Can I reduce the start current of a fridge compressor?

Question

Not a duplicate question: this question is about reducing start current, not about identifying the motor type.

I have a refrigerator that has a permanent split capacitor (PSC) motor, which I am powering from solar panels/battery/inverter.

My inverter is rated at 1000w (2000w peak), but unfortunately, the motor occationally trips the overload protection of the inverter, with the risk of spoiling the contents of the fridge if it goes unnoticed.

I am considering replacing the motor with a DC compressor (Danfoss BD35F), but it's quite expensive (about $350).

Before I do that, I want to experiment first, to see if I can get it to work with a consistently lower start current.

I realize it might break the motor, that it might not start consistently, but it's worth a try.

I've read about the following options:

• a) use a start capacitor, 150uF, for the first 1000ms
• b) use an NTC thermistor
• c) power resistors, for the first 1000ms (similar effect as NTC)
• d) voltage transformation to a lower voltage, e.g. from 230V to 200V

but:

• a) I'm having a hard time finding a start capacitor to try with, and random experimentation could be expensive, since they seem to cost about $10-15 a piece and can break if powered for more than a few seconds.
• b) I've tried a 10ohm NTC, but that made no difference at all.
• c) I'm going to try with various resistors at 50, 70, 100 ohms (rated at 50w).
• d) I'm not sure which type, which voltage to try etc..

ALSO:

I tested the motor with the following results:

• Disconnected for 24h, ~850w
• Connected for 20min, disconnected for 5min, ~600w
• Connected for 45min which included a self-restart, ~1100w
• Disconnected, then reconnected after 15min, ~800w

I'm puzzled by these results, because it seems that the motor draws more if left connected. But the thermostat disconnects the circuit. The only way this makes sense to me, is if the thermostat somehow causes an arc that draws an additional 300w when reconnecting. But I tested it by disconnecting for 15min, turning the thermostat to 0, reconnecting, then turning the thermostat to 3. The max power draw was only 782w.

Very strange... Any ideas?

Answer 1

I have a similar situation, off grid PV / battery storage system with a 1Kva 12-volt inverter and I run a small 230-volt standard fridge. It used to knock out my TV & Satellite at start-up and hit the battery bank voltage quite hard. I connected a VDR (voltage dependant resistor) in series with the live feed. This just takes the sting out of the start-up inrush current and overcame the problem for me

Answer 2

I solved this with a 2 farad 16v super capacitor on the input of the inverter> Although it doesnt reduce the start current it provides enough electrons to start the compressor without shutting down the inverter

Answer 3

Clearly, if your inverter cannot provide enough power for the fridge to start, the proper solution is to get a more powerful inverter. That said, if you know your inverted can provide the extra current for a short time during startup, you may just be able to replace its protection circuit by a slow-blow circuit breaker, which is specially designed for loads like motors and such.

Soft-starter can be another solution, but only if your compressor supports it (many big compressors do).

Answer 4

Reducing startup power is not needed for on grid use. The measures would only increase the cost of the fridge. To reduce the startup power some form of unloading is required. For big compressors this is done by lifting the suction valve(s). Sometimes a softstarter is also included. A better way in your situation would be to change to a socalled absorption type of fridge. They work with a small heating element and have no inrush current at all. It might be possible to work even without an inverter if you select the right type (12 or 24 VDC also needed for the Danfoss compressor).

Answer 5

First make sure that your inverter is performing as advertised. Some brands, such as the newer AIMS, have the surge time so short (40ms) that it is useless for pretty much everything. The second is to make sure that the battery Voltage isn't going low enough to shut down the inverter during start up.

Now the answer: Modern energy efficient motors seem to have very low resistance motor coils, which makes the start up surge much larger. These low resistance permanent split capacitor compressors seem to be able to restart immediately while the refrigerant is still pressurized, which is something that always used to lock up the compressor. A 1 Amp running compressor may draw 10 to 15 Amps with a locked rotor.

I haven't tried it, but I would recommend the resistor in series with the run winding with a 1 second timer relay, which shorts out the resistors after 1 second. You shouldn't use a capacitor as it would change the phase and defeat the effect of the permanent run capacitor. If you hook up both windings (by hooking up to the whole compressor supply wire) you should be able to use either a resistor or a capacitor to limit current while the 1 second relay is open.

