Start Capacitor and Inrush Facts & Myths – Part #3

Before we get into the parts that will ruffle some feathers, let's 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 for 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. The potential relay contacts are closed on startup, 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 risk damaging the start winding.

Various types of relays and controls 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 its 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, but that is not what's happening at all.

That increased voltage is actually being generated by the compressor motor, and we call that power “back EMF.” When we spin a motor using magnetism (which is how we spin a motor), the motor also acts as a generator when the magnetic fields from the rotor (the part that spins) interact 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 the motor draws high amps and produces no back EMF at startup.

As the motor begins spinning faster and faster, the back EMF and inductive reactance increase, which 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.

We 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 a 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 as the motor spins faster.

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


P.S. – Here are the previous installments of this series:

Start Capacitor & Inrush Facts and Myths – Part #1

Start Capacitor & Inrush Facts and Myths – Part #2


David White
David White
11/3/22 at 07:40 PM

Great articles; Where is Part 4?
Would love to know how one tests the circuit, especially the relay since NC. I have a current situation where the (C2S) + (C2R) = (R2S) but ohms off slightly. 1.59 + .67 = 2.16. The Start and Run Cap check out fine. The CSR coil and contacts Ohm out. No breaker is tripped. Line in and Load out are 240 and on each leg of the Line I get .29A.
Yet when I manually invoke load (contractor), I get nothing on any of the compressor leads. Even with Max set, I’m guessing it trips so fact the meter can’t pick it up.


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