard-start
https://www.amazon.com/hard-start-kit-air-conditioner/s?k=hard+start+kit+for+air+conditioner

Here is a thread from a year or so ago about H/L freq inverters (Home Power Mag direct link is dead):

https://forum.solar-electric.com/discussion/354120/high-frequency-vs-low-frequency-inverters

True "low frequency" inverters are pretty much obsolete. The H/L frequency design common today refers to a higher frequency switching electronics section driving a 60 (or 50 Hz) transformer. A true H/H frequency design drives a lighter weight transformer.

Magazine article on how to pick an inverter (almost 20 year old article, but the basics are still very much similar):

https://d3f7dpm96o8eu9.cloudfront.net/media/wysiwyg/Pump-Inverter.pdf

What needs to be done first is to measure/estimate the loads you want to run. Getting a Kill-a-Watt type meter to measure your daily loads and running Watts is needed to get a better idea of the loads so that the battery bank, solar array, AC inverter, charger, etc. can be properly sized.

https://www.amazon.com/s?k=kill+a+watt+meter&ref=nb_sb_noss

The problem is that large AC inverters can be very cheap these days (the Asian imports especially), but they have drawbacks. A major one is that getting an "oversized" AC inverter will run your loads just fine, but they can draw 20-40-80+ Watts just turned on without any loads. So, sizing the inverter to your loads can save you on your battery bank/solar array sizing (20 watts * 24 hours per day = 480 WH per day -- Or almost 1/2 the daily power consumption of an efficient refrigerator--Typically 1,000 to 1,5000 WH per day).

If you get "USA standard" appliances, they generally are the cheaper to manufacture induction motor type. In countries outside the US, you can get "inverter refrigerators" and even washers and driers that that are inverter based (I have a set from Fisher Paykel 15 years old, these are Permanent Magnet motors and use an VFD--Variable frequency drive--A version of 3 phase variable frequency inverter) to drive the main motors--However, at least in the USA, the ones I bought are no longer sold).

In many parts of the world, there are now Inverter-Refrigerators that have almost zero additional surge current--Those are great for use with off grid inverters. You do have to check the total power requirements (such as anti-sweat door heaters, and defrost heaters). While the compressor may only take 120 Watts, the defrost heater(s) may take 500-600 Watts--So your still have to account for that power draw (for XX minutes once or twice a day (defrost cycle).

Typically, once you have a full size "USA" style refrigerator, this pretty much pushes an off grid solar system in to a "mid-size" system. you install a 1,500 to 2,400 Watt AC inverter, and a large enough battery bank and solar array to run ~3,300 WH per day (refrigerator, washing machine, plus, LED lighting, a "solar friendly water/well pump", Laptop computer, cell phone charging, etc.). Something like a 650 AH @ 25 volt battery bank (good for a maximum of ~3,250 Watt AC inverter and ~3,250 Watt solar array maximum), any good quality inverter in the 1,500 to 3,000 watt range should be OK (watch Tare loading from the inverter).

I know that you are looking at Li Ion batteries (good for hot weather climates)--So the rules of thumbs for sizing the battery bank will ge different than the ones we use for FLA batteries...

If using Li Ion cells, you have to make sure the cells/battery/BMS (if any) are rated to supply the running/surge current of the inverter (especially larger inverters)... For example a 2,400 Watt 24 volt inverter would run/surge around:

• 2,400 Watts continuous * 1/0.85 AC inverter efficiency * 1/20.5 cutoff voltage (or whatever your Li Ion voltage is) = 138 Amps continuous
• 4,800 Watts surge * 1/0.85 AC inverter efficiency * 1/20.5 cutoff voltage (or whatever your Li Ion voltage is) = 275 Amps surge (for a few seconds?)

You end up with two or three things "driving" the sizing of the battery bank. Absolute maximum surge current (lots of current, larger bank). smaller loads running many hours per day--such as a refrigerator (120 Watts * 10 hours per day = 1,200 WH per day) (lots of stored energy needed for overnight, couple days of stored energy). And how many hours of sun you get per day (you have lots of sun, so don't need an oversized array for "winter).

You might have some other issues that apply to Jamaica... I am not sure, but are you looking at ~110 VAC @ 50 Hz? That is pretty rare for AC inverters... You will see ~120 VAC @ 60 Hz or 230 VAC @ 50 Hz. That may be an issue for some appliances(???).

-Bill