Also, if your inverter puts out a 2 step square wave or it puts out a 3 step square wave with very little dead time I don't recommend using it with a high efficiency motor. It will make the motor run hot and waste a lot of electricity. A 3 step square wave with 25% dead time is okay, but with most inverters, especially cheaper ones, as the battery Voltage drops and other loads are turned on that 25% gets reduced to increase the AC Voltage. Such inverters have no other way to regulate the output Voltage.

Answer 6

I think your best bet would be one of those power factor correction capacitors. A large motor run cap might be a suitable alternative. The only method I have of calculating the required size is from someone else (I haven't tried it) and is really hit and miss.

It goes like this: connect the capacitor in parallel with a filament bulb and connect both of these in series to the appliance while it is running. Keep increasing the capacitor size till the bulb goes out. Or you know you could use the saying: the bigger the gob the better the job.

Alternatively you could use a larger inverter to start it and use a change over switch to go to the correct size.

Finally. Use a DC inverter compressor refrigerator. Obviously you'll have to buy a new one.

Answer 7

You need to limit inrush current, it seems. Perhaps do a search on "mains inrush protection". One such item from the search is this

This device is useful where the current drawn from the mains supply by a piece of equipment at switch-on (eg. an audio power amplifier) is considerably higher than the steady state current consumption of the equipment. This current surge can be troublesome in that it may blow fuses or trip circuit breakers. The unit has IEC input and output connectors and limits the current at switch-on to an acceptable level for 200mS–400mS. Two LED’s on the front panel indicate the status of the unit. A voltage dependent resistor is included for transient suppression.

Answer 8

Sounds like a job for an inrush current limiter... I'm here thinking about off-grid living and the most inconvenient way to get a DC fridge is to make one! My naïve go at this would be to realize a fridge works by using a control board to turn the compressor motor on/off. what if it instead was modified...

Supply the control board of a fridge with the DC it needs (buck/boost converter) but change it from turning on a motor to turning on an inverter that turns on the motor. Remove the AC power cord and replace with DC screw terminals =)

Use a larger inverter? The "on time" for a fridge in kilowatt hours would be quite small compared to the 24/7 "on time" and standby current of an inverter. The problem just gets worse as the inverters get bigger.

For example on youtube: there is someone showing off a 4000 watt inverter they installed for a 3.2kw battery bank thinking it was sufficient until they saw the 50w draw from the inverter under no load. That's 37.5% of their battery bank every day just to have the inverter on! The solar panels they had would not power the inverter under no load for a day because of all the other losses involved with an off-grid system.

Another example on youtube: there is someone showing off 2 12v lead acid batteries wired in parallel and a 2000 watt inverter that keeps tripping on a small 1.7 ft3 galanz fridge that only needs 72w power... That fridge on a bad day would consume maybe .4 kw the inverter would be several times that because its "standing by". Off grid requires DC appliances which are extremely expensive and sub-par but maybe AC items can be retrofit.

My point: A DC fridge (24/7 appliance) only consuming power to run some control circuits and an on/off switch for a motor is more efficient in an incredible way. Off-grid people must use DC equipment period. Think of an inverter as a necessary evil to be switched on and off as needed for things like microwaves, toaster ovens etc (still need to limit the inrush current)!

Answer 9

Something to keep in mind with all AC motors, reducing the voltage actually causes motors to draw more current. Remember that the only thing that reduces the current when a motor is running is the back voltage that is generated by the motor spinning. If the voltage is too low you will burn out the motor, just as will happen if it is on too high a voltage. All AC motors have frequency and voltage specifications that it is designed to run properly. Also there are some motors that are designed specifically for inverter use, and can be speed adjusted over a range to increase and decrease the power consumption.

Just keep in mind that the higher the frequency the faster the motor will spin, and the higher the operating voltage. When the frequency goes down so must the voltage. This is how variable frequency drives work in the real world. Also with a soft start it is reducing the voltage for a very short time so that the motor can build up to speed, and then a set of contacts close to bypass the voltage reduction part of the circuit.

One of the problems that I have personally encountered is that some people connect motors and other things without even bothering to make sure the motors are connected for the correct voltage. Also one of the more important things to do when doing any connections is give all the connections a pull test before closing up anything (power off obviously for safety). That is to make sure all the connections are good and the wires have not opened on the inside between the ends. There is nothing more annoying than intermittent connections, and when you find a wire that stretches you found the problem most of the time.

Answer 10

Here is a simple fix for your problem that you can do for very little investment. Buy a surge protector suitable for the device and pinch off the noise maker. When your fridge draws hard the surge protector will supply the extra voltage then return to normal pass-through. Cheap easy fast available almost everywhere.