Tag: start winding

Take a look at the specs from this Copeland scroll compressor pulled from the Copeland Mobile App (which is an incredible app by the way).

This is a single-phase compressor so the amperages listed are based on an amperage reading from the wire connected to the common terminal.

LRA is locked rotor amperage which is the expected measurable starting amperage and RLA is rated load amps, meaning the amperage it will draw when running normally at its rated load. You may wonder why there are two different RLA ratings here… that’s not what this tech tip is about but if you get the app and click the i with the circle around it you can find out.

The point is we are always taught to measure amperage on common with single-phase motors, but do you know why?

A single-phase motor like the one shown above has three terminals (Common, Start and Run) but only two actual windings (Start and Run). The common terminal is just the “common” point between both of the windings so when we measure the amperage on common we see the total current of both windings.

In tradeschool we learn Ohms law which teaches us

VOLTS = AMPS X OHMS

However, when we try to apply that in the field we realize some things pretty quick that get in the way of applying that neat little formula

Namely –

  • Voltage (and therefore amperage) isn’t fixed in an alternating current so we measure RMS values not ACTUAL peak values
  • The total resistance (impedance) in an inductive (magnetic) load isn’t fixed and is a combination of the static resistance of the windings and the inductive reactance that builds as the magnetic fields expand and collapse and as back EMF is generated when motors spin.
  • Even in a simple DC light bulb circuit we cannot simply measure the resistance of the bulb with a meter and apply ohms law because the resistance of the filament increases as the filament heats up (try it sometime).

So to summarize….

YOU AREN’T GOING TO BE ABLE TO ACCURATELY APPLY OHMS LAW IN THE HVAC/R FIELD

When we measure the ohms of windings from terminal to terminal it is mostly meaningless because the readings are often very low anyway… sometimes so low that your meter becomes inaccurate.

Notice how low the resistances are of this same compressor.

The real resistance of the motor only shows up when it is energized with alternating current and the magnetic fields begin to interact, this total resistance when energized is called impedance.

We do know that the start winding has a higher static ohm value than the run winding and that when we add start to common and run to common together that it will equal run to start (which is a fairly obvious statement since common is just a center point) and that if the thermal overload is open we will measure OL between C-R and C-S but will read the combined value R-S.

These are all true and are reasons to pull out the meter but this still doesn’t tell us anything about the title of this article and you are probably wondering what the heck I’m driving at.

I’m making sure we are all on the same page before I drop a start winding fact bomb on you…

But one more thing we need to come to an agreement on.

The run winding is connected “Across The Line”, in other words with one leg of split phase power connected to Common and the other to run. The current that travels through that run winding is completely a function of the total impedance of that winding which has several factors including the static winding resistance, the inductive reactance of the windings and the back EMF that builds as the motor starts running.

In other words… the amperage starts high because the resistance starts low in the run winding and amperage goes down as the motor gets up to speed because the total impedance increases.

Remember, ohms law teaches us as resistance goes up, amperage goes down if the voltage stays the same.

The start winding is connected through a run capacitor and potentially some other start gear and not connected “across the line” like the run winding. This means that the current that moves through the start winding is limited by BOTH the total impedance of the winding AND the capacitance of the run capacitor and any other start gear.

 

Here is an image from an oscilloscope on this very same compressor referred to above with 197V applied, a proper run capacitor and no hard start kit…

Take a long close look.

Notice the blue line is the RUN WINDING CURRENT and the red line is the START WINDING CURRENT.

Notice that ALL of the true inrush current occurs on the run winding and the start winding current doesn’t go up until the run winding current starts to go down?

That’s because unless the start winding has some form of start capacitor it cannot draw any amperage higher than what the run capacitor will allow. In essence, the run capacitor becomes a ceiling or current limiter that allows only so much stored current per cycle and no more.

Try it sometime.

Measure the running amperage on the start winding with a capacitor slightly larger, slightly smaller and then with none at all. You will see higher amps, lower amps and then (obviously) no amps.

