There are two types of capacitors you’ll encounter in HVAC field. We have the run capacitor and the start capacitor. Both are measured in microfarads which you will see labeled as uf, µF, or mfd.

Capacitors are used only in single-phase motors. You will not see a capacitor in a three-phase unit.

Run Capacitor

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Max applied voltage is the maximum back emf that can be applied to the capacitor before the dielectric fluid inside the capacitor will break down. We use 370V and 440V. When you’re replacing a 370V, you can use either a 370V or a 440V since the 440V is a higher rated. You cannot use a 370V in place of a 440V since the 370V is a lower rating. You can only use a higher rated capacitor (440V) in place of a lower rated capacitor (370V).

When you measure across a capacitor with a multimeter, you will see a higher voltage than your applied voltage. That’s because you will also be measuring the back emf from the Start (secondary/auxiliary) winding.

Run capacitors are designed to be cooled by air passing over the body and filled with oil. The temperature limit is written on the label.

Dual Capacitor

We also use dual run capacitors which is just two capacitors in one. You will run into these quite a bit when working on condensers in air conditioning.

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Dual run capacitor terminals:

• Herm – Hermetically sealed compressor. That’s the auxiliary winding of the compressor and the higher of the listed microfarad rating. So in the 45/5uf capacitor above, it’s the 45uf
• Fan – Fan of the condenser and the lower of the two ratings. In the 45/5uf rating, it’s the 5uf
• C – Common point of the capacitor

The weak point is designed to give away if the capacitor overheats. It’s an intended weakness for safety.

Sometimes your capacitor will be so rusted you cannot make out the terminal labels. In that case, Common will usually have 4 terminal taps, Herm will have 3 terminal taps, and Fan will have 1 or two terminal taps.

It’s a common misconception that a run capacitor is used to boost voltage in a motor or that current will flow through a capacitor. While capacitors like that exist, we do not use them in a PSC (permanent split capacitor) motor.

A run capacitor is used to power the secondary winding of a motor in a single-phase application bringing that secondary winding back in phase with the run winding. This secondary winding can also be referred to as the auxiliary winding. In most residential heating and air conditioning, this would be the Start winding of the motor whereas the Run winding would be the primary winding.

If you recall from the, “What is Alternating Current” lesson, electrons reverse 60 times a second and are labeled as positive and negative on the sine wave. This shift is essential in understanding capacitors.

We’re going to be focusing on a compressor in a condenser using for electrical flow in this lesson.

• T1 of the contactor feeds through Common of the compressor, through Start (secondary/auxiliary) winding to Herm on the capacitor. That charges the Herm side of the capacitor. You can think of it as the electrons are just sitting on that terminal waiting to discharge backward and power the secondary winding
• Meanwhile, T2 of the contactor feeds to Common on the capacitor. Remember, this Common terminal on the capacitor is not connected to anything aside from T2
• Once we have a sign wave shift, electron flow reverses causing the capacitor to discharge back the way they came. So, the electrons that are stored on the Herm terminal reverse, flowing from Herm on the capacitor, through the secondary (start or auxiliary) winding of the compressor powering it

At no point does any current move through the capacitor. It only stores and discharges on the Herm and Fan side, although we’re just focusing on the Herm side now.

Start Capacitor

Start capacitors are used in conjunction with a potential relay (start relay). A start relay is left in the circuit until the motor reaches about 3/4 of the running speed. It happens in about a second or less. It has to be taken out of the circuit because they’re increasing torque much more than a run capacitor. The capacitor could explode or destroy the windings in the compressor if left in the circuit.

The start capacitor is used to increase the phase angle between the secondary (start or auxiliary) winding and the run (primary) winding.

Start capacitors are rated with a range, such as 189-227uf, which is quite a bit higher than a run capacitor.

The potential relay is used in all capacitor start/capacitor run motors. They’re also used in capacitor start/induction run motors. However, it’s rare to see that style of a motor in residential and lite commercial in our industry.

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The potential relay may also be called a start relay. Without it, the start capacitor could explode or destroy the windings in the motor within seconds. Using a start capacitor with a potential relay is known as a hard start kit.

