Soft Starters for motors ,Fridges etc.

Bob Steel

Solar Enthusiast
I have the problem that in the early hours of the morning when my battery is getting low my freezer turns on and pulls a huge start current out before it settles to a steady drain .
The Inverter handles the normal running current no problem but the huge drain to start it pushes the battery voltage down just enough to trip the low battery alarm and than I have to get up to turn the inverter off to silence it . Because it will do the same again I have to remove the freezer's plug to a grid supplied one before switching my inverter back on .

So is there anything I can build or buy to stop this sudden fall in voltage of my battery?
Is anyone using or have used a supercapacitor bank on their batteries to accommodate this ?

PS. I am aware of the NTC Thermistor 5D-25, 5 Ohm 8A InRush Thermistor Resistor approach and have a couple ordered to try .
 
Last edited:

svetz

Works in theory! Practice? That's something else
Staff member
Moderator
You could get a soft-starter and for some members like me they're great, but for the $$ why not some additional battery? Although I don't have one, a supercapacitor might work (ref):

Supercapacitors are ideal when a quick charge is needed to fill a short-term power need; whereas batteries are chosen to provide long-term energy. Combining the two into a hybrid battery satisfies both needs and reduces battery stress, which reflects in a longer service life.
 

Deveak

New Member
I have yet to see a soft starter that fits a refrigerator or small AC. I put one on my 3.5 ton AC. Cost 320ish bucks, from hyper engineering.
I don't have a good enough meter to check the inrush but I was able to run it on a 13 KW generator while before it would fail to start it (and dump it on my inverter!) According to the charts I had an inrush current of 72ish amps and it brought it down to I think somewhere around 30.
 

Larry619

Solar Enthusiast
Forgive my trivial question - Is it bad to put a timer, essentially cutting power, on a major appliance?

Both electrically and food safety wise, what are the impacts to the appliance or food long term? Do you think the internal temp will keep food cold or frozen just for the few hours overnight?

id love to do that even I don’t have my home off grid (yet). The fridge keeps running overnight even thought we aren’t opening the doors. I’ve measured my upright freezer consumption to be about 700W over 24hrs. Cutting about 6-8hrs overnight could cut consumption by 1/3rd. 😳
 

Hedges

Photon Sorcerer
Forgive my trivial question - Is it bad to put a timer, essentially cutting power, on a major appliance?

Both electrically and food safety wise, what are the impacts to the appliance or food long term? Do you think the internal temp will keep food cold or frozen just for the few hours overnight?

id love to do that even I don’t have my home off grid (yet). The fridge keeps running overnight even thought we aren’t opening the doors. I’ve measured my upright freezer consumption to be about 700W over 24hrs. Cutting about 6-8hrs overnight could cut consumption by 1/3rd. 😳

Turning off power to your freezer won't save energy. In a 24 hour day, the compressor still needs to pump out whatever heat leaked in through the thermal insulation.
The goal would be to avoid using battery to power freezer, rather using surplus PV while battery is fully charged during the day.

Fridge/freezer will hold temperatures for a while. More food in freezer and it will hold longer. Frozen water bottles would hold it against warming past 32F/0C until the ice melts, and frozen brine would have a colder phase change more appropriate to keep frozen foods frozen.

Cycling power to electronics causes stresses, probably shortens life.

An older fridge or freezer has an electromechanical thermostat which cycles power to the compressor, so shutting off power doesn't matter.
It probably has a defrost thermostat timer. If that is electromechanical, I wouldn't worry about it either.

For a newer fridge/freezer having electronic controls, there is the risk of harm from power cycling. Don't know if that would shorten appliance life or not. Perhaps modifying to let a timer disconnect power to motor/heater would be worthwhile.

There is also the question of whether the timer is more or less reliable than the freezer. You don't want food to spoil because a cheap timer failed.
I've thought about a second thermostat to re-enable freezer if temperature rises above a higher setpoint. That was meant to take care of warming overnight, but it would also bypass a failed thermostat.
 

Larry619

Solar Enthusiast
Turning off power to your freezer won't save energy. In a 24 hour day, the compressor still needs to pump out whatever heat leaked in through the thermal insulation.
The goal would be to avoid using battery to power freezer, rather using surplus PV while battery is fully charged during the day.

Fridge/freezer will hold temperatures for a while. More food in freezer and it will hold longer. Frozen water bottles would hold it against warming past 32F/0C until the ice melts, and frozen brine would have a colder phase change more appropriate to keep frozen foods frozen.

Cycling power to electronics causes stresses, probably shortens life.

An older fridge or freezer has an electromechanical thermostat which cycles power to the compressor, so shutting off power doesn't matter.
It probably has a defrost thermostat timer. If that is electromechanical, I wouldn't worry about it either.

For a newer fridge/freezer having electronic controls, there is the risk of harm from power cycling. Don't know if that would shorten appliance life or not. Perhaps modifying to let a timer disconnect power to motor/heater would be worthwhile.

There is also the question of whether the timer is more or less reliable than the freezer. You don't want food to spoil because a cheap timer failed.
I've thought about a second thermostat to re-enable freezer if temperature rises above a higher setpoint. That was meant to take care of warming overnight, but it would also bypass a failed thermostat.
Ok, so for me, it doesn’t sound like it’s worth the risk to save a few watts. Especially the fridge serves a family of 4 and lots of $$$ of food.
 

DThames

Solar Addict
You might consider putting some super capacitors in parallel with your battery, right at the inverter terminals. That will give a big current boost when a high demand load starts.
 

NwCali

New Member
How did you solve this? I agree with some others, more battery is the best long term solution as taking any of them (that I'm aware of) down this low shortens their lives.
 

Ampster

Renewable Energy Hobbyist
How did you solve this?
If the problem you are asking about is surge the ailution is in the title of the thread. A soft start device may be the simplest. An inverter with surge capacity is another solution when paired with the right battery. It does need a system approach and there are tradeoffs in cost that need to be evaluated. I am moving my inverter to a new home and my inverter does not have significant surge capacity. Fortunately my refrigerators are already inverter driven so natively they are soft start. I am upgrading the furnace to a heat Pump A/C so i will be sure that they are soft start.
This may be enough of a universal problem that a recent Tesla Powerwall iteration says that it is now capable of "soft start" what ever that means. I would guess that is marketing speak for increase surge capacity by incorporating some of the techniques in soft start devices.
 

paul12345

Solar Addict
I vote you revisit your inverter if that isn’t a big expensive one. My Victron inverter has “dynamic” and configurable cut off which uses a lower cutoff voltage for higher load currents, because of course they cause bigger transient voltage drops.
 
Top