Introduction to Capacitors
1.Understand the function of run capacitor
The run capacitor provides continuous phase-shifted current to the motor start winding, enabling the motor to run:
- with improved efficiency
- in the right direction
- with proper torque
Has a power factor close to “unity” (power factor close to 1.0) If the run capacitor fails, in the case of high torque motors such as compressors that usually cannot run, or have a fan running, they may run backwards and run slowly, Too high current, overheating, etc.
2.Understand the cause of the failure of the run capacitor
The run capacitor fails for the following reasons:
- poor manufacturing quality
- overheat
- Overvoltage (too high voltage)
3.Visual inspection
If the capacitor shows physical damage, such as bubbling from the top or leaking oil, it should be replaced. Normal rust is not a reason to replace the capacitor. Note the microfarad (MFD or µF) rating listed on the capacitor. The rated voltage is also worth noting; higher rated voltage capacitors can be used, but not lower.
4.Decide whether to load test or bench test
If the system is currently running, then load testing is probably best. Do not underload the blower capacitor, as the gauge leads may become wrapped around the rotating blower wheel. If the system isn’t running, then benchmarking will be your best bet.
5.Bench test or load test
Choose Under Load if possible, as it can be done under the real load conditions of the running system. Select Bench Test for simplicity or when the system is not running.
6.Load test
Measurements need to be made while the system is running. Please wear appropriate PPE and only do so when safe to do so. An accurate multimeter is required that can reliably measure voltage and current. In general, if the current clamp is receiving interference from other circuits, the measurement under load can be high. Measure the amperage on the starting wire with the wire in the center of the clip, then multiply by 2652. Now, measure the voltage across the capacitor and divide the amperage x 2652 by that voltage to get the capacitance in the MFD. If the underloaded MFD is below 10%, we recommend replacement. If the rated value is exceeded, it is usually a measurement error.If the capacitor is weak by load testing, proceed to bench testing.
7.Bench test
Bench testing simply removes the two leads from the run capacitor after safely disconnecting power and discharging the capacitor. Then place a meter for testing the capacitance between the terminals and note the reading. Be careful not to touch the gauge probes as you try to make a good, solid connection with the metal connection spade on the capacitor. If the measurement exceeds 10%, we recommend replacement.
8.Check the double run capacitor and pay attention to the connection position of each line
Taking pictures is one of the easiest ways to remember before removing the wires.
9.Power off
The power supply must be disconnected, the device is switched off and free from potential. Test with a multimeter with a pre-tested known voltage source and check L1 to L2, L1 to ground, and L-2 ground to ensure no voltage is present.
10.Discharge capacitor
Before testing, touching or removing the capacitor, it needs to be discharged. Do this by bridging the HERM and Fan terminals to C or a single capacitor with a high-impedance resistor.
NOTE: In practice it is very rare for a run capacitor to contain a charge in a normal operating system as it will discharge through the compressor windings unless one of the windings is open. For this reason, many technicians choose to discharge with a screwdriver, a controversial but common practice.
11.Disconnect the wires and remove the used dual run capacitor
To remove, disconnect the wires on top of the capacitor, then remove the clips holding the capacitor in place.
12.Replace the capacitor with the same MFD rating
During this process, the same MFD rated capacitor must be used. The value is on the side of the capacitor. Install the capacitor vertically with the terminals facing up. Test the new capacitance index through bench test before installation.
13.Readjust the size of the cable tie
Sometimes the new capacitor may be larger or smaller. At this point, use the metal strapping and create a new strapping for the capacitor. Cut it to the correct size and attach it to the correct area with self-tapping screws. Always check to make sure you don’t have any danger of puncturing the coil before using a self-tapping tool.
14.Connect the wires
At this point, reconnect the wires to the top of the dual run capacitor. Make sure the common, HERM (compressor) and FAN are connected to the capacitor. You should always double check to make sure all the wires are in the correct place. Make sure the terminals fit snugly; squeeze them with needle nose pliers to make sure they are very tight before installing.
15.Test system
Reconnect power or turn circuit breaker back on. Check to make sure all aspects of the system are running and the compressor and fans are running at the proper amperage. A good addition is to use a dynamometer and test the motor power factor to make sure it is close to unity.
16.Clean and reinstall the condenser panel
Make sure to clean the area and reattach the panel to the condenser.
