Typical running amps

[acnerd]

Let's start a discussion on compressor running amps:

I admit I am not sure how to read if a compressor is running at the correct amps. You can read off the data plate what the locked rotor amps (LRA) are, and the full load amps (FLA)......

For example the LRA may be 30A and the FLA can be 10A; as long as the running amps is below the 10A, I am happy it's not overworking. But if you study refrigeration literature, it mentions that for eg. high subcooling, higher amps, short of refrigerant, lower amps ....

The question then is : Is there a scientific means of ensuring you have the right running amps for various load applications, or am I trying to be over-the-top?

Bonus question: what is the maximum allowable difference in voltage and current draw between phases on a 3-phase compressor?

[nike123]

Let's start a discussion on compressor running amps:

I admit I am not sure how to read if a compressor is running at the correct amps. You can read off the data plate what the locked rotor amps (LRA) are, and the full load amps (FLA)......

For example the LRA may be 30A and the FLA can be 10A; as long as the running amps is below the 10A, I am happy it's not overworking. But if you study refrigeration literature, it mentions that for eg. high subcooling, higher amps, short of refrigerant, lower amps ....

The question then is : Is there a scientific means of ensuring you have the right running amps for various load applications, or am I trying to be over-the-top?

Every manufacturer gives data about compressor performance in various working conditions (read evaporation and condensation saturation temperatures).
Example:

If your measured data corespond with that in such tables/charts you have OK compressor.

[Tesla]

I thought the rule of thumb was 7 - normal running current + 1/7 of LRA.
Each manufacturer will vary but for an 06E carrier the limmits are 2% for voltage and 10% for current.

[Frikkie]

The 7 to 1 connection between LRA and FLA is a rule of thumb. There can be big differences between single and three phase motors, standard and high efficiency motors and even different motor constructions like internal and external rotor configerations.

I think the the voltage (whilst running with load ) should be within 10% and the current through each of the phases should also be within 10% of each other. The normal running current should also be at least 10% less that the FLA written on the engraving plate of the motor to allow for the effects of small voltage dips. Try not to oversize a motor too much because it can be very innefficient if only running at low load and cost a lot in electricity bills, this is assuming motor is separate from compressor. Efficiency is lot higher at nearer full load. If the compressor has a temperature / Amps load graph from the manufacturer I would guess it should also be accurate to 10% at any given point.

This is all rule of thumb, if the booklet says different then stick to what it says.

I don't think there is a scientific means you can use for knowing if current is correct, I think there are too many variables to consider like power factor which can vary with different loads, supply harmonics present, etc. I think a clamp ammeter, the engraving information and the load graph if one is available is the best you will find without great expense of power analyzers and other intelligent test equipment.

[acnerd]

Every manufacturer gives data about compressor performance in various working conditions (read evaporation and condensation saturation temperatures).

If your measured data corespond with that in such tables/charts you have OK compressor.

Thanks Nike. With all the compressor manufacturers and sizes out there, it will be a lot of graphs to carry. Hopefully the manufacturers have websites with these details?

[acnerd]

I thought the rule of thumb was 7 - normal running current + 1/7 of LRA.
Each manufacturer will vary but for an 06E carrier the limmits are 2% for voltage and 10% for current.

Cheers Tesla, I'll try that formula out on a fridge system tomorrow.

[acnerd]

The 7 to 1 connection between LRA and FLA is a rule of thumb. There can be big differences between single and three phase motors, standard and high efficiency motors and even different motor constructions like internal and external rotor configerations.

I think the the voltage (whilst running with load ) should be within 10% and the current through each of the phases should also be within 10% of each other. The normal running current should also be at least 10% less that the FLA written on the engraving plate of the motor to allow for the effects of small voltage dips. Try not to oversize a motor too much because it can be very innefficient if only running at low load and cost a lot in electricity bills, this is assuming motor is separate from compressor. Efficiency is lot higher at nearer full load. If the compressor has a temperature / Amps load graph from the manufacturer I would guess it should also be accurate to 10% at any given point.

This is all rule of thumb, if the booklet says different then stick to what it says.

I don't think there is a scientific means you can use for knowing if current is correct, I think there are too many variables to consider like power factor which can vary with different loads, supply harmonics present, etc. I think a clamp ammeter, the engraving information and the load graph if one is available is the best you will find without great expense of power analyzers and other intelligent test equipment.

Thanks for the answer Frikkie. I wasn't aware of the graphs that Nike showed us..... combined with what you have said, it will be a little more scientific at least!
Mooi bly!!

[nike123]

Thanks Nike. With all the compressor manufacturers and sizes out there, it will be a lot of graphs to carry. Hopefully the manufacturers have websites with these details?

Yes, they usually have! You need laptop and mobile internet access (and base of links or base of data on your Laptop HD) and you could easily find out all relevant data for most of compressors.

[nike123]

Cheers Tesla, I'll try that formula out on a fridge system tomorrow.

Tesla speaking here about ratio of LRA (locked rotor amperage) and FLA (or MRA = maximum runing amperage). That two values you will probably never see in action.

As already pointed that is not formula, that is rule of thumb and you cannot measure it without min/max function on multimeter/clamp meter capable to make sampling faster than 1/10 of second or more.
But with true Rms clamp meter you could very accurately measure running current and compare that with manufacturer data. Compression ratio is variable what determine compressor running current change and tables gives you right current for your conditions.