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
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.
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.