Mpp and lock rotor amps

cisumma

New Member
Joined
Apr 2, 2021
Messages
112
Lock rotor amps.
I am writing today to explore options concerning the " all in one mpp"
Although, my ac output is minimal right now it is set up to run off of a metal bus bar;And, it runs a radio, chargers and small loads.
Now, I have not tried to run a 1 amp fridge; but, it may be possible but locked rotor amps and bus bars do not seem safe.
Pointedly, is it possible to come out of the ac output with a bus bar for small loads and then also come out of the AC output with a hard wire to make a psuedo homerun to a fridge to eliminate a bus bar exploding under lock rotary amps?
Thanks
 

RCinFLA

Photon Sorcerer
Joined
Jun 21, 2020
Messages
1,139
Most refrig compressors have startup surge less then 1kVA to 2kVA. They can exceed this if restarted too soon after cycling off, due to extra refrigerant head presure that has not equalized and/or startup capacitor thermister not cooled down.

Many use positive temp coefficient thermistor to control their start capacitor. The thermistor is in series with start capacitor and starts out cool and low in resistance to engage the starter capacitor at startup. It then heats up and increases its series resistance effectively taking start capacitor out of circuit.

If you don't allow time for this thermistor to cool back down between run cycles then the compressor is effectively starting up without the aid of starter capacitor. In worse case the compressor just 'grunts' at locked rotor until the internal klixon thermal overload switch in compressor opens. It then takes a long time for the hot compressor to cool down enough for klixon to reset itself.

Problem with high frequency inverters is surge overload on DC to HV DC converter can cause its ferrite transformer core to hard saturate. When this happens the DC-DC converter MOSFET current shoots through the roof and can damage them. There is usually some protection circuitry to detect this and shuts down the inverter but sometimes the protection circuitry is not fast enough, particularly when going from low load current to high surge current where some of the paralleled MOSFET's have been standby'd to save inverter no-load/light load current.

Heavy iron, low freq inverters' transformers have a softer saturation onset, allowing more time for protection circuitry to react.
 
Last edited:

cisumma

New Member
Joined
Apr 2, 2021
Messages
112
Interesting DC current inverter stuff. I am not clear on the difference between The use of vac when it comes to talking about volts; so, I would assume I could fuse the wire for the frig to protect the wire if the compressor locks up; and, I guess that the inverter surge system will have to maintain the inverter fields.
Thanks.
 

Texas-Mark

Solar Addict
Joined
Aug 4, 2021
Messages
514
Now, I have not tried to run a 1 amp fridge; but, it may be possible but locked rotor amps and bus bars do not seem safe.
Why not safe? Most if not all fridge compressors have a built in thermal switch in the windings that opens if the compressor stays in a locked rotor state.

The inverter would probably shut down too under an overload. And then of course there should be a fuse on the DC side of the inverter.
 

rcrracer

Solar Enthusiast
Joined
Apr 10, 2021
Messages
86
Most refrig compressors have startup surge less then 1kVA to 2kVA. They can exceed this if restarted too soon after cycling off, due to extra refrigerant head presure that has not equalized and/or startup capacitor thermister not cooled down.

Many use positive temp coefficient thermistor to control their start capacitor. The thermistor is in series with start capacitor and starts out cool and low in resistance to engage the starter capacitor at startup. It then heats up and increases its series resistance effectively taking start capacitor out of circuit.

If you don't allow time for this thermistor to cool back down between run cycles then the compressor is effectively starting up without the aid of starter capacitor. In worse case the compressor just 'grunts' at locked rotor until the internal klixon thermal overload switch in compressor opens. It then takes a long time for the hot compressor to cool down enough for klixon to reset itself.

Problem with high frequency inverters is surge overload on DC to HV DC converter can cause its ferrite transformer core to hard saturate. When this happens the DC-DC converter MOSFET current shoots through the roof and can damage them. There is usually some protection circuitry to detect this and shuts down the inverter but sometimes the protection circuitry is not fast enough, particularly when going from low load current to high surge current where some of the paralleled MOSFET's have been standby'd to save inverter no-load/light load current.

Heavy iron, low freq inverters' transformers have a softer saturation onset, allowing more time for protection circuitry to react.
Can something like these, wired in series, https://www.ebay.com/itm/3120308435...U%3D|ampid:PL_CLK|clp:2563228&epid=1242221288 have a beneficial effect when using with high frequency inverters?
 

cisumma

New Member
Joined
Apr 2, 2021
Messages
112
Why not safe? Most if not all fridge compressors have a built in thermal switch in the windings that opens if the compressor stays in a locked rotor state.

The inverter would probably shut down too under an overload. And then of course there should be a fuse on the DC side of the inverter.
I just have always learned not to run fridge on a extension chord and I have seen those power strips catch fire.
 

Texas-Mark

Solar Addict
Joined
Aug 4, 2021
Messages
514
I just have always learned not to run fridge on a extension chord and I have seen those power strips catch fire.

Anything that draws too much current for the wire size will cause issues. Crappy power strips can melt from a high wattage light bulb. I have a 50ft 10awg extension cord that I run my full size fridge off of when there is a power outage and it does not even get warm, and only drops a couple of volts. If the compressor locks up, the internal thermal switches will open first. But that is what fuses/breakers are for too.
 

cisumma

New Member
Joined
Apr 2, 2021
Messages
112
Thanks for all the feedback.
It is sounding like extension cords and power strips are the norm for AC output; so , I will run a power strip for smaller loads and wire in a separate output plug to energize the 1 amp refrigerator; unless having 2@ AC outputs from the inverter is a bad thing.
Thanks again.
 
Top