Tag: run capacitor

One of the most common parts to fail on a single phase HVAC system is a run capacitor, so much so that we sometimes refer to junior techs as “capacitor changers”. While capacitors may be easy to diagnose and replace, here are some things many techs may not know.

Capacitors Don’t “Boost” the Voltage 

A capacitor is a device that stores a differential charge on opposing metal plates. While capacitors can be used in circuits that boost voltage they don’t actually increase voltage themselves. We often see higher voltage across a capacitor than the line voltage, but this is due to the Back EMF (Counter electromotive force) generated by the motor, not the capacitor.

Current Doesn’t Flow Through The Capacitor, Just in and Out of It 

Techs notice that the one side of power is connected to the C terminal or the side opposite the run winding. Many techs imagine that this power “feeds” into the terminal, get’s boosted or shifted and then enters the compressor or motor through the other side. While that may make sense it isn’t actually how a capacitor works at all.

A typical HVAC run capacitor is just two long sheets of really thin metal, insulated with an insulation barrier of very thin plastic and immersed in an oil to help dissipate heat. Just like the primary and secondary of a transformer the two sheets of metal never actually touch, but electrons do gather and discharge with every cycle of the alternating current.  For example, the electrons that gather on the “C” side of the capacitor never go “through” the plastic insulation barrier over to the “HERM” or “FAN” side. The two forces simply attract and release in and out of the capacitor on the same side they entered.

The Higher the Capacitance, the Higher the Current on the Start Winding 

On a properly wired PSC (Permanent Split Capacitor) motor, the only way the start winding can have any current move through is if the capacitor stores and discharges. The higher the MFD of the capacitor, the greater the stored energy and the greater the start winding amperage. If the capacitor is completely failed with 0 capacitance it is the same as having an open start winding. Next time you find a failed run capacitor (with no start capacitor) read the amperage on the start winding with a clamp to see what I mean.

This is why oversizing a capacitor can quickly cause damage to a compressor. By increasing the current on the start winding the compressor start winding will be much more prone to early failure.

The Voltage Rating is What it Can Handle, Not What it Will Produce

Many techs think they must replace a 370v capacitor with a 370v capacitor. The voltage rating displays the “not to exceed” rating, which means you can replace a 370v with a 440v but you cannot replace a 440v with a 370v. This misconception is so common that many capacitor manufactures began stamping 440v capacitors with 370/440 just to eliminate confusion.

You Can Test a Capacitor While the Unit is Running

You simply measure the current (amps) of the motor start winding coming off of the capacitor and multiply it times 2652 (on 60hz power 3183 on 50hz power) and then divide that number by the voltage you measure across the capacitor. For a full write up on the process, you can look here

 

 

 

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

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