Inverter peak continuous wattage

kansas · June 2015

I would like to have a dehumidifier in the cellar under my cabin. The most efficient I've been able to identify is a Frigidaire that is rated at "3.2 amps(cool)" and "320 watts(cool)." Two questions: 1) does anyone know what "cool" wattage and amperage are? I've emailed Frigidaire but so far no response. 2) What would happen if I attempted to run the dehumidifier using my Morningstar 300 watt Sure SIne inverter? Would it run hot or shut down, or both? The temperature in the cellar is a constant 60%. Thanks - Bill

inetdog · June 2015

The "cool" readings assume that the temperature of the air being dehumidified is comfortable room temperature.
The dehumidifier, if I am analyzing correctly, may actually use less energy per quart of water removed if the air temperature is higher. But the compressor will be working against a higher average pressure, so the straight wattage would be higher at higher temperatures.
You also need to allow for the unit cycling on and off under the control of a humidistat if you want to know the energy consumption over 24 hours rather than just the power.
A motor with a 320W running load will pull several times the current when starting, so you 300W inverter is likely to trip (turn off) every time the humidifier tries to start.

As for the 320W versus 300W, the inverter may drop the output voltage slightly as it nears its peak rated output. Seeing the lower voltage the compressor motor will try to keep producing the same output by drawing more current, causing the inverter to shut down on overtemperature.

mike95490 · June 2015

De-humidifiers have a small refrigerator / air conditioner compressor in them, to chill the air and condense the water out. So the starting load will be much higher than the running load, and I doubt a suresine can start it . However, the suresine could be mounted on a beefy heat sink, which wold keep it cooler, and you may get longer run times before it overheats.

kansas · June 2015

Thanks for your comments. The SureSine is rated at peak power of 600 watts for 10 minutes. From what I'm reading and your comments, a dehumidifier is most efficient at a relatively high temperature and high humidity level. I have the second in my cellar but not the first. On the other hand, the relatively cool temperature of the cellar should (?) help keep the inverter cool. I realize that the 320 watt rating of the dehumidifier may not be accurate and probably is lower than the actual energy use. Still, it seems tantalizingly close to the capacity of the SureSine. I use my solar system very very little in the summer when there is lots of solar energy available and a 100% battery bank all the time. I guess the only option is a larger inverter. Hmmm . . .

Johann · June 2015

The compressor motor on the dehumidifier will use at least 4 times the current or wattage at start up. You should expect to need a inverter that can handle a lot more watts.
The rule is that motors can take up to 10 times the current/watts at start up. Up to means that all depends on the kind of motor and the load attached to it at the time of starting the motor. So it looks like you will need a higher watt inverter. In reality we are talking about in the neighbor hood of needing 4 to 5 times the power rating at start up.

Your signature says that you have two 140 watt panels and that is not enough to support the combined power requirements of your dehumidifier and batteries.

kansas · June 2015

Understood. My setup does everything it was designed to do. Just wishful thinking,I guess. Cheers.

Stix · June 2015

You could put a hard start kit on it to make it less of a draw on the inverter. You would want it to be the type that cuts out after startup.

BB. · June 2015

I think you have to be very careful with "hard start" kits... Many just simply let more Amperage go to the compressor. That is pretty much the opposite of what you want when trying to make things "easier" on the inverter.

You have to really study them to figure out what type you are getting (many call the generic replacement parts kit for a failed starter assembly a "hard start kit" (I believe--I am not in the industry).

-Bill

inetdog · June 2015

I believe that some member (maybe Mike?) did some experiments with changing the size of the factory starting capacitor to reduce starting current, as long as the motor did not have to start under full load.

Stix · June 2015

BB. wrote: »

I think you have to be very careful with "hard start" kits... Many just simply let more Amperage go to the compressor. That is pretty much the opposite of what you want when trying to make things "easier" on the inverter.

You have to really study them to figure out what type you are getting (many call the generic replacement parts kit for a failed starter assembly a "hard start kit" (I believe--I am not in the industry).

-Bill

The power is stored in the capacitor so it lightens the load on the Line. Also it will get the compressor to running amps about 50% faster so even less strain on the inverter.

inetdog · June 2015

Stix wrote: »

The power is stored in the capacitor so it lightens the load on the Line.

That is fine if and only if the motor is operating at a low power factor (high inductive current) during the starting surge.

BB. · June 2015

No--Not really. The power stored in the capacitor is only "advancing" the current phase to the start winding by ~90 degrees or a 1/4 cycle. The change in the current phase for the start winding is needed to create a "rotating field" inside the induction motor--To "drag" the rotor to turn (starting torque) for single phase induction motors.

Three phase motors do not a starting capacitor/windings--The three phase power is already "naturally" a rotating field in a three phase motor.

The capacitor does not store multiple cycles or multiple seconds of energy--So it cannot reduce the starting current unless the new capacitor is smaller (lower capacitance) than the original capacitor. (the capacitor, besides "shifting" the timing of the current is also helping to limit the actual current flow to the start windings).

-Bill

notes · June 2015

I will add my 2 cents on this as I am new to off grid solar but am in the commercial refrigeration service industry for 40 plus years. Start and run capacitors matched to the proper relay are fine, but must be matched to compressor or your problem will be worse and probably burn out compressor. It's all about the load. We use a pump down selinoid valve, it's purpose is to let the tstat drop out valve and let compressor turn off on a pressure switch. When that calls for a command, compressor starts with no load and slow comes up to full RLA. It works, I have 3 house frig/freezer running at my shop that way as experiment. Not a fan of propane frig. Typical readings before 1200 WH per day to 500 WH per day. Although I did disconnect all defrost heaters and frame heaters. Nothing wrong with manual defrost. Also static condenser only.

Stix · June 2015

notes wrote: »

I will add my 2 cents on this as I am new to off grid solar but am in the commercial refrigeration service industry for 40 plus years. Start and run capacitors matched to the proper relay are fine, but must be matched to compressor or your problem will be worse and probably burn out compressor. It's all about the load. We use a pump down selinoid valve, it's purpose is to let the tstat drop out valve and let compressor turn off on a pressure switch. When that calls for a command, compressor starts with no load and slow comes up to full RLA. It works, I have 3 house frig/freezer running at my shop that way as experiment. Not a fan of propane frig. Typical readings before 1200 WH per day to 500 WH per day. Although I did disconnect all defrost heaters and frame heaters. Nothing wrong with manual defrost. Also static condenser only.

That is all fine but a dehumidifier will not come that way and you would need to modify it to do so. Easier to just get a variable speed compressor with little load on start up. A dual capacitor say 35/5 35micro for compressor 5 for fan gets an added benefit from the hard start cap of 450 micro for a short burst to make it easier to start under low power conditions or heavy compressor load/pressure.

Inverter peak continuous wattage

Comments

Categories