How to Check a Start Capacitor

How to Check a Start Capacitor

A start capacitor is a variant of a capacitor which is specially designed to give an instant boost to the working motor. Start capacitor allows a motor to be cycled on and off rapidly by increasing its starting torque. It is used to give the initial boost, and it is not meant to be used for more than a few seconds, which allows it to stay energized for a long time. However, its design doesn’t allow it to dissipate the heat, which may create from the continuous operation of the motor. Though it is long lasting, it may get worse, and you need to start capacitor test.

How  to check a start capacitor , you need to discharge the capacitor first. To unplug a capacitor, you’ll need


* A Screwdriver with an insulated handle

* A needle-nose pliers

* A pair of rubber gloves


After discharging it, check the capacitor carefully. While testing a start capacitor check the top(cap) of the capacitor carefully but most of the time the top stays the same without any notable visible sign of damage. Let’s not be worried. There are several ways of checking it. 


Testing a capacitor in the motor and start the motor in the usual way. If the motor responses (sounds like growling) but it is not working, try to rotate the fans of the motor manually (which does the capacitor, giving the initial power to start the motor). Be careful while doing this as it may get you hurt. If the motor starts spinning, then, undoubtedly, the capacitor is bad, and you need to replace it as early as possible.

**If a motor energized for a minute or two is a bad starter capacitor but taking an extended period is very bad which may end up ruining the motor permanently. 


Method- 2 


If you own a Multimeter, you can check the capacitor more conveniently and accurately. After discharging the capacitor from your appliance, connect the Multimeter leads to the terminals of the capacitor. Select the Meter on Ohm (select the higher range of Ohms). Select the higher range of Ohm in Analog Multimeter. Now check for the result.


* If the resistance is low, it means your capacitor is SHORT.

* If there is no deflection on Ohm meter, your capacitor is OPEN.

* If the meter shows low resistance initially but increases to the infinity gradually, it means your capacitor is in GOOD condition.


Method- 3


Discharge the capacitor carefully. Now check the Voltage rating (normally it is printed on it. Check the top/bottom if you do not find it). Charge the capacitor for a few seconds. Make sure to connect the Positive lead to the Positive lead of the voltage source and Negative lead to the Negative lead). Make sure that the Voltage source has less voltage than the capacitor. Set the Voltmeter to the DC and connect the testing capacitor by connecting the positive lead to positive lead of the meter and negative to negative. Observe and note the voltage shown on Voltmeter. If the result is close to the printed numbers on the capacitor, then your capacitor is in GOOD condition. If it indicates far little from the printed value, then it is dead. 

**The Voltmeter will show the value for a very little time and as soon as the capacitor discharges, its volt in the Voltmeter will be normal again. 


Besides start capacitor, there is another kind of capacitor in an HVAC unit commonly known as ‘run capacitor.’ The start capacitor provides the initial power (extra voltage), which makes the fan start, and the run capacitor provides energy to keep them running continuously. In simple words, start capacitor functions only at the starting of the motor generating the initial power while the run capacitor performs throughout the cycle relentlessly. The dual run capacitor is used heat pumps and air conditioners as it connects both the compressor and fan. Run capacitors may go bad and need to be replaced like the start capacitor. To start a run capacitor test, you’ll need –

* Screwdrivers

* Nut drivers

* Needle-nose pliers 

* Safety glasses

* Rubber gloves

* Multimeter


Capacitors store electrical energy to trigger. It may store lethal charge too. So you need to be cautious. Make sure to discharge the capacitor before touching the terminals or to connect the wire to the terminals. The quickest and convenient way of discharging a capacitor is to short the capacitor terminals together by using a well insulated screwdriver. It may create a loud crack while arcing discharge. Even the capacitors can explode if it is faulty. Remember to put one of your hands in your pocket, and you’ll never become a ground for the electricity.


Testing a Run Capacitor:

* Discharge the capacitor first. If you failed to discharge it, it might give you a lethal electric shock. Even it may destroy your Voltmeter.

* Remove the leads from the capacitor. Use needle-nose pliers if necessary.

* Set the Multimeter to Ohms which may sign as Ω (omega sign).  Set the range (which often uses the sign R) to 1000 or higher (if possible).

* Touch the probes of meter to the terminal of the capacitor. The needle of the meter should be zero (0) at first and then slowly drift to upwards. If it happens, it is a good and working capacitor. If the meter doesn’t react in this manner, it is bad and needs to be replaced.


Besides this, you can use Heathkit capacitor tester and Capacitor leakage tester kit to check your capacitor efficiency. Harbor Freight produces specialized tools to check the capacitors more efficiently, safely and conveniently, and you can use those to start capacitor test.

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