Tag: inrush

Now is the part where we get specific about Start capacitors and inrush. If you haven’t read the first three parts please do so before reading this one or it may not make sense.

I’m going to come out and say it so you keep reading.

What you were taught about hard start kits decreasing inrush amperage is wrong.

Look at the oscilloscope image above. It’s a 3 ton reciprocation compressor with equalized pressures and 230V applied. Now keep in mind the voltage value shown here is RMS and the amperage is PEAK so if the amps seem high in these charts that is why.

So this compressor starts up at locked rotor (on the run winding remember) and it gets up to speed at around 180 milliseconds of run time. That is pretty typical of a good, equalized compressor under normal conditions.

Sometimes compressors do struggle to start and this can be due to.

  • Low input voltage (usually due to voltage drop)
  • Starting unequalized due to short cycling and or non-bleed (Hard Shut Off) expansion valves
  • Long line lengths
  • Refrigerant migration into the crankcase
  • Compressor wear

Here we show a system with low voltage applied (187v) and it HAS a hard start in place (not necessarily the same compressor as the last so don’t try to compare apples to apples)

In this case, the compressor doesn’t get up to speed until about 550 milliseconds and at that point both the run and start winding amperage drops.

This proves that the hard start kit is working in that case.

You can tell this because the only amperage that can enter the start winding is dictated by the run and start capacitors and like we said before… the capacity of a capacitor is dictated completely by the capacitance (mfd) and the voltage across it.

For the amperage of the start winding to go UP at 580 milliseconds, it can only be due to an increase in back EMF as that motor gets up to speed. Then at 600 milliseconds, the potential relay removes the start capacitor and the amperage drops down to the run capacitor only level.

Conclusion #1

Fixing a voltage drop issue does more good than adding a hard start kit.

A valuable test is measuring the voltage feeding your compressor with it under load and rectifying poor line voltage connections or undersized conductors.

Look at these two identical compressors, one with a hard start kit and one without.

Techs are often taught that measuring inrush on the compressor common wire is a way to show how a hard start kit decreases startup amps.

This isn’t what we are measuring when we see a big difference in inrush amps.

What we are measuring is how QUICKLY the compressor starts not the true inrush amps at start. I’ve looked at the specs on several high-end ammeters that measure inrush and they read at 100 milliseconds.

If you look at the two examples above the compressor with a hard start will read lower at 100 milliseconds because it is already almost at full speed and the LRA has nearly passed.

LRA (locked rotor amps) is essentially the amperage the compressor runs when the run winding is functioning as a heater rather than a motor. The quicker the motor starts turning the quicker it gets out of the LRA range.

Now consider the start winding. Take a look at the start winding with no hard start…. it stays the same even though the compressor is essentially locked all the way up to 600 milliseconds. So even though the compressor is locked the current through the start winding is limited to what the run capacitor can hold and release.

With the hard start in place, the start winding amperage peaks right up until the potential relay takes it out at 100 milliseconds.

The problem is that we use tools that measure at 100 milliseconds when that may be before or after the motor has hit that 80% speed


Conclusion #2

Locked rotor amps are what they are… that’s why the manufacturer can publish it on the data tag. It’s the amperage that motor will draw when it’s locked at the rated voltage.

When we measure inrush with a meter we are really just taking a snapshot at a particular point in time that may or may not line up with what the manufacturer published depending on how close the motor to full speed at that time.

Conclusion #3

Hard start kits don’t decrease starting amps at the moment of start, they can’t.

What they can do is reduce the time it takes to get the motor started, So in a time-averaged sense, a hard start kit very well may reduce amperage and wattage.

This is why a hard start is often specified for long line applications and non-bleed TXVs. It’s also why Techs have found that adding a hard start can reduce light dimming complaints by speeding up the amount of time the compressor remains at LRA.

Conclusion #4

Hard start kits do increase the current on the start winding, in fact, that’s essentially all they do differently than a run capacitor. They both provide a phase shifted current to the start winding, a start capacitor just does MORE of it.

The argument that hard start kits reduce wear and stress on the start winding is false. With no hard start the load on the start winding is constant and very low. Adding in a hard start adds in more current at start and the possibility of potential relay sticking that can definitely cause start winding stress.

This isn’t to say start kits are a problem when sized properly. They can and do reduce LRA on the run winding by adding more phase shifted current to start.

Conclusion #5

Start capacitor sizing and potential relay voltage ratings are really important and should be selected to do the job of starting the compressor quickly without staying in too long or providing more start winding current than needed.

— Bryan

Before we get into the parts that will ruffle some feathers lets talk a bit about what a “Start” capacitor is and what it does.

First, let’s review that both start and run capacitors connect between the leg of power opposite of compressor common and the start winding.

Even though it seems like a run capacitor should connect to the run winding, it doesn’t… it connects to the start winding just like a start capacitor

Most start capacitors have a much higher MFD (Microfarad) rating than the run capacitor meaning they can store and release much more current. They are also generally electrolytic capacitors instead of oil filled metal film type like a run capacitor.

All this adds up to a start capacitor being able to store and release a lot of current into the start winding but it only stays in the circuit a short period of time without damaging itself because it cannot dissipate heat easily like the metal film run capacitor.