Try taking an inrush reading on the start wire of a compressor with no hard start and see what you get.

Then try it with a hard start.

Notice anything different on the start winding amps? Can you see the moment the back EMF removed the hard start from the circuit? Was the TOTAL amperage actually lower with a hard start or was the time to start decreased and more current shifted to the start winding?

— Bryan

 

 

 

 

Thought Experiment #3 – The Start Winding Has No “Inrush” with a run capacitor only 

The name “start winding” is an antiquated term for the single phase residential industry.

It’sa left over from the days when CSIR (Capacitor start, induction run) motors were still used commonly. In a CSIR motor the start relay removes the start winding when the motor gets near full speed and then the motor would “run” completely on one winding (like the diagram shown above).

I wish we would call the run winding the “primary” winding and the start winding the “auxilliary” or “supplemental”… But alas my last name is Orr not Westinghouse, Tesla or Edison so what do I know…

If you were to check the amperage on one of these CSIR motors on the Start winding (not common) you would see a current for the first few hundred milliseconds and then the relay would open and take the “start” winding out of the circuit completely. So after the first split second, you would have zero amps on the start winding.

This is NOT how a modern single phase compressor works.

For a modern single phase A/C system the motors are primarily PSC (Permanent split capacitor) with a run capacitor that stays in the circuit all the time and connects between the start terminal and the same line of power that feeds run.

Go ahead and measure inrush current on the wire that connects to “Herm” on a dual run capacitor on the next system you work on that has a run capacitor and no start capacitor. In most cases, your meter won’t pickup inrush at all on the start winding. Even if you do pick up a reading it will be the same as when the compressor is running…. or maybe even a little higher as the compressor gets up to speed and back EMF kicks in (more about that later).

Thought Experiment #4 – But Wait… I Know Inrush Occurs at Motor Start!

Absolutely! a motor will draw higher amperage at startup when measured on Run or Common. This is because the run and common on a single phase motor are connected “across the line” from one side of the power supply to the other. In the run winding, the current is regulated only by the resistance in the run winding.

When that run winding first gets hit with the full applied voltage it is really nothing but a heater. If you take an ohm reading on a compressor and try to work Ohm’s to calculate the current you will notice that it is VERY high. This is because the majority of the electrical impedance (total resistance) is generated once the motor starts spinning and the magnetic field inside the motor starts to push back against the magnetic field being generated by the current moving through the windings.

This can, does and MUST occur on the run winding. The amperage will jump way up to the LRA (locked rotor amps) at first until the motor gets up to speed and then it will drop back down to the RLA (run load amps)… But only on the run winding when the unit has a run capacitor only.

The start winding has that darn membrane in the way (the run capacitor) and that membrane limits how much current can go in and out of that start winding.

Thought Experiment #5 – So I Bet a Failed Capacitor Causes Start Winding Failure…. Oh… Wait

So you walk up on a unit with a run capacitor and no start capacitor and the run capacitor is failed open and looks like a bloated toad. Would that failed open capacitor result in start winding stress?

Nope…

That failed open run capacitor (when there is no start capacitor) results in ZERO current moving through the start winding which means zero heat in the start winding itself.

A failed run capacitor causes stress on the RUN WINDING because now the run winding will keep drawing LRA and going out on thermal overload until that capacitor gets replaced or the overload or run winding fails.

The start winding will just sit there open with no current load whatsoever.

Before you say it (because I know some of you are thinking it), What happens if the run capacitor fails SHORTED? While that may be a theoretical possibility it is not possible in a practical sense because of the way that metal film run capacitors are made. the metallic coating on the internal windings is so thin that it vaporizes whenever there is a high current event like a short circuit. The possibility of a modern HVAC run capacitor actually staying shorted is slim to none.

Part #3 is coming soon…. we haven’t even gotten to start capacitors yet

— 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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