With the metal rotor turning and surrounded by motor windings, this generates a significant voltage called back emf. As back emf is generated across the start winding, a small current will flow through the start winding and across the coil of the potential relay. With this happening, the potential relay takes the start capacitor out of the circuit using this back emf when the motor reaches about 3/4 of its speed.

With power removed from the motor, the back emf generating across the start winding of the motor will become too low and the contacts that were opened will again close.

The potential relay has a coil wired between terminals 2 and 5 and NC (normally closed) contacts between terminals 1, and 2. 4 and 6 are dummy/ convenience terminals.

Testing the relay

Checking a potential relay is quite simple as there’s not much to them.

Coil – Between terminals 2 and 5

The coil of the relay is between terminals 2 and 5. It’s very high resistance, so use a high setting on your meter if it doesn’t self-adjust. You should have a reading in the thousands of ohms. I’ve typically seen them around 10kΩ – 12kΩ. OL or infinity would indicate no continuity through the coil meaning a broken wire in the coil, and this potential relay would need replacement.

NC Contacts – Between terminals 1 and 2

With power off to the appliance, disconnect the wires on terminals 1 and 2. Using an Ohm meter, check resistance across the two terminals. You should read near 0Ω or very very close. While set to continuity with the beep, you should hear the beep sound indicating the contacts are closed.

If you read OL or infinity, that means the NC contacts are not open. They would be stuck open. You would need to replace this potential relay.

Relay

We’ll be using the most common type of relay in residential service, a 24v relay. A relay is an electronically operated switch that opens and closes using an electromagnet. The electromagnet is a coil of wire that when powered acts as a magnet to close/open a switch.

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Terminal 1 of this relay is the common terminal. It’s “common” to terminal 2 and 3. Terminal 1 will be our terminal that will pass on power to and/or complete the circuit between terminal 2 or 3 when 24v is applied to the electromagnet.

Terminal 2 of the relay is a NC (normally closed) contact which you will usually see written on diagrams and equipment as NC. So when no 24v is powering this relay, the switch between terminals 1+2 is closed completing that side of the circuit.

Now when you power the coil with 24v, the switch between terminals 1+2 open and break the NC connection.

Example: If you had 120v to terminal 1 and terminal 2 was connected to a light, that light would be on as that circuit is NC. As soon as the relay is powered by 24v, the switch opens and the NC connection is now open breaking the circuit and the light goes out.

Terminal 3 of the relay is a NO (normally open) contact which you will usually see written on diagrams and equipment as NO. So when no 24v is powering this relay, the switch between terminals 1+3 is open with no completion of that circuit.

Now when you power the coil with 24v, the switch between terminals 1+3 closes and completes the NO circuit making it closed allowing for power to pass.

Example: You have no 24v powering the relay. You do have 120v to terminal 1. Terminal 3 is connected to a light. So this light would be off as that circuit between 1+3 is NO. As soon as the relay is powered with 24v, the switch between 1+3 closes and the NO connection is now closed making the circuit, and the light goes on.

As you can see with a powered relay, the NO changes to NC and the NC changes to NO.

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Terminals 4, 5, and 6 are controlled the same as terminals 1, 2, and 3. It just gives you the option to control another piece of equipment at the same time with one relay instead of two.

A switch opens and closes to allow the flow of electric current or to stop it. There are many types of switches including, but not limited to manual switch you would have to turn on an off, switches reacting to pressure, heat, or to water or air.

Switches are designed to be either normally open (NO) or normally closed (NC). They will either make or break a circuit.

Example: A pressure switch is designed to sense unsafe operation of a furnace. It’s a NO (normally open) switch that closes on the rated negative pressure. When exhaust venting becomes plugged, the pressure the switch senses is not correct and breaks the circuit.

Wiring in the circuit connects to the contacts of the switch to move current through the circuit when the pole comes into contact with the contacts.

SPST – Single Pole, Single Throw

The switch is either open or closed like a furnace door switch and has one pole and one contact.

SPDT – Single Pole, Double Throw

Similar to the SPST, but with an extra throw (contact). The SPDT has a pole that can move to one of two contacts

DPST – Double Pole, Single Throw

Like the SPST but with an extra pole and contact. 240v appliances are controlled with one pole wired to L1 and the other pole wired to L2 with each pole being 120v.

DPDT – Double Pole, Double Throw

Similar to the DPST, but with an extra throw and extra contact per pole.