The start capacitor is wired in parallel with the run capacitor as shown in the image above. On startup, the potential relay contacts are closed which means the capacitor is in the circuit with all that electron storage capacity. When the compressor contactor closes a large amount of current can move into that start winding because there is a larger “membrane” (see part 1 of this series) that can store and release energy. This extra current moving through the start winding helps get the compressor started more quickly,

But the start capacitor must be pulled out of the circuit very quickly to avoid overheating itself or damaging the compressor start winding

Single phase compressor start windings are not designed to carry high continuous current like the run winding. If the start capacitor were to stay in the circuit too long the current on the start winding will stay high and will risk damaging the start winding.

There are various types of relays and controls that can remove a start capacitor from the circuit but the most common is a potential relay. The potential relay coil is either connected between start and common or start and run and it is sized to open up when the motor reaches about 80% of it’s full speed.

The potential relay opens based on an effect called “Back EMF” which leads us to our next thought experiment.

Thought Experiment #6 – Where Does Back EMF Come from?

Next time you check a dual capacitor, measure voltage (Safely) between the C and HERM terminals on the capacitor, Now measure between L1 and L2 at the contactor (line in). You will notice that the voltage at the capacitor is significantly higher than the input voltage when the compressor is running.

This has led many techs to conclude that the capacitor somehow “boosts” voltage like a transformer… that is not what’s happening at all.

That increased voltage is actually being generated by the compressor motor and we that power “back EMF”. When we spin a motor using magnetism (which is how we spin a motor) the motor also acts like a generator as the magnetic fields from the rotor (the part that spins) interacts with the stator (the part that stays still with the windings in it). When the motor is still it generates no back EMF or Inductive reactance (magnetic resistance) and this is why at startup the motor draws high amps and produces no back EMF.

As the motor begins spinning faster and faster the back EMF and inductive reactance increases, this causes the amperage to drop on the run winding and the back EMF to increase. Now you generally won’t see the back EMF when you measure between Run and Common because they are connected to the line power which dissipates this returned energy very quickly. You do measure it between start and common and run and start because the start winding is only connected between the lines capacitively.

When use this back EMF to our advantage to open up the relay contacts on the potential relay and get that start capacitor out of the start winding circuit as soon as the motor approaches full speed.

Let’s do a bit of recap…

  • The start winding and run windings are not the same and do not function in the same way in single phase air conditioning compressors
  • Common is a point not a winding
  • A capacitor functions like a membrane or storage tank for current
  • The current that can move in and out of the start winding is dictated by the voltage across the capacitor and the size of the capacitor and has nothing to do with the load on the compressor, LRA or anything else.
  • Locked Rotor amps occur on the run winding not on the start winding. When we measure LRA on common we are seeing the combination of start and run but without a start capacitor, the vast majority of the amperage will be on the run winding.
  • If your capacitors are failed open you can have no current through the start winding
  • A start capacitor increases the amount of current that can move through the start winding for the first few hundred milliseconds after startup
  • The Back EMF we measure at the capacitor is generated by the motor and increases the faster the motor spins

We wrap it all up in the next and final article in the series.

— Bryan



This series of articles is one of those that will bug a lot of people because it will go against a lot of what you’ve been told about compressors, start capacitors and inrush current. It is for this reason that I want you to work through a few thought experiments first and maybe even stop and try it on your own unit before you get worked up.

Thought Experiment #1 – A Capacitor as a Balloon or Membrane 

A capacitor stores electrical energy, the amount of electrical energy stored depends on the pressure (voltage) across the capacitor as well as the size of the capacitors (Measured in Microfarads for our purposes).

Think of the capacitor like a membrane that a water pump pushes against. Water can’t go through the membrane but it can be “stored” in the membrane to some extent on the higher pressure side. If the membrane is larger (Higher microfarads) or if the pressure differential across it is higher (Voltage) then the capacitor can hold more energy.

In alternating current the pressure shifts from positive to negative 60 times per second (60hz) so that stored energy builds up and is released to and from the capacitor back and forth 60 times per second but at 90 degrees out of phase from the way the energy would be distributed if there were no membrane at all.

The point of this analogy is to anchor the reality that the amount of “water” that can move in and out of this membrane is completely dependant on the size of the membrane (capacitor) and the pressure (voltage) across it.

Wanna test for yourself? connect 120v from a plug across a 20mfd capacitor and measure the amperage. Then do the same with a 40mfd capacitor and measure the amperage. Do it safely with proper PPE of course, but what do you think you will find based on the analogy of the membrane?

Remember, current is just a measure of the quantity of electron flow. The larger capacitor will show a higher current even though no real “work” is being done by that current. It is just moving in and out of the capacitor like a pressure tank.

Thought Experiment #2 – There is no “Common” winding

What happens when you walk up to a residential system and the run capacitor is failed open when there is no start capacitor?

The compressor draws “Locked Rotor Amps” amps on the common right?

But what is common? it is the point that connects the start and run windings, then connects to the C terminal on the compressor after going through the thermal overload.

The common is a point that connects back to the line but it is not a “winding”, there are only two windings (wraps of copper wire around the motor stator) and those are run and start.

When the system has a failed run capacitor and you measure “LRA” on common, what do you think the amperage will be on Run? What about Start?

Try it next time you have a failed capacitor, or go out and safely disconnect your run capacitor and test.

Your run and common will read exactly the same (what goes in must come out) and your start will read zero amps. This is pretty obvious and intuitive, if your capacitor is failed the only winding in the circuit is the Run so common and run will be the same. The start winding can have no amperage because the capacitor is failed open.

So the current on the start winding of a single phase compressor is completely dependant on whether there is capacitor in the circuit and the size of that capacitor.

Part #2 is coming tomorrow…

— Bryan





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