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How to reduce motor start current

How to reduce motor start current

How to reduce motor start current

It is time to head off to the camp and I have modified a small chest freezer with a temperature control to use as a refrigerator.  Because of the top load and better insulation, these consume less than 100 watt hours a day compared to 1KWH for a normal fridge. Thought this would be great for my 150W solar panels. The compressor is a capacitor run motor with that capacitor shorted by a PTTC for starting.  Normal wattage is 100W running and 150W starting for about the first 45 seconds.  Thought this would run great on my small 200W inverter.   

Inverter immediately goes into overload.  I put the VAW meter into peak amp hold and it reads about 17A on house current.   I tried a 300uF cap in the starting circuit instead of the short and that didn't help much.  I have a small quiet ONAN 450 watt gas generator and even that immediately dies when the freezer is plugged in.  Anyone have any tricks to reduce the starting current?  Hate to have to buy a 1200W inverter just for this.  

RE: How to reduce motor start current

The freezer may be 150W starting but I bet it's around 600kVA. Being a single phase motor it could even be more.


RE: How to reduce motor start current

Do your 150W solar panels get 100W steady out of them?  I'll take a wild guess that they don't -- unless you live in nevada or arizona.

RE: How to reduce motor start current


...I bet it's around 600kVA. ...
Check you math there my friend...

Unfortunately, there isn't much that can be done for 1 phase cap start motors. Anything that reduces voltage electronically will likely be damaged by the capacitor charging current (di/dt). You might be able to cobble together a makeshift autotransformer or reactor or primary resistance type starter arrangement. I've never tried it, but I heard someone say that it worked. Nothing like that exists for 1 phase motors that I know of, but if you get creative with the components you may be able to pull it off.  

RE: How to reduce motor start current

I was supposed to go on vacation last Tuesday, now if lucky I will get out next Wednesday. The home projects have been on the back burner.  I did just try the generator again with a 3 ohm resistor in series.  That dropped the peak amps to 8.  The generator struggled for about 30 seconds and then recovered. That at least gives me a backup.

The refrigerator should run about 5 minutes every hour. The solar panels are operated at the power point.  Three hours of sun should keep the whole thing going.  That isn't as easy as it sounds.  I will be storing extra cold when sunny and let temp rise at night.  That works out better than more batteries.

RE: How to reduce motor start current

Sorry, I missed the "hours" in 100 watt hours in the original post.

RE: How to reduce motor start current

Here's the real kicker... Since this is a totally enclosed fully hermetic compressor what happens is they will regularly stall while starting.  They have no expansion valves, they have only a capillary tube.  This means if when the compressor shuts off the piston comes to a stop anywhere within a about 20 degrees of top dead center, the motor will not have enough torque to start the compressor.

  On a start attempt the motor stalls and sits there drawing LRA.  This goes on for about 2 seconds until a klickson trips preventing motor meltdown.  The switch then takes approximately 1 minute to cool down eneough to reclose. When it does the same exact thing happens again.  But each time it happens the piston advances another few degrees and the presuure it created leaks on thru the capilary tube.  After several of these cycles the compressor will succesfully start.

So, if in your case you have a really weak supply then this torque issue will be greatly increased.  Now there may be a  100 degree area that will cause the starting stall issue.  Then when it does happen each hammering will gain only a small improvement in the march to top-dead-center.

In reality if you don't have enough available power you may toast the motor outright as the time it takes the thermal to trip is an engineered system.  It could be with reduced current the motor windings will toast before the klickson trips.

Keith Cress
kcress - http://www.flaminsystems.com

RE: How to reduce motor start current


It could be with reduced current the motor windings will toast before the klickson trips.
I agree with Keith (itsmoked) here, based on on experience with distribution circuit single phase conditions with a wye delta transformer bank somewhere on the circuit.
This was a system that tied the primary star point to the system neutral on wye delta transformer banks.
With a single phase energized, the two unenergized phases would share normal voltage. That is 50% voltage on each phase if the loads were balanced.
With two phases energized, the third phase would get about 85% to 95% of rated voltage (depending on the load).
Almost every phase loss event would result in a failed refrigerator or freezer somewhere on the circuit.

"Why not the best?"
Jimmy Carter

RE: How to reduce motor start current

could you fit it with a pull rope :)?
if you could get it moving then let the juice in your problem would be solved.

RE: How to reduce motor start current

Yes, I meant 600VA.

But then, 17A of current is 2040VA... not going to start that on a 200W inverter.


RE: How to reduce motor start current

There are "hard start" kits available for hermetic compressors that might help your situation.  There are small ones for domestic refrigerators and freezers and larger ones for larger single phase refrigeration compressors as well.  

The small ones have a start relay,a start capacitor and an overload klixon built in and replace the standard split phase start relay and klixon. On PSC compressors,the unit is paralleled with the run capacitor and converts the PSC to a capacitor start/capacitor run motor.  

RE: How to reduce motor start current

You can make a simple soft-starter with a voltage-controlled switch that slowly charges an upstream R-C circuit.  When the bus voltage builds up, short out the resistor. Then you should have enough energy ready before kicking the motor on.  

Alternatively, you can put the switch resistors into the motor legs to soft start the motor each time it kicks on.

Personally, I prefer the first approach.  You might want to diode protect either one.

Vince Socci

RE: How to reduce motor start current

I don't see how soft starting can possibly work here.  In fact the slow start is exactly what's causing the problem!  This is not a centrifugal pump situation.

The problem is not enough torque to get over the very first TOP DEAD CENTER of the rotation.

Keith Cress
kcress - http://www.flaminsystems.com

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