Hi, I'm Vance and welcome to Spec. Sense.
Capacitors are an essential component in many single phase motors.
This includes furnace blowers, air conditioner fans, compressors, and pumps.
Capacitors store electrical energy.
As an electrical charge builds and releases between the dielectric plates,
the voltage will be delayed slightly as it lags behind the charging current.
This creates a magnetic phase shift in the start winding
providing the torque needed to turn the rotor.
Now if the motor isn't starting,
or if it starts slowly then its likely that the capacitor has failed.
Sometimes the capacitor will bulge or leak when it fails.
In other cases the failure is internal and can't be seen.
In this episode, we'll look at how to safely discharge a capacitor,
and how to test capacitance with a multimeter.
On the capacitor you'll see 2 ratings, Capacitance and Voltage.
The voltage rating refers to the maximum voltage that the capacitor can be exposed to.
You can use a capacitor with a equal or higher voltage rating
but never with a lower rating as this can result in capacitor failure.
The capacitance, expressed in microfarads, is a rating of how much energy can be stored in the capacitor.
This should be matched exactly.
Start capacitors have a high capacitance, anywhere from 40 to 300 microfarads.
They are used only briefly during the startup sequence in Capacitor Start motors.
Run capacitors have a lower capacitance, around 5 to 80 mircofarads.
They are permanently wired into the circuit, and are used in Permanent Split Capacitor motors.
Dual run capacitors are used to support 2 electric motors
like in air conditioners that use both a compressor and blower fan.
They will have 3 terminals marked, Common, Fan, and HERM, or hermetically sealed compressor.
They will have 2 microfarad ratings.
The higher rating is usually for the compressor, and the lower rating is for the fan.
Always be safe, first disconnect the power and gas before accessing your capacitor.
Be aware that even when the power is off, they can still hold a charge.
Always assume capacitors are charged, as there is a risk of a serious electrical shock.
Before testing, discharge any stored electrical energy in the capacitor.
You can place a resistor or a screwdriver with an insulated handle across each terminal.
This will short out and discharge the capacitor.
On a dual capacitor, first touch the screwdriver from FAN to Common,
and then from HERM to Common to discharge both capacitors.
To test the capacitor, use a multimeter with a capacitance testing function.
Now touch the probes to each terminal.
If the capacitor is good, then the multimeter will show a reading very close to the rated value.
Capacitors can lose capacitance over time and outside a 5% tolerance,
any change in capacitance will negatively affect the motor performance.
If the multimeter shows a significantly different reading, then its best to replace the capacitor.
For dual run capacitors first test from Herm to common. Then test from Fan to common.
These readings should also match closely to the 2 rated values.
You can also test if the capacitor has shorted to ground.
Set the multimeter to ohms and touch the probes from each terminal to the side of the housing.
The multimeter should display OL or open loop, as No terminal should have continuity.
If there's any resistance, then the capacitor has shorted to ground and should be replaced.
Now you know how to safely test a motor capacitor.
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