The article provides an overview of the three main types of capacitors used in HVACR systems: run capacitors, dual capacitors, and start capacitors. It explains their functions, applications, physical characteristics, and importance in motor operation while also highlighting safety considerations, such as the use of bleed resistors to prevent motor damage and reduce electrical shock risks.
Three types of capacitors are frequently used in HVACR equipment:
â– Run capacitors
â– Dual capacitors
â– Start capacitors.
Run Capacitor
A run capacitor (see Figure 1) is identified by its case. It has a metal or gray plastic case. The start capacitor has a black plastic or Bakelite case. Run capacitors can be round, oval, square, or rectangular. The round or oval design is the most common.
Figure 1 This run capacitor has a rating of 30 mfd and a maximum operating voltage of 440 V.
If a motor has a start and run winding, the run capacitor is installed in series with the start winding and parallel with the run winding as shown in Figure 2. This design increases starting torque (twisting motion) and improves running efficiency by reducing running amps. The main purpose of the run capacitor is to reduce the running load amperage. The starting torque is a side benefit. The run capacitor does not provide the maximum starting torque as seen with the start capacitor.
Figure 2 This permanent split capacitor motor requires a capacitor to start and run. The run capacitor is wired in series with the start winding and parallel with the run winding.
TECH TIP
Most motors need a capacitor that is designed, tested, and selected to operate a specific motor application. Some motors only have a run winding, while others have a start and run winding. Our industry also has motors that only use start capacitors.
Dual Capacitor
As the name expresses, the dual capacitor is two capacitors in one shell. It serves the purpose of two run capacitors. A common application is in a condensing unit. A dual capacitor is used with the unit’s condenser fan motor and compressor motor. It has a common power connection. Figure 3 shows a dual capacitor. The top of the dual run capacitor is illustrated in Figure 4.
Figure 3 Note that this dual capacitor has two microfarad ratings, 35 and 7.5 mfd. The voltage rating is 440 V.
Figure 4 The top of the dual capacitor has embossed labels indicating the connection. The letter C is for a common power connection. The HERM is the connection to the compressor motor start winding. The FAN connection goes to the condenser fan motor start winding.
Start Capacitor
The start capacitor is enclosed in a round, black plastic or BakeliteTM case that is not conductive. Unlike the run capacitor it is easily identified, as shown in Figure 5. The start capacitor is used to create a 90-degree phase shift between the starting current and the voltage. The 90- degree phase shift will give the motor the maximum starting torque. In selecting the correct capacitor, note that the start capacitor must match the motor design. A capacitor that is too small will not develop enough starting torque. An oversized capacitor will overshoot the 90-degree phase shift and reduce the starting torque. Sizing within 10% must be maintained for the best starting torque.
Figure 5 Start capacitors are round with a black plastic or BakeliteTM case.
A start capacitor should not be left in a circuit for more than a second or two. Start capacitors are not designed to remove internal heat like a run capacitor. Some switching device must be used to remove the start capacitor from the circuit very quickly once the motor reaches 75% of its speed. Two types of motors use a start capacitor. One motor type is the capacitor start and induction run motor, abbreviated CSIR. The other motor type is the capacitor start and capacitor run motor, abbreviated CSCR. Motors will be studied in more detail in future units. At that time the importance of capacitors will become more apparent.
Safety Tip
Some start and run capacitors have a bleed resistor across the terminals, as shown in Figures 5 and 6. The main purpose of a bleed resistor is motor protection. A bleed resistor discharges the capacitor quickly in case there is a short cycle (on and off quickly) condition. A charged capacitor in a short cycling condition will discharge back into the motor. This can damage the motor. A second benefit of this resistor is to reduce shock risk to the technician. If a bleed resistor is installed, it should be used any time the motor is in operation.
Figure 6 A bleed resistor is installed across the terminals of a run or start capacitor. The purpose of the bleed resistor is to remove the charge from a capacitor so that it does not damage the motor in a short cycling condition.
Capacitors in HVAC Key Takeaways
In HVACR systems, capacitors play a critical role in ensuring efficient motor operation, reliable performance, and energy savings. Run capacitors enhance running efficiency by reducing power consumption, start capacitors provide the necessary torque for motor startup, and dual capacitors optimize space and simplify wiring in condensing units. Their proper selection and maintenance are essential for the longevity and functionality of HVAC equipment, making them indispensable components in heating, ventilation, and air conditioning systems.
