Here is a question to ponder: What need to the amperage attract be that a compressor in i m sorry the nameplate claims the RLA (rated pack amperage) is 59.6, yet the compressor is just pulling 45 amps? Is over there a trouble here? perform we need to change the compressor?

These room questions typically raised throughout Copeland Compressor Operation and also Service Seminars (COSS).

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The to run or operation amperage of a refrigeration or air conditioning compressor is straight related to the fill put ~ above the compressor motor.

Various Motor tons

One an approach of engine loading would certainly be the mechanical load. Think the the relatively small amount of friction produced by the matched contents of the compressor, such as the crankshaft come bearings, connecting rods to the crankshaft, pistons to cylinders, etc.

Another an approach of motor loading would certainly be the pressure entering and also exiting the compression chamber, basically the suction and discharge pressures.


Of these 2 loads, the mechanical pack is a given. As long as the compressor is mechanically OK, the amp draw would it is in appropriate.

Now let"s think about the engine load created by suction and discharge pressures. If a 43-psig suction pressure rises to 76 psig, how would this influence the fill on the motor? clearly it would be increased, which way the engine would need to work harder. When a motor needs to work harder, what wake up to the amp draw? that goes up. And also so we are earlier to the initial question, "What should the amperage be?"


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Figure 1.
This curve addresses compressors that operate at a 230-V current.

Determining Amperage

Some think the compressor"s rated pack amperage (RLA) is its running amperage. This is not so. Think around what us have questioned so far: The mechanical load is a continuous and should not change. The pressure pack will fluctuate up and down depending on such things as the evaporator load and ambient temperature. Us must recognize what the pressure are when we check out the amperage.

An amp reading alone tells united state nothing. When you gather the suction pressure, discharge pressure, and amperage readings, record this information in addition to the voltage measured at the compressor terminals. Then apply these 4 readings come the compressor curve sheet. Every compressor model has its own details curve.

Let"s look in ~ Copeland compressor design 3DS3-1500-TFC operating v HCFC-22. The suction pressure at the compressor is 43 psig and also the condensing push at the compressor is 260 psig. We review 43 amps on our amperage meter and 230 V at the compressor terminals. Now we must use this details to the curve sheet.

To understand exactly how to read a curve sheet, monitor along very closely using figure 1. The saturated suction temperatures and also pressures are provided along the bottom the the chart. Vertically come the appropriate of the chart, you will view the saturated condensing temperatures and pressures. Left the the chart is the amperage column. This particular curve addresses compressors that run at a current of 230 V. An alert the multiplier for various other voltages (listed above the graph in figure 1).

Now let"s look at our conditions. First, looking across the bottom of the chart, let"s uncover 43-psig evaporating pressure; look at the vertical line expanding upwards indigenous this point. On the ideal side, find the 260-psig condensing pressure and also follow the curved sloping line directly left the this condition. Ar a dot wherein it intersects or crosses the vertical line that corresponds with the evaporating pressure.

From this dot, go directly and also horizontally to the left; you have to be at 44 amps. This is the suitable amp draw for this compressor at these conditions.

Unfortunately, the RLA in our example is 59.6 because that the 3DS3-1500-TFC. Why is the compressor illustration low amps?


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Numbers call A Story

Remember, the meter reading at the compressor was 43 amps. The allowable yongin is ±10 percent, so the compressor really is not illustration low amps. This is the proper amperage because that the operating problems we recorded. Let"s say we are still operating at 43 psig and 260 psig, and also our amp analysis is 59 (RLA is 59.6).

What is this informing us? many technicians would certainly think 59 amps is the exactly reading, due to the fact that the RLA is 59.6. However, we should recognize that the compressor is drawing an extreme amount that amperage. Other is overloading the motor, and it is no our pressures - we room using the pressure to set the period on the curve sheet.

Think earlier on mechanically motor loading. The overloading can be a mechanically problem, choose worn bearings, worn rods, etc. If we were looking in ~ a single-phase compressor, the high amperage might be brought about by a bad run capacitor. There are other reasons too, however that"s for one more article.

Let"s look at at one more scenario. The suction push is approximately 84 psig and also the discharge has actually increased come 337 psig. Look again in ~ the curve and place a brand-new dot follow to these higher pressures. Note that the compressor have to be illustration 57 amps. We are within 10 percent the the RLA. Due to the fact that of the high suction pressure, however, many business technicians assume the compressor is not pumping and therefore requirements to be adjusted out.

Again, this is incorrect. We are assessing the very same compressor on the same system with fully different amperage draws. Both amp readings are correct since the pressures have actually changed.

Let"s assume the compressor has actually one damaged rod, which could cause a high suction pressure. Under this condition, what would occur to the amp draw? It would be much less than where we ar our dot because of a reduced mechanical load. In this instance, the amp attract would be about 30 percent lower. However, in the previous instance we were illustration the suitable amperage.

We need to uncover out why the pressures changed so severely. Space the pressures high or low? Which set of pressures is closest to the common operating conditions for this kind of system?

Remember, we space working top top the system because of part reported problem. What we room trying to perform is diagnose whether or not we must replace the compressor. The amp draw will, to a good extent, help us in that determination.

Don"t acquire Sidetracked

RLA is not the running amperage that the compressor; the is a mathematical formula and also is used for such things as sizing fuses, wires, and contactors. The published RLA worth does not change as the motor load changes.

Different articles and also even some instructors have declared that high evaporator loads will reason high amp draws. This can easily be misconstrued. If a greater load will boost the amp draw, it does no equate to improper amps even if the compressor trips ~ above its overload. Think when again around what we have discussed. If the evaporator load increased, how would that impact the suction pressure? It would certainly increase.

How does greater suction pressure affect amp draw? Yes, the amp draw is higher - however is it appropriate according to engine conditions?

It"s an ext important to uncover out why the suction press is high. Return the suction press to its ideal level and also the amp attract will follow. Amperage curves room an invaluable tool in diagnosing compressor problems.

In this case, if you had actually a high amp draw and decided to change the compressor, the brand-new compressor would have operated the same as the vault one, till you found out why the suction push was high and you repair it.

Voltage at the compressor terminals will also affect amp draw. Over there is an inverse correlation between the two: as voltage drops, amperage increases, and vice versa. If the voltage is 5 percent low, then amperage will certainly be 5 percent high. Figure 2 has the formula because that making adjustments because that either high or short voltage.

Curve sheets are available from different sources. A an excellent source is with your regional factory-authorized wholesaler, who can often administer the sheets or aid you with reading them based on your tape-recorded information.By looking virtual or contacting the compressor manufacturer, girlfriend can likewise obtain paper copies or a CD containing this information. You can even it is in surprised to find that your office currently has compressor curves because that reference.

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No matter where you obtain the information, using amperage together a diagnostic tool is smart. Usage it to store from an altering a compressor the otherwise may have been misdiagnosed.

Dave Bell is a cultivate specialist through Copeland Corp., an Emerson Climate technologies company. The has more than 30 years of experience in the industry, instructing top top topics varying from refrigeration system diagnosing and troubleshooting, and also compressor operation, etc. For more information, contact Bell at Dave.Bell